diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2c0f4af9..d67f2585 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,9 @@ Rhai Release Notes
 Version 0.19.14
 ===============
 
+This version runs faster due to optimizations done on AST node structures. It also fixes a number of
+panic bugs related to passing shared values as function call arguments.
+
 Bug fixes
 ---------
 
@@ -27,18 +30,24 @@ Breaking changes
 * `Module::update_fn_metadata` input parameter is changed.
 * Function keywords (e.g. `type_of`, `eval`, `Fn`) can no longer be overloaded. It is more trouble than worth. To disable these keywords, use `Engine::disable_symbol`.
 * `is_def_var` and `is_def_fn` are now reserved keywords.
-* `Engine::id` field is removed.
+* `Engine::id` field is removed because it is never used.
 * `num-traits` is now a required dependency.
+* The `in` operator is now implemented on top of the `contains` function and is no longer restricted to a few specific types.
+* `EvalAltResult::ErrorInExpr` is removed because the `in` operator now calls `contains`.
+* The methods `AST::walk`, `Expr::walk`, `Stmt::walk` and `ASTNode::walk` and the callbacks they take now return `bool` to optionally terminate the recursive walk.
 
 Enhancements
 ------------
 
+* Layout of AST nodes is optimized to reduce redirections, so speed is improved.
 * Function calls are more optimized and should now run faster.
 * `range` function now supports negative step and decreasing streams (i.e. to < from).
 * More information is provided to the error variable captured by the `catch` statement in an _object map_.
 * Previously, `private` functions in an `AST` cannot be called with `call_fn` etc. This is inconvenient when trying to call a function inside a script which also serves as a loadable module exporting part (but not all) of the functions. Now, all functions (`private` or not) can be called in an `AST`. The `private` keyword is relegated to preventing a function from being exported.
 * `Dynamic::as_unit` just for completeness sake.
 * `bytes` method added for strings to get length quickly (if the string is ASCII-only).
+* `FileModuleResolver` can now enable/disable caching.
+* Recursively walking an `AST` can now be terminated in the middle.
 
 
 Version 0.19.13
diff --git a/src/ast.rs b/src/ast.rs
index 81b77671..9ce4c8bf 100644
--- a/src/ast.rs
+++ b/src/ast.rs
@@ -47,7 +47,7 @@ pub enum FnAccess {
 #[derive(Debug, Clone)]
 pub struct ScriptFnDef {
     /// Function body.
-    pub body: Stmt,
+    pub body: StmtBlock,
     /// Encapsulated running environment, if any.
     pub lib: Option<Shared<Module>>,
     /// Encapsulated imported modules.
@@ -61,9 +61,9 @@ pub struct ScriptFnDef {
     pub params: StaticVec<ImmutableString>,
     /// Access to external variables.
     #[cfg(not(feature = "no_closure"))]
-    pub externals: Vec<ImmutableString>,
+    pub externals: StaticVec<ImmutableString>,
     /// Function doc-comments (if any).
-    pub comments: Vec<String>,
+    pub comments: StaticVec<String>,
 }
 
 impl fmt::Display for ScriptFnDef {
@@ -149,7 +149,7 @@ pub struct AST {
     /// Source of the [`AST`].
     source: Option<ImmutableString>,
     /// Global statements.
-    statements: Vec<Stmt>,
+    body: StmtBlock,
     /// Script-defined functions.
     functions: Shared<Module>,
     /// Embedded module resolver, if any.
@@ -162,7 +162,7 @@ impl Default for AST {
     fn default() -> Self {
         Self {
             source: None,
-            statements: Vec::with_capacity(16),
+            body: Default::default(),
             functions: Default::default(),
             #[cfg(not(feature = "no_module"))]
             resolver: None,
@@ -179,7 +179,10 @@ impl AST {
     ) -> Self {
         Self {
             source: None,
-            statements: statements.into_iter().collect(),
+            body: StmtBlock {
+                statements: statements.into_iter().collect(),
+                pos: Position::NONE,
+            },
             functions: functions.into(),
             #[cfg(not(feature = "no_module"))]
             resolver: None,
@@ -194,7 +197,10 @@ impl AST {
     ) -> Self {
         Self {
             source: Some(source.into()),
-            statements: statements.into_iter().collect(),
+            body: StmtBlock {
+                statements: statements.into_iter().collect(),
+                pos: Position::NONE,
+            },
             functions: functions.into(),
             #[cfg(not(feature = "no_module"))]
             resolver: None,
@@ -231,7 +237,7 @@ impl AST {
     #[cfg(not(feature = "internals"))]
     #[inline(always)]
     pub(crate) fn statements(&self) -> &[Stmt] {
-        &self.statements
+        &self.body.statements
     }
     /// _(INTERNALS)_ Get the statements.
     /// Exported under the `internals` feature only.
@@ -239,13 +245,13 @@ impl AST {
     #[deprecated = "this method is volatile and may change"]
     #[inline(always)]
     pub fn statements(&self) -> &[Stmt] {
-        &self.statements
+        &self.body.statements
     }
     /// Get a mutable reference to the statements.
     #[cfg(not(feature = "no_optimize"))]
     #[inline(always)]
-    pub(crate) fn statements_mut(&mut self) -> &mut Vec<Stmt> {
-        &mut self.statements
+    pub(crate) fn statements_mut(&mut self) -> &mut StaticVec<Stmt> {
+        &mut self.body.statements
     }
     /// Get the internal shared [`Module`] containing all script-defined functions.
     #[cfg(not(feature = "internals"))]
@@ -333,7 +339,7 @@ impl AST {
         functions.merge_filtered(&self.functions, &filter);
         Self {
             source: self.source.clone(),
-            statements: Default::default(),
+            body: Default::default(),
             functions: functions.into(),
             #[cfg(not(feature = "no_module"))]
             resolver: self.resolver.clone(),
@@ -345,7 +351,7 @@ impl AST {
     pub fn clone_statements_only(&self) -> Self {
         Self {
             source: self.source.clone(),
-            statements: self.statements.clone(),
+            body: self.body.clone(),
             functions: Default::default(),
             #[cfg(not(feature = "no_module"))]
             resolver: self.resolver.clone(),
@@ -515,20 +521,19 @@ impl AST {
         filter: impl Fn(FnNamespace, FnAccess, bool, &str, usize) -> bool,
     ) -> Self {
         let Self {
-            statements,
-            functions,
-            ..
+            body, functions, ..
         } = self;
 
-        let ast = match (statements.is_empty(), other.statements.is_empty()) {
+        let merged = match (body.is_empty(), other.body.is_empty()) {
             (false, false) => {
-                let mut statements = statements.clone();
-                statements.extend(other.statements.iter().cloned());
-                statements
+                let mut body = body.clone();
+                body.statements
+                    .extend(other.body.statements.iter().cloned());
+                body
             }
-            (false, true) => statements.clone(),
-            (true, false) => other.statements.clone(),
-            (true, true) => vec![],
+            (false, true) => body.clone(),
+            (true, false) => other.body.clone(),
+            (true, true) => Default::default(),
         };
 
         let source = other.source.clone().or_else(|| self.source.clone());
@@ -537,9 +542,9 @@ impl AST {
         functions.merge_filtered(&other.functions, &filter);
 
         if let Some(source) = source {
-            Self::new_with_source(ast, functions, source)
+            Self::new_with_source(merged.statements, functions, source)
         } else {
-            Self::new(ast, functions)
+            Self::new(merged.statements, functions)
         }
     }
     /// Combine one [`AST`] with another.  The second [`AST`] is consumed.
@@ -599,7 +604,10 @@ impl AST {
         other: Self,
         filter: impl Fn(FnNamespace, FnAccess, bool, &str, usize) -> bool,
     ) -> &mut Self {
-        self.statements.extend(other.statements.into_iter());
+        self.body
+            .statements
+            .extend(other.body.statements.into_iter());
+
         if !other.functions.is_empty() {
             shared_make_mut(&mut self.functions).merge_filtered(&other.functions, &filter);
         }
@@ -673,45 +681,51 @@ impl AST {
     /// Clear all statements in the [`AST`], leaving only function definitions.
     #[inline(always)]
     pub fn clear_statements(&mut self) {
-        self.statements = vec![];
+        self.body = Default::default();
     }
     /// Recursively walk the [`AST`], including function bodies (if any).
+    /// Return `false` from the callback to terminate the walk.
     #[cfg(not(feature = "internals"))]
     #[cfg(not(feature = "no_module"))]
     #[inline(always)]
-    pub(crate) fn walk(&self, on_node: &mut impl FnMut(&[ASTNode])) {
-        self.statements()
-            .iter()
-            .chain({
-                #[cfg(not(feature = "no_function"))]
-                {
-                    self.iter_fn_def().map(|f| &f.body)
-                }
-                #[cfg(feature = "no_function")]
-                {
-                    crate::stdlib::iter::empty()
-                }
-            })
-            .for_each(|stmt| stmt.walk(&mut Default::default(), on_node));
+    pub(crate) fn walk(&self, on_node: &mut impl FnMut(&[ASTNode]) -> bool) -> bool {
+        let path = &mut Default::default();
+
+        for stmt in self.statements() {
+            if !stmt.walk(path, on_node) {
+                return false;
+            }
+        }
+        #[cfg(not(feature = "no_function"))]
+        for stmt in self.iter_fn_def().flat_map(|f| f.body.statements.iter()) {
+            if !stmt.walk(path, on_node) {
+                return false;
+            }
+        }
+
+        true
     }
     /// _(INTERNALS)_ Recursively walk the [`AST`], including function bodies (if any).
+    /// Return `false` from the callback to terminate the walk.
     /// Exported under the `internals` feature only.
     #[cfg(feature = "internals")]
     #[inline(always)]
-    pub fn walk(&self, on_node: &mut impl FnMut(&[ASTNode])) {
-        self.statements()
-            .iter()
-            .chain({
-                #[cfg(not(feature = "no_function"))]
-                {
-                    self.iter_fn_def().map(|f| &f.body)
-                }
-                #[cfg(feature = "no_function")]
-                {
-                    crate::stdlib::iter::empty()
-                }
-            })
-            .for_each(|stmt| stmt.walk(&mut Default::default(), on_node));
+    pub fn walk(&self, on_node: &mut impl FnMut(&[ASTNode]) -> bool) -> bool {
+        let path = &mut Default::default();
+
+        for stmt in self.statements() {
+            if !stmt.walk(path, on_node) {
+                return false;
+            }
+        }
+        #[cfg(not(feature = "no_function"))]
+        for stmt in self.iter_fn_def().flat_map(|f| f.body.statements.iter()) {
+            if !stmt.walk(path, on_node) {
+                return false;
+            }
+        }
+
+        true
     }
 }
 
@@ -804,6 +818,41 @@ impl<'a> From<&'a Expr> for ASTNode<'a> {
     }
 }
 
+/// _(INTERNALS)_ A statements block.
+/// Exported under the `internals` feature only.
+///
+/// # Volatile Data Structure
+///
+/// This type is volatile and may change.
+#[derive(Clone, Hash, Default)]
+pub struct StmtBlock {
+    pub statements: StaticVec<Stmt>,
+    pub pos: Position,
+}
+
+impl StmtBlock {
+    /// Is this statements block empty?
+    #[inline(always)]
+    pub fn is_empty(&self) -> bool {
+        self.statements.is_empty()
+    }
+    /// Number of statements in this statements block.
+    #[inline(always)]
+    pub fn len(&self) -> usize {
+        self.statements.len()
+    }
+}
+
+impl fmt::Debug for StmtBlock {
+    #[inline(always)]
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if !self.pos.is_none() {
+            write!(f, "{} @ ", self.pos)?;
+        }
+        fmt::Debug::fmt(&self.statements, f)
+    }
+}
+
 /// _(INTERNALS)_ A statement.
 /// Exported under the `internals` feature only.
 ///
@@ -815,32 +864,36 @@ pub enum Stmt {
     /// No-op.
     Noop(Position),
     /// `if` expr `{` stmt `}` `else` `{` stmt `}`
-    If(Expr, Box<(Stmt, Option<Stmt>)>, Position),
+    If(Expr, Box<(StmtBlock, StmtBlock)>, Position),
     /// `switch` expr `{` literal or _ `=>` stmt `,` ... `}`
     Switch(
         Expr,
         Box<(
-            HashableHashMap<u64, Stmt, StraightHasherBuilder>,
-            Option<Stmt>,
+            HashableHashMap<u64, StmtBlock, StraightHasherBuilder>,
+            StmtBlock,
         )>,
         Position,
     ),
     /// `while` expr `{` stmt `}`
-    While(Option<Expr>, Box<Stmt>, Position),
+    While(Expr, Box<StmtBlock>, Position),
     /// `do` `{` stmt `}` `while`|`until` expr
-    Do(Box<Stmt>, Expr, bool, Position),
+    Do(Box<StmtBlock>, Expr, bool, Position),
     /// `for` id `in` expr `{` stmt `}`
-    For(Expr, Box<(String, Stmt)>, Position),
+    For(Expr, Box<(String, StmtBlock)>, Position),
     /// \[`export`\] `let` id `=` expr
-    Let(Box<Ident>, Option<Expr>, bool, Position),
+    Let(Expr, Ident, bool, Position),
     /// \[`export`\] `const` id `=` expr
-    Const(Box<Ident>, Option<Expr>, bool, Position),
+    Const(Expr, Ident, bool, Position),
     /// expr op`=` expr
     Assignment(Box<(Expr, Expr, Option<OpAssignment>)>, Position),
     /// `{` stmt`;` ... `}`
     Block(Vec<Stmt>, Position),
     /// `try` `{` stmt; ... `}` `catch` `(` var `)` `{` stmt; ... `}`
-    TryCatch(Box<(Stmt, Option<Ident>, Stmt)>, Position, Position),
+    TryCatch(
+        Box<(StmtBlock, Option<Ident>, StmtBlock)>,
+        Position,
+        Position,
+    ),
     /// [expression][Expr]
     Expr(Expr),
     /// `continue`
@@ -848,10 +901,10 @@ pub enum Stmt {
     /// `break`
     Break(Position),
     /// `return`/`throw`
-    Return((ReturnType, Position), Option<Expr>, Position),
+    Return(ReturnType, Option<Expr>, Position),
     /// `import` expr `as` var
     #[cfg(not(feature = "no_module"))]
-    Import(Expr, Option<Box<Ident>>, Position),
+    Import(Expr, Option<Ident>, Position),
     /// `export` var `as` var `,` ...
     #[cfg(not(feature = "no_module"))]
     Export(Vec<(Ident, Option<Ident>)>, Position),
@@ -867,6 +920,27 @@ impl Default for Stmt {
     }
 }
 
+impl From<Stmt> for StmtBlock {
+    #[inline(always)]
+    fn from(stmt: Stmt) -> Self {
+        match stmt {
+            Stmt::Block(block, pos) => Self {
+                statements: block.into(),
+                pos,
+            },
+            Stmt::Noop(pos) => Self {
+                statements: Default::default(),
+                pos,
+            },
+            _ => {
+                let pos = stmt.position();
+                let statements = vec![stmt].into();
+                Self { statements, pos }
+            }
+        }
+    }
+}
+
 impl Stmt {
     /// Is this statement [`Noop`][Stmt::Noop]?
     #[inline(always)]
@@ -889,7 +963,7 @@ impl Stmt {
             | Self::While(_, _, pos)
             | Self::Do(_, _, _, pos)
             | Self::For(_, _, pos)
-            | Self::Return((_, pos), _, _)
+            | Self::Return(_, _, pos)
             | Self::Let(_, _, _, pos)
             | Self::Const(_, _, _, pos)
             | Self::TryCatch(_, pos, _) => *pos,
@@ -918,7 +992,7 @@ impl Stmt {
             | Self::While(_, _, pos)
             | Self::Do(_, _, _, pos)
             | Self::For(_, _, pos)
-            | Self::Return((_, pos), _, _)
+            | Self::Return(_, _, pos)
             | Self::Let(_, _, _, pos)
             | Self::Const(_, _, _, pos)
             | Self::TryCatch(_, pos, _) => *pos = new_pos,
@@ -938,6 +1012,31 @@ impl Stmt {
 
         self
     }
+    /// Does this statement return a value?
+    pub fn returns_value(&self) -> bool {
+        match self {
+            Self::If(_, _, _) | Self::Switch(_, _, _) | Self::Block(_, _) | Self::Expr(_) => true,
+
+            Self::Noop(_)
+            | Self::While(_, _, _)
+            | Self::Do(_, _, _, _)
+            | Self::For(_, _, _)
+            | Self::TryCatch(_, _, _) => false,
+
+            Self::Let(_, _, _, _)
+            | Self::Const(_, _, _, _)
+            | Self::Assignment(_, _)
+            | Self::Continue(_)
+            | Self::Break(_)
+            | Self::Return(_, _, _) => false,
+
+            #[cfg(not(feature = "no_module"))]
+            Self::Import(_, _, _) | Self::Export(_, _) => false,
+
+            #[cfg(not(feature = "no_closure"))]
+            Self::Share(_) => unreachable!("Stmt::Share should not be parsed"),
+        }
+    }
     /// Is this statement self-terminated (i.e. no need for a semicolon terminator)?
     pub fn is_self_terminated(&self) -> bool {
         match self {
@@ -976,23 +1075,29 @@ impl Stmt {
             Self::Expr(expr) => expr.is_pure(),
             Self::If(condition, x, _) => {
                 condition.is_pure()
-                    && x.0.is_pure()
-                    && x.1.as_ref().map(Stmt::is_pure).unwrap_or(true)
+                    && x.0.statements.iter().all(Stmt::is_pure)
+                    && x.1.statements.iter().all(Stmt::is_pure)
             }
             Self::Switch(expr, x, _) => {
                 expr.is_pure()
-                    && x.0.values().all(Stmt::is_pure)
-                    && x.1.as_ref().map(Stmt::is_pure).unwrap_or(true)
+                    && x.0
+                        .values()
+                        .flat_map(|block| block.statements.iter())
+                        .all(Stmt::is_pure)
+                    && x.1.statements.iter().all(Stmt::is_pure)
             }
-            Self::While(Some(condition), block, _) | Self::Do(block, condition, _, _) => {
-                condition.is_pure() && block.is_pure()
+            Self::While(condition, block, _) | Self::Do(block, condition, _, _) => {
+                condition.is_pure() && block.statements.iter().all(Stmt::is_pure)
+            }
+            Self::For(iterable, x, _) => {
+                iterable.is_pure() && x.1.statements.iter().all(Stmt::is_pure)
             }
-            Self::While(None, block, _) => block.is_pure(),
-            Self::For(iterable, x, _) => iterable.is_pure() && x.1.is_pure(),
             Self::Let(_, _, _, _) | Self::Const(_, _, _, _) | Self::Assignment(_, _) => false,
             Self::Block(block, _) => block.iter().all(|stmt| stmt.is_pure()),
             Self::Continue(_) | Self::Break(_) | Self::Return(_, _, _) => false,
-            Self::TryCatch(x, _, _) => x.0.is_pure() && x.2.is_pure(),
+            Self::TryCatch(x, _, _) => {
+                x.0.statements.iter().all(Stmt::is_pure) && x.2.statements.iter().all(Stmt::is_pure)
+            }
 
             #[cfg(not(feature = "no_module"))]
             Self::Import(_, _, _) => false,
@@ -1003,53 +1108,150 @@ impl Stmt {
             Self::Share(_) => false,
         }
     }
-    /// Recursively walk this statement.
+    /// Is this statement _pure_ within the containing block?
+    ///
+    /// An internally pure statement only has side effects that disappear outside the block.
+    ///
+    /// Only variable declarations (i.e. `let` and `const`) and `import`/`export` statements
+    /// are internally pure.
     #[inline(always)]
-    pub fn walk<'a>(&'a self, path: &mut Vec<ASTNode<'a>>, on_node: &mut impl FnMut(&[ASTNode])) {
+    pub fn is_internally_pure(&self) -> bool {
+        match self {
+            Self::Let(expr, _, _, _) | Self::Const(expr, _, _, _) => expr.is_pure(),
+
+            #[cfg(not(feature = "no_module"))]
+            Self::Import(expr, _, _) => expr.is_pure(),
+            #[cfg(not(feature = "no_module"))]
+            Self::Export(_, _) => true,
+
+            _ => self.is_pure(),
+        }
+    }
+    /// Does this statement break the current control flow through the containing block?
+    ///
+    /// Currently this is only true for `return`, `throw`, `break` and `continue`.
+    ///
+    /// All statements following this statement will essentially be dead code.
+    #[inline(always)]
+    pub fn is_control_flow_break(&self) -> bool {
+        match self {
+            Self::Return(_, _, _) | Self::Break(_) | Self::Continue(_) => true,
+            _ => false,
+        }
+    }
+    /// Recursively walk this statement.
+    /// Return `false` from the callback to terminate the walk.
+    pub fn walk<'a>(
+        &'a self,
+        path: &mut Vec<ASTNode<'a>>,
+        on_node: &mut impl FnMut(&[ASTNode]) -> bool,
+    ) -> bool {
         path.push(self.into());
-        on_node(path);
+
+        if !on_node(path) {
+            return false;
+        }
 
         match self {
-            Self::Let(_, Some(e), _, _) | Self::Const(_, Some(e), _, _) => e.walk(path, on_node),
+            Self::Let(e, _, _, _) | Self::Const(e, _, _, _) => {
+                if !e.walk(path, on_node) {
+                    return false;
+                }
+            }
             Self::If(e, x, _) => {
-                e.walk(path, on_node);
-                x.0.walk(path, on_node);
-                if let Some(ref s) = x.1 {
-                    s.walk(path, on_node);
+                if !e.walk(path, on_node) {
+                    return false;
+                }
+                for s in &x.0.statements {
+                    if !s.walk(path, on_node) {
+                        return false;
+                    }
+                }
+                for s in &x.1.statements {
+                    if !s.walk(path, on_node) {
+                        return false;
+                    }
                 }
             }
             Self::Switch(e, x, _) => {
-                e.walk(path, on_node);
-                x.0.values().for_each(|s| s.walk(path, on_node));
-                if let Some(ref s) = x.1 {
-                    s.walk(path, on_node);
+                if !e.walk(path, on_node) {
+                    return false;
+                }
+                for s in x.0.values().flat_map(|block| block.statements.iter()) {
+                    if !s.walk(path, on_node) {
+                        return false;
+                    }
+                }
+                for s in &x.1.statements {
+                    if !s.walk(path, on_node) {
+                        return false;
+                    }
                 }
             }
-            Self::While(Some(e), s, _) | Self::Do(s, e, _, _) => {
-                e.walk(path, on_node);
-                s.walk(path, on_node);
+            Self::While(e, s, _) | Self::Do(s, e, _, _) => {
+                if !e.walk(path, on_node) {
+                    return false;
+                }
+                for s in &s.statements {
+                    if !s.walk(path, on_node) {
+                        return false;
+                    }
+                }
             }
-            Self::While(None, s, _) => s.walk(path, on_node),
             Self::For(e, x, _) => {
-                e.walk(path, on_node);
-                x.1.walk(path, on_node);
+                if !e.walk(path, on_node) {
+                    return false;
+                }
+                for s in &x.1.statements {
+                    if !s.walk(path, on_node) {
+                        return false;
+                    }
+                }
             }
             Self::Assignment(x, _) => {
-                x.0.walk(path, on_node);
-                x.1.walk(path, on_node);
+                if !x.0.walk(path, on_node) {
+                    return false;
+                }
+                if !x.1.walk(path, on_node) {
+                    return false;
+                }
+            }
+            Self::Block(x, _) => {
+                for s in x {
+                    if !s.walk(path, on_node) {
+                        return false;
+                    }
+                }
             }
-            Self::Block(x, _) => x.iter().for_each(|s| s.walk(path, on_node)),
             Self::TryCatch(x, _, _) => {
-                x.0.walk(path, on_node);
-                x.2.walk(path, on_node);
+                for s in &x.0.statements {
+                    if !s.walk(path, on_node) {
+                        return false;
+                    }
+                }
+                for s in &x.2.statements {
+                    if !s.walk(path, on_node) {
+                        return false;
+                    }
+                }
+            }
+            Self::Expr(e) | Self::Return(_, Some(e), _) => {
+                if !e.walk(path, on_node) {
+                    return false;
+                }
             }
-            Self::Expr(e) | Self::Return(_, Some(e), _) => e.walk(path, on_node),
             #[cfg(not(feature = "no_module"))]
-            Self::Import(e, _, _) => e.walk(path, on_node),
+            Self::Import(e, _, _) => {
+                if !e.walk(path, on_node) {
+                    return false;
+                }
+            }
             _ => (),
         }
 
         path.pop().unwrap();
+
+        true
     }
 }
 
@@ -1102,7 +1304,7 @@ pub struct OpAssignment {
 /// # Volatile Data Structure
 ///
 /// This type is volatile and may change.
-#[derive(Debug, Clone, Copy, Eq, PartialEq, Hash, Default)]
+#[derive(Clone, Copy, Eq, PartialEq, Hash, Default)]
 pub struct FnHash {
     /// Pre-calculated hash for a script-defined function ([`None`] if native functions only).
     script: Option<u64>,
@@ -1110,6 +1312,20 @@ pub struct FnHash {
     native: u64,
 }
 
+impl fmt::Debug for FnHash {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if let Some(script) = self.script {
+            if script == self.native {
+                write!(f, "({}=={})", script, self.native)
+            } else {
+                write!(f, "({}, {})", script, self.native)
+            }
+        } else {
+            write!(f, "{}", self.native)
+        }
+    }
+}
+
 impl FnHash {
     /// Create a [`FnHash`] with only the native Rust hash.
     #[inline(always)]
@@ -1185,6 +1401,7 @@ pub struct FloatWrapper(FLOAT);
 
 #[cfg(not(feature = "no_float"))]
 impl Hash for FloatWrapper {
+    #[inline(always)]
     fn hash<H: crate::stdlib::hash::Hasher>(&self, state: &mut H) {
         self.0.to_ne_bytes().hash(state);
     }
@@ -1192,6 +1409,7 @@ impl Hash for FloatWrapper {
 
 #[cfg(not(feature = "no_float"))]
 impl AsRef<FLOAT> for FloatWrapper {
+    #[inline(always)]
     fn as_ref(&self) -> &FLOAT {
         &self.0
     }
@@ -1199,6 +1417,7 @@ impl AsRef<FLOAT> for FloatWrapper {
 
 #[cfg(not(feature = "no_float"))]
 impl AsMut<FLOAT> for FloatWrapper {
+    #[inline(always)]
     fn as_mut(&mut self) -> &mut FLOAT {
         &mut self.0
     }
@@ -1208,6 +1427,7 @@ impl AsMut<FLOAT> for FloatWrapper {
 impl crate::stdlib::ops::Deref for FloatWrapper {
     type Target = FLOAT;
 
+    #[inline(always)]
     fn deref(&self) -> &Self::Target {
         &self.0
     }
@@ -1215,6 +1435,7 @@ impl crate::stdlib::ops::Deref for FloatWrapper {
 
 #[cfg(not(feature = "no_float"))]
 impl crate::stdlib::ops::DerefMut for FloatWrapper {
+    #[inline(always)]
     fn deref_mut(&mut self) -> &mut Self::Target {
         &mut self.0
     }
@@ -1222,6 +1443,7 @@ impl crate::stdlib::ops::DerefMut for FloatWrapper {
 
 #[cfg(not(feature = "no_float"))]
 impl fmt::Debug for FloatWrapper {
+    #[inline(always)]
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         fmt::Display::fmt(self, f)
     }
@@ -1229,6 +1451,7 @@ impl fmt::Debug for FloatWrapper {
 
 #[cfg(not(feature = "no_float"))]
 impl fmt::Display for FloatWrapper {
+    #[inline(always)]
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         #[cfg(feature = "no_std")]
         use num_traits::Float;
@@ -1244,6 +1467,7 @@ impl fmt::Display for FloatWrapper {
 
 #[cfg(not(feature = "no_float"))]
 impl From<FLOAT> for FloatWrapper {
+    #[inline(always)]
     fn from(value: FLOAT) -> Self {
         Self::new(value)
     }
@@ -1253,6 +1477,7 @@ impl From<FLOAT> for FloatWrapper {
 impl FromStr for FloatWrapper {
     type Err = <FLOAT as FromStr>::Err;
 
+    #[inline(always)]
     fn from_str(s: &str) -> Result<Self, Self::Err> {
         FLOAT::from_str(s).map(Into::<Self>::into)
     }
@@ -1260,6 +1485,7 @@ impl FromStr for FloatWrapper {
 
 #[cfg(not(feature = "no_float"))]
 impl FloatWrapper {
+    #[inline(always)]
     pub const fn new(value: FLOAT) -> Self {
         Self(value)
     }
@@ -1298,18 +1524,16 @@ pub enum Expr {
     Unit(Position),
     /// Variable access - (optional index, optional (hash, modules), variable name)
     Variable(Box<(Option<NonZeroUsize>, Option<(u64, NamespaceRef)>, Ident)>),
-    /// Property access - (getter, hash, setter, hash, prop)
-    Property(Box<(ImmutableString, u64, ImmutableString, u64, Ident)>),
-    /// { [statement][Stmt] }
-    Stmt(Box<StaticVec<Stmt>>, Position),
+    /// Property access - ((getter, hash), (setter, hash), prop)
+    Property(Box<((ImmutableString, u64), (ImmutableString, u64), Ident)>),
+    /// { [statement][Stmt] ... }
+    Stmt(Box<StmtBlock>),
     /// func `(` expr `,` ... `)`
     FnCall(Box<FnCallExpr>, Position),
     /// lhs `.` rhs
     Dot(Box<BinaryExpr>, Position),
     /// expr `[` expr `]`
     Index(Box<BinaryExpr>, Position),
-    /// lhs `in` rhs
-    In(Box<BinaryExpr>, Position),
     /// lhs `&&` rhs
     And(Box<BinaryExpr>, Position),
     /// lhs `||` rhs
@@ -1329,6 +1553,7 @@ impl Expr {
     /// Get the [`Dynamic`] value of a constant expression.
     ///
     /// Returns [`None`] if the expression is not constant.
+    #[inline]
     pub fn get_constant_value(&self) -> Option<Dynamic> {
         Some(match self {
             Self::DynamicConstant(x, _) => x.as_ref().clone(),
@@ -1379,6 +1604,7 @@ impl Expr {
         }
     }
     /// Get the [position][Position] of the expression.
+    #[inline]
     pub fn position(&self) -> Position {
         match self {
             #[cfg(not(feature = "no_float"))]
@@ -1392,12 +1618,12 @@ impl Expr {
             Self::FnPointer(_, pos) => *pos,
             Self::Array(_, pos) => *pos,
             Self::Map(_, pos) => *pos,
-            Self::Property(x) => (x.4).pos,
-            Self::Stmt(_, pos) => *pos,
+            Self::Property(x) => (x.2).pos,
+            Self::Stmt(x) => x.pos,
             Self::Variable(x) => (x.2).pos,
             Self::FnCall(_, pos) => *pos,
 
-            Self::And(x, _) | Self::Or(x, _) | Self::In(x, _) => x.lhs.position(),
+            Self::And(x, _) | Self::Or(x, _) => x.lhs.position(),
 
             Self::Unit(pos) => *pos,
 
@@ -1407,6 +1633,7 @@ impl Expr {
         }
     }
     /// Override the [position][Position] of the expression.
+    #[inline]
     pub fn set_position(&mut self, new_pos: Position) -> &mut Self {
         match self {
             #[cfg(not(feature = "no_float"))]
@@ -1421,10 +1648,10 @@ impl Expr {
             Self::Array(_, pos) => *pos = new_pos,
             Self::Map(_, pos) => *pos = new_pos,
             Self::Variable(x) => (x.2).pos = new_pos,
-            Self::Property(x) => (x.4).pos = new_pos,
-            Self::Stmt(_, pos) => *pos = new_pos,
+            Self::Property(x) => (x.2).pos = new_pos,
+            Self::Stmt(x) => x.pos = new_pos,
             Self::FnCall(_, pos) => *pos = new_pos,
-            Self::And(_, pos) | Self::Or(_, pos) | Self::In(_, pos) => *pos = new_pos,
+            Self::And(_, pos) | Self::Or(_, pos) => *pos = new_pos,
             Self::Unit(pos) => *pos = new_pos,
             Self::Dot(_, pos) | Self::Index(_, pos) => *pos = new_pos,
             Self::Custom(_, pos) => *pos = new_pos,
@@ -1435,17 +1662,18 @@ impl Expr {
     /// Is the expression pure?
     ///
     /// A pure expression has no side effects.
+    #[inline]
     pub fn is_pure(&self) -> bool {
         match self {
             Self::Array(x, _) => x.iter().all(Self::is_pure),
 
             Self::Map(x, _) => x.iter().map(|(_, v)| v).all(Self::is_pure),
 
-            Self::Index(x, _) | Self::And(x, _) | Self::Or(x, _) | Self::In(x, _) => {
+            Self::Index(x, _) | Self::And(x, _) | Self::Or(x, _) => {
                 x.lhs.is_pure() && x.rhs.is_pure()
             }
 
-            Self::Stmt(x, _) => x.iter().all(Stmt::is_pure),
+            Self::Stmt(x) => x.statements.iter().all(Stmt::is_pure),
 
             Self::Variable(_) => true,
 
@@ -1461,6 +1689,7 @@ impl Expr {
         }
     }
     /// Is the expression a constant?
+    #[inline]
     pub fn is_constant(&self) -> bool {
         match self {
             #[cfg(not(feature = "no_float"))]
@@ -1480,17 +1709,11 @@ impl Expr {
             // An map literal is constant if all items are constant
             Self::Map(x, _) => x.iter().map(|(_, expr)| expr).all(Self::is_constant),
 
-            // Check in expression
-            Self::In(x, _) => match (&x.lhs, &x.rhs) {
-                (Self::StringConstant(_, _), Self::StringConstant(_, _))
-                | (Self::CharConstant(_, _), Self::StringConstant(_, _)) => true,
-                _ => false,
-            },
-
             _ => false,
         }
     }
     /// Is a particular [token][Token] allowed as a postfix operator to this expression?
+    #[inline]
     pub fn is_valid_postfix(&self, token: &Token) -> bool {
         match token {
             #[cfg(not(feature = "no_object"))]
@@ -1507,14 +1730,13 @@ impl Expr {
             | Self::IntegerConstant(_, _)
             | Self::CharConstant(_, _)
             | Self::FnPointer(_, _)
-            | Self::In(_, _)
             | Self::And(_, _)
             | Self::Or(_, _)
             | Self::Unit(_) => false,
 
             Self::StringConstant(_, _)
             | Self::FnCall(_, _)
-            | Self::Stmt(_, _)
+            | Self::Stmt(_)
             | Self::Dot(_, _)
             | Self::Index(_, _)
             | Self::Array(_, _)
@@ -1544,29 +1766,68 @@ impl Expr {
         }
     }
     /// Recursively walk this expression.
-    #[inline(always)]
-    pub fn walk<'a>(&'a self, path: &mut Vec<ASTNode<'a>>, on_node: &mut impl FnMut(&[ASTNode])) {
+    /// Return `false` from the callback to terminate the walk.
+    pub fn walk<'a>(
+        &'a self,
+        path: &mut Vec<ASTNode<'a>>,
+        on_node: &mut impl FnMut(&[ASTNode]) -> bool,
+    ) -> bool {
         path.push(self.into());
-        on_node(path);
+
+        if !on_node(path) {
+            return false;
+        }
 
         match self {
-            Self::Stmt(x, _) => x.iter().for_each(|s| s.walk(path, on_node)),
-            Self::Array(x, _) => x.iter().for_each(|e| e.walk(path, on_node)),
-            Self::Map(x, _) => x.iter().for_each(|(_, e)| e.walk(path, on_node)),
-            Self::Index(x, _)
-            | Self::Dot(x, _)
-            | Expr::In(x, _)
-            | Expr::And(x, _)
-            | Expr::Or(x, _) => {
-                x.lhs.walk(path, on_node);
-                x.rhs.walk(path, on_node);
+            Self::Stmt(x) => {
+                for s in &x.statements {
+                    if !s.walk(path, on_node) {
+                        return false;
+                    }
+                }
+            }
+            Self::Array(x, _) => {
+                for e in x.as_ref() {
+                    if !e.walk(path, on_node) {
+                        return false;
+                    }
+                }
+            }
+            Self::Map(x, _) => {
+                for (_, e) in x.as_ref() {
+                    if !e.walk(path, on_node) {
+                        return false;
+                    }
+                }
+            }
+            Self::Index(x, _) | Self::Dot(x, _) | Expr::And(x, _) | Expr::Or(x, _) => {
+                if !x.lhs.walk(path, on_node) {
+                    return false;
+                }
+                if !x.rhs.walk(path, on_node) {
+                    return false;
+                }
+            }
+            Self::FnCall(x, _) => {
+                for e in &x.args {
+                    if !e.walk(path, on_node) {
+                        return false;
+                    }
+                }
+            }
+            Self::Custom(x, _) => {
+                for e in &x.keywords {
+                    if !e.walk(path, on_node) {
+                        return false;
+                    }
+                }
             }
-            Self::FnCall(x, _) => x.args.iter().for_each(|e| e.walk(path, on_node)),
-            Self::Custom(x, _) => x.keywords.iter().for_each(|e| e.walk(path, on_node)),
             _ => (),
         }
 
         path.pop().unwrap();
+
+        true
     }
 }
 
@@ -1583,8 +1844,8 @@ mod tests {
         assert_eq!(size_of::<Position>(), 4);
         assert_eq!(size_of::<ast::Expr>(), 16);
         assert_eq!(size_of::<Option<ast::Expr>>(), 16);
-        assert_eq!(size_of::<ast::Stmt>(), 32);
-        assert_eq!(size_of::<Option<ast::Stmt>>(), 32);
+        assert_eq!(size_of::<ast::Stmt>(), 40);
+        assert_eq!(size_of::<Option<ast::Stmt>>(), 40);
         assert_eq!(size_of::<FnPtr>(), 32);
         assert_eq!(size_of::<Scope>(), 48);
         assert_eq!(size_of::<LexError>(), 56);
diff --git a/src/bin/rhai-repl.rs b/src/bin/rhai-repl.rs
index c5c91a18..fedd1dc0 100644
--- a/src/bin/rhai-repl.rs
+++ b/src/bin/rhai-repl.rs
@@ -1,8 +1,5 @@
 use rhai::{Dynamic, Engine, EvalAltResult, Module, Scope, AST};
 
-#[cfg(not(feature = "no_optimize"))]
-use rhai::OptimizationLevel;
-
 use std::{
     env,
     fs::File,
@@ -56,35 +53,38 @@ fn print_help() {
 }
 
 fn main() {
-    let mut engine = Engine::new();
-
     println!("Rhai REPL tool");
     println!("==============");
     print_help();
 
-    // Load init scripts
+    // Initialize scripting engine
+    let mut engine = Engine::new();
 
     #[cfg(not(feature = "no_module"))]
     {
+        // Set a file module resolver without caching
+        let mut resolver = rhai::module_resolvers::FileModuleResolver::new();
+        resolver.enable_cache(false);
+        engine.set_module_resolver(resolver);
+
+        // Load init scripts
         let mut contents = String::new();
         let mut has_init_scripts = false;
 
         for filename in env::args().skip(1) {
-            {
-                contents.clear();
+            contents.clear();
 
-                let mut f = match File::open(&filename) {
-                    Err(err) => {
-                        eprintln!("Error reading script file: {}\n{}", filename, err);
-                        exit(1);
-                    }
-                    Ok(f) => f,
-                };
-
-                if let Err(err) = f.read_to_string(&mut contents) {
-                    println!("Error reading script file: {}\n{}", filename, err);
+            let mut f = match File::open(&filename) {
+                Err(err) => {
+                    eprintln!("Error reading script file: {}\n{}", filename, err);
                     exit(1);
                 }
+                Ok(f) => f,
+            };
+
+            if let Err(err) = f.read_to_string(&mut contents) {
+                println!("Error reading script file: {}\n{}", filename, err);
+                exit(1);
             }
 
             let module = match engine
@@ -119,9 +119,8 @@ fn main() {
     }
 
     // Setup Engine
-
     #[cfg(not(feature = "no_optimize"))]
-    engine.set_optimization_level(OptimizationLevel::None);
+    engine.set_optimization_level(rhai::OptimizationLevel::None);
 
     let mut scope = Scope::new();
 
@@ -130,8 +129,10 @@ fn main() {
     let mut ast_u: AST = Default::default();
     let mut ast: AST = Default::default();
 
-    // REPL loop
+    // Make Engine immutable
+    let engine = engine;
 
+    // REPL loop
     'main_loop: loop {
         print!("rhai-repl> ");
         stdout().flush().expect("couldn't flush stdout");
@@ -233,7 +234,7 @@ fn main() {
 
                 #[cfg(not(feature = "no_optimize"))]
                 {
-                    ast = engine.optimize_ast(&scope, r, OptimizationLevel::Simple);
+                    ast = engine.optimize_ast(&scope, r, rhai::OptimizationLevel::Simple);
                 }
 
                 #[cfg(feature = "no_optimize")]
diff --git a/src/engine.rs b/src/engine.rs
index 30f6d1a4..973cf56b 100644
--- a/src/engine.rs
+++ b/src/engine.rs
@@ -1,6 +1,6 @@
 //! Main module defining the script evaluation [`Engine`].
 
-use crate::ast::{Expr, FnCallExpr, FnHash, Ident, OpAssignment, ReturnType, Stmt};
+use crate::ast::{Expr, FnCallExpr, FnHash, Ident, OpAssignment, ReturnType, Stmt, StmtBlock};
 use crate::dynamic::{map_std_type_name, AccessMode, Union, Variant};
 use crate::fn_native::{
     CallableFunction, IteratorFn, OnDebugCallback, OnPrintCallback, OnProgressCallback,
@@ -17,7 +17,6 @@ use crate::stdlib::{
     collections::{HashMap, HashSet},
     fmt, format,
     hash::{Hash, Hasher},
-    iter::empty,
     num::{NonZeroU64, NonZeroU8, NonZeroUsize},
     ops::DerefMut,
     string::{String, ToString},
@@ -25,12 +24,12 @@ use crate::stdlib::{
 use crate::syntax::CustomSyntax;
 use crate::utils::{get_hasher, StraightHasherBuilder};
 use crate::{
-    calc_fn_hash, Dynamic, EvalAltResult, FnPtr, ImmutableString, Module, Position, RhaiResult,
-    Scope, Shared, StaticVec,
+    Dynamic, EvalAltResult, FnPtr, ImmutableString, Module, Position, RhaiResult, Scope, Shared,
+    StaticVec,
 };
 
 #[cfg(not(feature = "no_index"))]
-use crate::Array;
+use crate::{calc_fn_hash, stdlib::iter::empty, Array};
 
 #[cfg(not(feature = "no_index"))]
 pub const TYPICAL_ARRAY_SIZE: usize = 8; // Small arrays are typical
@@ -203,9 +202,15 @@ pub const FN_IDX_GET: &str = "index$get$";
 pub const FN_IDX_SET: &str = "index$set$";
 #[cfg(not(feature = "no_function"))]
 pub const FN_ANONYMOUS: &str = "anon$";
-#[cfg(any(not(feature = "no_index"), not(feature = "no_object")))]
+
+/// Standard equality comparison operator.
 pub const OP_EQUALS: &str = "==";
 
+/// Standard method function for containment testing.
+///
+/// The `in` operator is implemented as a call to this method.
+pub const OP_CONTAINS: &str = "contains";
+
 /// Method of chaining.
 #[cfg(any(not(feature = "no_index"), not(feature = "no_object")))]
 #[derive(Debug, Clone, Copy, Eq, PartialEq, Hash)]
@@ -223,11 +228,11 @@ pub enum ChainType {
 #[derive(Debug, Clone, Hash)]
 pub enum ChainArgument {
     /// Dot-property access.
-    Property,
+    Property(Position),
     /// Arguments to a dot-function call.
-    FnCallArgs(StaticVec<Dynamic>),
+    FnCallArgs(StaticVec<Dynamic>, StaticVec<Position>),
     /// Index value.
-    IndexValue(Dynamic),
+    IndexValue(Dynamic, Position),
 }
 
 #[cfg(any(not(feature = "no_index"), not(feature = "no_object")))]
@@ -241,8 +246,10 @@ impl ChainArgument {
     #[cfg(not(feature = "no_index"))]
     pub fn as_index_value(self) -> Dynamic {
         match self {
-            Self::Property | Self::FnCallArgs(_) => panic!("expecting ChainArgument::IndexValue"),
-            Self::IndexValue(value) => value,
+            Self::Property(_) | Self::FnCallArgs(_, _) => {
+                panic!("expecting ChainArgument::IndexValue")
+            }
+            Self::IndexValue(value, _) => value,
         }
     }
     /// Return the `StaticVec<Dynamic>` value.
@@ -252,27 +259,29 @@ impl ChainArgument {
     /// Panics if not `ChainArgument::FnCallArgs`.
     #[inline(always)]
     #[cfg(not(feature = "no_object"))]
-    pub fn as_fn_call_args(self) -> StaticVec<Dynamic> {
+    pub fn as_fn_call_args(self) -> (StaticVec<Dynamic>, StaticVec<Position>) {
         match self {
-            Self::Property | Self::IndexValue(_) => panic!("expecting ChainArgument::FnCallArgs"),
-            Self::FnCallArgs(value) => value,
+            Self::Property(_) | Self::IndexValue(_, _) => {
+                panic!("expecting ChainArgument::FnCallArgs")
+            }
+            Self::FnCallArgs(values, positions) => (values, positions),
         }
     }
 }
 
 #[cfg(any(not(feature = "no_index"), not(feature = "no_object")))]
-impl From<StaticVec<Dynamic>> for ChainArgument {
+impl From<(StaticVec<Dynamic>, StaticVec<Position>)> for ChainArgument {
     #[inline(always)]
-    fn from(value: StaticVec<Dynamic>) -> Self {
-        Self::FnCallArgs(value)
+    fn from((values, positions): (StaticVec<Dynamic>, StaticVec<Position>)) -> Self {
+        Self::FnCallArgs(values, positions)
     }
 }
 
 #[cfg(any(not(feature = "no_index"), not(feature = "no_object")))]
-impl From<Dynamic> for ChainArgument {
+impl From<(Dynamic, Position)> for ChainArgument {
     #[inline(always)]
-    fn from(value: Dynamic) -> Self {
-        Self::IndexValue(value)
+    fn from((value, pos): (Dynamic, Position)) -> Self {
+        Self::IndexValue(value, pos)
     }
 }
 
@@ -395,7 +404,11 @@ impl<'a> Target<'a> {
     }
     /// Update the value of the `Target`.
     #[cfg(any(not(feature = "no_object"), not(feature = "no_index")))]
-    pub fn set_value(&mut self, new_val: Dynamic, pos: Position) -> Result<(), Box<EvalAltResult>> {
+    pub fn set_value(
+        &mut self,
+        new_val: Dynamic,
+        _pos: Position,
+    ) -> Result<(), Box<EvalAltResult>> {
         match self {
             Self::Ref(r) => **r = new_val,
             #[cfg(not(feature = "no_closure"))]
@@ -411,7 +424,7 @@ impl<'a> Target<'a> {
                     Box::new(EvalAltResult::ErrorMismatchDataType(
                         "char".to_string(),
                         err.to_string(),
-                        pos,
+                        _pos,
                     ))
                 })?;
 
@@ -893,6 +906,7 @@ impl Engine {
     }
 
     /// Search for a module within an imports stack.
+    #[inline]
     pub(crate) fn search_imports(
         &self,
         mods: &Imports,
@@ -932,7 +946,7 @@ impl Engine {
         match expr {
             Expr::Variable(v) => match v.as_ref() {
                 // Qualified variable
-                (_, Some((hash_var, modules)), Ident { name, pos }) => {
+                (_, Some((hash_var, modules)), Ident { name, pos, .. }) => {
                     let module = self.search_imports(mods, state, modules).ok_or_else(|| {
                         EvalAltResult::ErrorModuleNotFound(
                             modules[0].name.to_string(),
@@ -971,7 +985,7 @@ impl Engine {
         this_ptr: &'s mut Option<&mut Dynamic>,
         expr: &Expr,
     ) -> Result<(Target<'s>, Position), Box<EvalAltResult>> {
-        let (index, _, Ident { name, pos }) = match expr {
+        let (index, _, Ident { name, pos, .. }) = match expr {
             Expr::Variable(v) => v.as_ref(),
             _ => unreachable!("Expr::Variable expected, but gets {:?}", expr),
         };
@@ -1152,9 +1166,9 @@ impl Engine {
                     // xxx.fn_name(arg_expr_list)
                     Expr::FnCall(x, pos) if x.namespace.is_none() && new_val.is_none() => {
                         let FnCallExpr { name, hash, .. } = x.as_ref();
-                        let args = idx_val.as_fn_call_args();
+                        let mut args = idx_val.as_fn_call_args();
                         self.make_method_call(
-                            mods, state, lib, name, *hash, target, args, *pos, level,
+                            mods, state, lib, name, *hash, target, &mut args, *pos, level,
                         )
                     }
                     // xxx.fn_name(...) = ???
@@ -1167,7 +1181,7 @@ impl Engine {
                     }
                     // {xxx:map}.id op= ???
                     Expr::Property(x) if target_val.is::<Map>() && new_val.is_some() => {
-                        let Ident { name, pos } = &x.4;
+                        let Ident { name, pos, .. } = &x.2;
                         let index = name.clone().into();
                         let val = self.get_indexed_mut(
                             mods, state, lib, target_val, index, *pos, true, is_ref, false, level,
@@ -1180,7 +1194,7 @@ impl Engine {
                     }
                     // {xxx:map}.id
                     Expr::Property(x) if target_val.is::<Map>() => {
-                        let Ident { name, pos } = &x.4;
+                        let Ident { name, pos, .. } = &x.2;
                         let index = name.clone().into();
                         let val = self.get_indexed_mut(
                             mods, state, lib, target_val, index, *pos, false, is_ref, false, level,
@@ -1190,7 +1204,7 @@ impl Engine {
                     }
                     // xxx.id = ???
                     Expr::Property(x) if new_val.is_some() => {
-                        let (_, _, setter, hash_set, Ident { pos, .. }) = x.as_ref();
+                        let (_, (setter, hash_set), Ident { pos, .. }) = x.as_ref();
                         let hash = FnHash::from_native(*hash_set);
                         let mut new_val = new_val;
                         let mut args = [target_val, &mut (new_val.as_mut().unwrap().0).0];
@@ -1202,7 +1216,7 @@ impl Engine {
                     }
                     // xxx.id
                     Expr::Property(x) => {
-                        let (getter, hash_get, _, _, Ident { pos, .. }) = x.as_ref();
+                        let ((getter, hash_get), _, Ident { pos, .. }) = x.as_ref();
                         let hash = FnHash::from_native(*hash_get);
                         let mut args = [target_val];
                         self.exec_fn_call(
@@ -1215,7 +1229,7 @@ impl Engine {
                     Expr::Index(x, x_pos) | Expr::Dot(x, x_pos) if target_val.is::<Map>() => {
                         let mut val = match &x.lhs {
                             Expr::Property(p) => {
-                                let Ident { name, pos } = &p.4;
+                                let Ident { name, pos, .. } = &p.2;
                                 let index = name.clone().into();
                                 self.get_indexed_mut(
                                     mods, state, lib, target_val, index, *pos, false, is_ref, true,
@@ -1225,9 +1239,9 @@ impl Engine {
                             // {xxx:map}.fn_name(arg_expr_list)[expr] | {xxx:map}.fn_name(arg_expr_list).expr
                             Expr::FnCall(x, pos) if x.namespace.is_none() => {
                                 let FnCallExpr { name, hash, .. } = x.as_ref();
-                                let args = idx_val.as_fn_call_args();
+                                let mut args = idx_val.as_fn_call_args();
                                 let (val, _) = self.make_method_call(
-                                    mods, state, lib, name, *hash, target, args, *pos, level,
+                                    mods, state, lib, name, *hash, target, &mut args, *pos, level,
                                 )?;
                                 val.into()
                             }
@@ -1250,7 +1264,7 @@ impl Engine {
                         match &x.lhs {
                             // xxx.prop[expr] | xxx.prop.expr
                             Expr::Property(p) => {
-                                let (getter, hash_get, setter, hash_set, Ident { pos, .. }) =
+                                let ((getter, hash_get), (setter, hash_set), Ident { pos, .. }) =
                                     p.as_ref();
                                 let hash_get = FnHash::from_native(*hash_get);
                                 let hash_set = FnHash::from_native(*hash_set);
@@ -1303,9 +1317,9 @@ impl Engine {
                             // xxx.fn_name(arg_expr_list)[expr] | xxx.fn_name(arg_expr_list).expr
                             Expr::FnCall(f, pos) if f.namespace.is_none() => {
                                 let FnCallExpr { name, hash, .. } = f.as_ref();
-                                let args = idx_val.as_fn_call_args();
+                                let mut args = idx_val.as_fn_call_args();
                                 let (mut val, _) = self.make_method_call(
-                                    mods, state, lib, name, *hash, target, args, *pos, level,
+                                    mods, state, lib, name, *hash, target, &mut args, *pos, level,
                                 )?;
                                 let val = &mut val;
                                 let target = &mut val.into();
@@ -1364,6 +1378,7 @@ impl Engine {
                 let Ident {
                     name: var_name,
                     pos: var_pos,
+                    ..
                 } = &x.2;
 
                 self.inc_operations(state, *var_pos)?;
@@ -1422,23 +1437,26 @@ impl Engine {
 
         match expr {
             Expr::FnCall(x, _) if parent_chain_type == ChainType::Dot && x.namespace.is_none() => {
+                let mut arg_positions: StaticVec<_> = Default::default();
+
                 let arg_values = x
                     .args
                     .iter()
                     .map(|arg_expr| {
+                        arg_positions.push(arg_expr.position());
                         self.eval_expr(scope, mods, state, lib, this_ptr, arg_expr, level)
                             .map(Dynamic::flatten)
                     })
                     .collect::<Result<StaticVec<_>, _>>()?;
 
-                idx_values.push(arg_values.into());
+                idx_values.push((arg_values, arg_positions).into());
             }
             Expr::FnCall(_, _) if parent_chain_type == ChainType::Dot => {
                 unreachable!("function call in dot chain should not be namespace-qualified")
             }
 
-            Expr::Property(_) if parent_chain_type == ChainType::Dot => {
-                idx_values.push(ChainArgument::Property)
+            Expr::Property(x) if parent_chain_type == ChainType::Dot => {
+                idx_values.push(ChainArgument::Property(x.2.pos))
             }
             Expr::Property(_) => unreachable!("unexpected Expr::Property for indexing"),
 
@@ -1447,29 +1465,33 @@ impl Engine {
 
                 // Evaluate in left-to-right order
                 let lhs_val = match lhs {
-                    Expr::Property(_) if parent_chain_type == ChainType::Dot => {
-                        ChainArgument::Property
+                    Expr::Property(x) if parent_chain_type == ChainType::Dot => {
+                        ChainArgument::Property(x.2.pos)
                     }
                     Expr::Property(_) => unreachable!("unexpected Expr::Property for indexing"),
                     Expr::FnCall(x, _)
                         if parent_chain_type == ChainType::Dot && x.namespace.is_none() =>
                     {
-                        x.args
+                        let mut arg_positions: StaticVec<_> = Default::default();
+
+                        let arg_values = x
+                            .args
                             .iter()
                             .map(|arg_expr| {
+                                arg_positions.push(arg_expr.position());
                                 self.eval_expr(scope, mods, state, lib, this_ptr, arg_expr, level)
                                     .map(Dynamic::flatten)
                             })
-                            .collect::<Result<StaticVec<Dynamic>, _>>()?
-                            .into()
+                            .collect::<Result<StaticVec<_>, _>>()?;
+
+                        (arg_values, arg_positions).into()
                     }
                     Expr::FnCall(_, _) if parent_chain_type == ChainType::Dot => {
                         unreachable!("function call in dot chain should not be namespace-qualified")
                     }
                     _ => self
-                        .eval_expr(scope, mods, state, lib, this_ptr, lhs, level)?
-                        .flatten()
-                        .into(),
+                        .eval_expr(scope, mods, state, lib, this_ptr, lhs, level)
+                        .map(|v| (v.flatten(), lhs.position()).into())?,
                 };
 
                 // Push in reverse order
@@ -1486,9 +1508,8 @@ impl Engine {
             }
 
             _ => idx_values.push(
-                self.eval_expr(scope, mods, state, lib, this_ptr, expr, level)?
-                    .flatten()
-                    .into(),
+                self.eval_expr(scope, mods, state, lib, this_ptr, expr, level)
+                    .map(|v| (v.flatten(), expr.position()).into())?,
             ),
         }
 
@@ -1603,63 +1624,6 @@ impl Engine {
         }
     }
 
-    // Evaluate an 'in' expression.
-    fn eval_in_expr(
-        &self,
-        scope: &mut Scope,
-        mods: &mut Imports,
-        state: &mut State,
-        lib: &[&Module],
-        this_ptr: &mut Option<&mut Dynamic>,
-        lhs: &Expr,
-        rhs: &Expr,
-        level: usize,
-    ) -> RhaiResult {
-        self.inc_operations(state, rhs.position())?;
-
-        let lhs_value = self.eval_expr(scope, mods, state, lib, this_ptr, lhs, level)?;
-        let rhs_value = self
-            .eval_expr(scope, mods, state, lib, this_ptr, rhs, level)?
-            .flatten();
-
-        match rhs_value {
-            #[cfg(not(feature = "no_index"))]
-            Dynamic(Union::Array(mut rhs_value, _)) => {
-                // Call the `==` operator to compare each value
-                let hash = calc_fn_hash(empty(), OP_EQUALS, 2);
-                for value in rhs_value.iter_mut() {
-                    let args = &mut [&mut lhs_value.clone(), value];
-                    let pos = rhs.position();
-
-                    if self
-                        .call_native_fn(mods, state, lib, OP_EQUALS, hash, args, false, false, pos)?
-                        .0
-                        .as_bool()
-                        .unwrap_or(false)
-                    {
-                        return Ok(true.into());
-                    }
-                }
-
-                Ok(false.into())
-            }
-            #[cfg(not(feature = "no_object"))]
-            Dynamic(Union::Map(rhs_value, _)) => match lhs_value {
-                // Only allows string or char
-                Dynamic(Union::Str(s, _)) => Ok(rhs_value.contains_key(&s).into()),
-                Dynamic(Union::Char(c, _)) => Ok(rhs_value.contains_key(&c.to_string()).into()),
-                _ => EvalAltResult::ErrorInExpr(lhs.position()).into(),
-            },
-            Dynamic(Union::Str(rhs_value, _)) => match lhs_value {
-                // Only allows string or char
-                Dynamic(Union::Str(s, _)) => Ok(rhs_value.contains(s.as_str()).into()),
-                Dynamic(Union::Char(c, _)) => Ok(rhs_value.contains(c).into()),
-                _ => EvalAltResult::ErrorInExpr(lhs.position()).into(),
-            },
-            _ => EvalAltResult::ErrorInExpr(rhs.position()).into(),
-        }
-    }
-
     /// Evaluate an expression.
     pub(crate) fn eval_expr(
         &self,
@@ -1691,16 +1655,11 @@ impl Engine {
                 .map(|(val, _)| val.take_or_clone()),
 
             // Statement block
-            Expr::Stmt(x, _) => self.eval_stmt_block(
-                scope,
-                mods,
-                state,
-                lib,
-                this_ptr,
-                x.as_ref().as_ref(),
-                true,
-                level,
-            ),
+            Expr::Stmt(x) if x.is_empty() => Ok(Dynamic::UNIT),
+            Expr::Stmt(x) => {
+                let statements = &x.statements;
+                self.eval_stmt_block(scope, mods, state, lib, this_ptr, statements, true, level)
+            }
 
             // lhs[idx_expr]
             #[cfg(not(feature = "no_index"))]
@@ -1745,13 +1704,13 @@ impl Engine {
             Expr::FnCall(x, pos) if x.namespace.is_none() => {
                 let FnCallExpr {
                     name,
-                    capture: cap_scope,
+                    capture,
                     hash,
                     args,
                     ..
                 } = x.as_ref();
                 self.make_function_call(
-                    scope, mods, state, lib, this_ptr, name, args, *hash, *pos, *cap_scope, level,
+                    scope, mods, state, lib, this_ptr, name, args, *hash, *pos, *capture, level,
                 )
             }
 
@@ -1771,10 +1730,6 @@ impl Engine {
                 )
             }
 
-            Expr::In(x, _) => {
-                self.eval_in_expr(scope, mods, state, lib, this_ptr, &x.lhs, &x.rhs, level)
-            }
-
             Expr::And(x, _) => {
                 Ok((self
                     .eval_expr(scope, mods, state, lib, this_ptr, &x.lhs, level)?
@@ -1844,6 +1799,10 @@ impl Engine {
         restore_prev_state: bool,
         level: usize,
     ) -> RhaiResult {
+        if statements.is_empty() {
+            return Ok(Dynamic::UNIT);
+        }
+
         let mut _extra_fn_resolution_cache = false;
         let prev_always_search = state.always_search;
         let prev_scope_len = scope.len();
@@ -2065,23 +2024,43 @@ impl Engine {
             }
 
             // Block scope
+            Stmt::Block(statements, _) if statements.is_empty() => Ok(Dynamic::UNIT),
             Stmt::Block(statements, _) => {
                 self.eval_stmt_block(scope, mods, state, lib, this_ptr, statements, true, level)
             }
 
             // If statement
             Stmt::If(expr, x, _) => {
-                let (if_block, else_block) = x.as_ref();
+                let (
+                    StmtBlock {
+                        statements: if_stmt,
+                        ..
+                    },
+                    StmtBlock {
+                        statements: else_stmt,
+                        ..
+                    },
+                ) = x.as_ref();
                 self.eval_expr(scope, mods, state, lib, this_ptr, expr, level)?
                     .as_bool()
                     .map_err(|err| self.make_type_mismatch_err::<bool>(err, expr.position()))
                     .and_then(|guard_val| {
                         if guard_val {
-                            self.eval_stmt(scope, mods, state, lib, this_ptr, if_block, level)
-                        } else if let Some(stmt) = else_block {
-                            self.eval_stmt(scope, mods, state, lib, this_ptr, stmt, level)
+                            if !if_stmt.is_empty() {
+                                self.eval_stmt_block(
+                                    scope, mods, state, lib, this_ptr, if_stmt, true, level,
+                                )
+                            } else {
+                                Ok(Dynamic::UNIT)
+                            }
                         } else {
-                            Ok(Dynamic::UNIT)
+                            if !else_stmt.is_empty() {
+                                self.eval_stmt_block(
+                                    scope, mods, state, lib, this_ptr, else_stmt, true, level,
+                                )
+                            } else {
+                                Ok(Dynamic::UNIT)
+                            }
                         }
                     })
             }
@@ -2097,36 +2076,55 @@ impl Engine {
                     value.hash(hasher);
                     let hash = hasher.finish();
 
-                    table
-                        .get(&hash)
-                        .map(|stmt| self.eval_stmt(scope, mods, state, lib, this_ptr, stmt, level))
+                    table.get(&hash).map(|StmtBlock { statements, .. }| {
+                        if !statements.is_empty() {
+                            self.eval_stmt_block(
+                                scope, mods, state, lib, this_ptr, statements, true, level,
+                            )
+                        } else {
+                            Ok(Dynamic::UNIT)
+                        }
+                    })
                 } else {
                     // Non-hashable values never match any specific clause
                     None
                 }
                 .unwrap_or_else(|| {
                     // Default match clause
-                    def_stmt.as_ref().map_or_else(
-                        || Ok(Dynamic::UNIT),
-                        |def_stmt| {
-                            self.eval_stmt(scope, mods, state, lib, this_ptr, def_stmt, level)
-                        },
-                    )
+                    let def_stmt = &def_stmt.statements;
+                    if !def_stmt.is_empty() {
+                        self.eval_stmt_block(
+                            scope, mods, state, lib, this_ptr, def_stmt, true, level,
+                        )
+                    } else {
+                        Ok(Dynamic::UNIT)
+                    }
                 })
             }
 
             // While loop
-            Stmt::While(expr, body, _) => loop {
-                let condition = if let Some(expr) = expr {
-                    self.eval_expr(scope, mods, state, lib, this_ptr, expr, level)?
-                        .as_bool()
-                        .map_err(|err| self.make_type_mismatch_err::<bool>(err, expr.position()))?
-                } else {
-                    true
-                };
+            Stmt::While(expr, body, _) => {
+                let body = &body.statements;
+                loop {
+                    let condition = if !expr.is_unit() {
+                        self.eval_expr(scope, mods, state, lib, this_ptr, expr, level)?
+                            .as_bool()
+                            .map_err(|err| {
+                                self.make_type_mismatch_err::<bool>(err, expr.position())
+                            })?
+                    } else {
+                        true
+                    };
 
-                if condition {
-                    match self.eval_stmt(scope, mods, state, lib, this_ptr, body, level) {
+                    if !condition {
+                        return Ok(Dynamic::UNIT);
+                    }
+                    if body.is_empty() {
+                        continue;
+                    }
+
+                    match self.eval_stmt_block(scope, mods, state, lib, this_ptr, body, true, level)
+                    {
                         Ok(_) => (),
                         Err(err) => match *err {
                             EvalAltResult::LoopBreak(false, _) => (),
@@ -2134,40 +2132,46 @@ impl Engine {
                             _ => return Err(err),
                         },
                     }
-                } else {
-                    return Ok(Dynamic::UNIT);
                 }
-            },
+            }
 
             // Do loop
-            Stmt::Do(body, expr, is_while, _) => loop {
-                match self.eval_stmt(scope, mods, state, lib, this_ptr, body, level) {
-                    Ok(_) => (),
-                    Err(err) => match *err {
-                        EvalAltResult::LoopBreak(false, _) => continue,
-                        EvalAltResult::LoopBreak(true, _) => return Ok(Dynamic::UNIT),
-                        _ => return Err(err),
-                    },
-                }
+            Stmt::Do(body, expr, is_while, _) => {
+                let body = &body.statements;
 
-                if self
-                    .eval_expr(scope, mods, state, lib, this_ptr, expr, level)?
-                    .as_bool()
-                    .map_err(|err| self.make_type_mismatch_err::<bool>(err, expr.position()))?
-                {
-                    if !*is_while {
-                        return Ok(Dynamic::UNIT);
+                loop {
+                    if !body.is_empty() {
+                        match self
+                            .eval_stmt_block(scope, mods, state, lib, this_ptr, body, true, level)
+                        {
+                            Ok(_) => (),
+                            Err(err) => match *err {
+                                EvalAltResult::LoopBreak(false, _) => continue,
+                                EvalAltResult::LoopBreak(true, _) => return Ok(Dynamic::UNIT),
+                                _ => return Err(err),
+                            },
+                        }
                     }
-                } else {
-                    if *is_while {
-                        return Ok(Dynamic::UNIT);
+
+                    if self
+                        .eval_expr(scope, mods, state, lib, this_ptr, expr, level)?
+                        .as_bool()
+                        .map_err(|err| self.make_type_mismatch_err::<bool>(err, expr.position()))?
+                    {
+                        if !*is_while {
+                            return Ok(Dynamic::UNIT);
+                        }
+                    } else {
+                        if *is_while {
+                            return Ok(Dynamic::UNIT);
+                        }
                     }
                 }
-            },
+            }
 
             // For loop
             Stmt::For(expr, x, _) => {
-                let (name, stmt) = x.as_ref();
+                let (name, StmtBlock { statements, pos }) = x.as_ref();
                 let iter_obj = self
                     .eval_expr(scope, mods, state, lib, this_ptr, expr, level)?
                     .flatten();
@@ -2216,9 +2220,15 @@ impl Engine {
                             *loop_var = value;
                         }
 
-                        self.inc_operations(state, stmt.position())?;
+                        self.inc_operations(state, *pos)?;
 
-                        match self.eval_stmt(scope, mods, state, lib, this_ptr, stmt, level) {
+                        if statements.is_empty() {
+                            continue;
+                        }
+
+                        match self.eval_stmt_block(
+                            scope, mods, state, lib, this_ptr, statements, true, level,
+                        ) {
                             Ok(_) => (),
                             Err(err) => match *err {
                                 EvalAltResult::LoopBreak(false, _) => (),
@@ -2244,10 +2254,20 @@ impl Engine {
 
             // Try/Catch statement
             Stmt::TryCatch(x, _, _) => {
-                let (try_body, err_var, catch_body) = x.as_ref();
+                let (
+                    StmtBlock {
+                        statements: try_body,
+                        ..
+                    },
+                    err_var,
+                    StmtBlock {
+                        statements: catch_body,
+                        ..
+                    },
+                ) = x.as_ref();
 
                 let result = self
-                    .eval_stmt(scope, mods, state, lib, this_ptr, try_body, level)
+                    .eval_stmt_block(scope, mods, state, lib, this_ptr, try_body, true, level)
                     .map(|_| Dynamic::UNIT);
 
                 match result {
@@ -2306,8 +2326,9 @@ impl Engine {
                             scope.push(unsafe_cast_var_name_to_lifetime(&name), err_value);
                         }
 
-                        let result =
-                            self.eval_stmt(scope, mods, state, lib, this_ptr, catch_body, level);
+                        let result = self.eval_stmt_block(
+                            scope, mods, state, lib, this_ptr, catch_body, true, level,
+                        );
 
                         state.scope_level -= 1;
                         scope.rewind(orig_scope_len);
@@ -2328,7 +2349,7 @@ impl Engine {
             }
 
             // Return value
-            Stmt::Return((ReturnType::Return, pos), Some(expr), _) => {
+            Stmt::Return(ReturnType::Return, Some(expr), pos) => {
                 let value = self
                     .eval_expr(scope, mods, state, lib, this_ptr, expr, level)?
                     .flatten();
@@ -2336,12 +2357,12 @@ impl Engine {
             }
 
             // Empty return
-            Stmt::Return((ReturnType::Return, pos), None, _) => {
+            Stmt::Return(ReturnType::Return, None, pos) => {
                 EvalAltResult::Return(Default::default(), *pos).into()
             }
 
             // Throw value
-            Stmt::Return((ReturnType::Exception, pos), Some(expr), _) => {
+            Stmt::Return(ReturnType::Exception, Some(expr), pos) => {
                 let value = self
                     .eval_expr(scope, mods, state, lib, this_ptr, expr, level)?
                     .flatten();
@@ -2349,38 +2370,34 @@ impl Engine {
             }
 
             // Empty throw
-            Stmt::Return((ReturnType::Exception, pos), None, _) => {
+            Stmt::Return(ReturnType::Exception, None, pos) => {
                 EvalAltResult::ErrorRuntime(Dynamic::UNIT, *pos).into()
             }
 
             // Let/const statement
-            Stmt::Let(var_def, expr, export, _) | Stmt::Const(var_def, expr, export, _) => {
+            Stmt::Let(expr, Ident { name, .. }, export, _)
+            | Stmt::Const(expr, Ident { name, .. }, export, _) => {
                 let entry_type = match stmt {
                     Stmt::Let(_, _, _, _) => AccessMode::ReadWrite,
                     Stmt::Const(_, _, _, _) => AccessMode::ReadOnly,
                     _ => unreachable!("should be Stmt::Let or Stmt::Const, but gets {:?}", stmt),
                 };
 
-                let value = if let Some(expr) = expr {
-                    self.eval_expr(scope, mods, state, lib, this_ptr, expr, level)?
-                        .flatten()
-                } else {
-                    Dynamic::UNIT
-                };
+                let value = self
+                    .eval_expr(scope, mods, state, lib, this_ptr, expr, level)?
+                    .flatten();
+
                 let (var_name, _alias): (Cow<'_, str>, _) = if state.is_global() {
                     (
-                        var_def.name.to_string().into(),
-                        if *export {
-                            Some(var_def.name.clone())
-                        } else {
-                            None
-                        },
+                        name.to_string().into(),
+                        if *export { Some(name.clone()) } else { None },
                     )
                 } else if *export {
                     unreachable!("exported variable not on global level");
                 } else {
-                    (unsafe_cast_var_name_to_lifetime(&var_def.name).into(), None)
+                    (unsafe_cast_var_name_to_lifetime(name).into(), None)
                 };
+
                 scope.push_dynamic_value(var_name, entry_type, value);
 
                 #[cfg(not(feature = "no_module"))]
@@ -2392,7 +2409,7 @@ impl Engine {
 
             // Import statement
             #[cfg(not(feature = "no_module"))]
-            Stmt::Import(expr, alias, _pos) => {
+            Stmt::Import(expr, export, _pos) => {
                 // Guard against too many modules
                 #[cfg(not(feature = "unchecked"))]
                 if state.modules >= self.max_modules() {
@@ -2419,14 +2436,14 @@ impl Engine {
                         })
                         .unwrap_or_else(|| self.module_resolver.resolve(self, &path, expr_pos))?;
 
-                    if let Some(name_def) = alias {
+                    if let Some(name) = export.as_ref().map(|x| x.name.clone()) {
                         if !module.is_indexed() {
                             // Index the module (making a clone copy if necessary) if it is not indexed
                             let mut module = crate::fn_native::shared_take_or_clone(module);
                             module.build_index();
-                            mods.push(name_def.name.clone(), module);
+                            mods.push(name, module);
                         } else {
-                            mods.push(name_def.name.clone(), module);
+                            mods.push(name, module);
                         }
                     }
 
@@ -2441,14 +2458,13 @@ impl Engine {
             // Export statement
             #[cfg(not(feature = "no_module"))]
             Stmt::Export(list, _) => {
-                for (Ident { name, pos: id_pos }, rename) in list.iter() {
+                for (Ident { name, pos, .. }, rename) in list.iter() {
                     // Mark scope variables as public
                     if let Some(index) = scope.get_index(name).map(|(i, _)| i) {
                         let alias = rename.as_ref().map(|x| &x.name).unwrap_or_else(|| name);
                         scope.add_entry_alias(index, alias.clone());
                     } else {
-                        return EvalAltResult::ErrorVariableNotFound(name.to_string(), *id_pos)
-                            .into();
+                        return EvalAltResult::ErrorVariableNotFound(name.to_string(), *pos).into();
                     }
                 }
                 Ok(Dynamic::UNIT)
@@ -2482,6 +2498,7 @@ impl Engine {
 
     /// Check a result to ensure that the data size is within allowable limit.
     #[cfg(not(feature = "unchecked"))]
+    #[inline(always)]
     fn check_data_size(&self, result: RhaiResult, pos: Position) -> RhaiResult {
         // Simply return all errors
         if result.is_err() {
diff --git a/src/engine_api.rs b/src/engine_api.rs
index 9ac38a97..7218582d 100644
--- a/src/engine_api.rs
+++ b/src/engine_api.rs
@@ -926,8 +926,9 @@ impl Engine {
                     if !resolver.contains_path(s) && !imports.contains(s) =>
                 {
                     imports.insert(s.clone());
+                    true
                 }
-                _ => (),
+                _ => true,
             });
         }
 
@@ -1832,7 +1833,7 @@ impl Engine {
         let lib = Default::default();
 
         let stmt = crate::stdlib::mem::take(ast.statements_mut());
-        crate::optimize::optimize_into_ast(self, scope, stmt, lib, optimization_level)
+        crate::optimize::optimize_into_ast(self, scope, stmt.into_vec(), lib, optimization_level)
     }
     /// Generate a list of all registered functions.
     ///
diff --git a/src/fn_builtin.rs b/src/fn_builtin.rs
index 8a1d96d8..c592bb0e 100644
--- a/src/fn_builtin.rs
+++ b/src/fn_builtin.rs
@@ -1,5 +1,6 @@
 //! Built-in implementations for common operators.
 
+use crate::engine::OP_CONTAINS;
 use crate::fn_native::{FnCallArgs, NativeCallContext};
 use crate::stdlib::{any::TypeId, format, string::ToString};
 use crate::{Dynamic, ImmutableString, RhaiResult, INT};
@@ -77,6 +78,13 @@ pub fn get_builtin_binary_op_fn(
                 Ok((x $op y).into())
             })
         };
+        ($xx:ident . $func:ident ( $yy:ty )) => {
+            return Some(|_, args| {
+                let x = &*args[0].read_lock::<$xx>().unwrap();
+                let y = &*args[1].read_lock::<$yy>().unwrap();
+                Ok(x.$func(y).into())
+            })
+        };
         ($func:ident ( $op:tt )) => {
             return Some(|_, args| {
                 let (x, y) = $func(args);
@@ -259,6 +267,24 @@ pub fn get_builtin_binary_op_fn(
             ">=" => impl_op!(get_s1s2(>=)),
             "<" => impl_op!(get_s1s2(<)),
             "<=" => impl_op!(get_s1s2(<=)),
+            OP_CONTAINS => {
+                return Some(|_, args| {
+                    let s = &*args[0].read_lock::<ImmutableString>().unwrap();
+                    let c = args[1].as_char().unwrap();
+                    Ok((s.contains(c)).into())
+                })
+            }
+            _ => return None,
+        }
+    }
+
+    // map op string
+    #[cfg(not(feature = "no_object"))]
+    if types_pair == (TypeId::of::<crate::Map>(), TypeId::of::<ImmutableString>()) {
+        use crate::Map;
+
+        match op {
+            OP_CONTAINS => impl_op!(Map.contains_key(ImmutableString)),
             _ => return None,
         }
     }
@@ -342,6 +368,13 @@ pub fn get_builtin_binary_op_fn(
             ">=" => impl_op!(ImmutableString >= ImmutableString),
             "<" => impl_op!(ImmutableString < ImmutableString),
             "<=" => impl_op!(ImmutableString <= ImmutableString),
+            OP_CONTAINS => {
+                return Some(|_, args| {
+                    let s1 = &*args[0].read_lock::<ImmutableString>().unwrap();
+                    let s2 = &*args[1].read_lock::<ImmutableString>().unwrap();
+                    Ok((s1.contains(s2.as_str())).into())
+                })
+            }
             _ => return None,
         }
     }
diff --git a/src/fn_call.rs b/src/fn_call.rs
index 63fe6079..8a008ee1 100644
--- a/src/fn_call.rs
+++ b/src/fn_call.rs
@@ -493,6 +493,10 @@ impl Engine {
 
         self.inc_operations(state, pos)?;
 
+        if fn_def.body.is_empty() {
+            return Ok(Dynamic::UNIT);
+        }
+
         // Check for stack overflow
         #[cfg(not(feature = "no_function"))]
         #[cfg(not(feature = "unchecked"))]
@@ -539,10 +543,10 @@ impl Engine {
         }
 
         // Evaluate the function
-        let stmt = &fn_def.body;
+        let body = &fn_def.body.statements;
 
         let result = self
-            .eval_stmt(scope, mods, state, unified_lib, this_ptr, stmt, level)
+            .eval_stmt_block(scope, mods, state, unified_lib, this_ptr, body, true, level)
             .or_else(|err| match *err {
                 // Convert return statement to return value
                 EvalAltResult::Return(x, _) => Ok(x),
@@ -722,6 +726,10 @@ impl Engine {
 
             let func = func.get_fn_def();
 
+            if func.body.is_empty() {
+                return Ok((Dynamic::UNIT, false));
+            }
+
             let scope: &mut Scope = &mut Default::default();
 
             // Move captured variables into scope
@@ -831,6 +839,7 @@ impl Engine {
     }
 
     /// Evaluate a text script in place - used primarily for 'eval'.
+    #[inline]
     fn eval_script_expr_in_place(
         &self,
         scope: &mut Scope,
@@ -884,7 +893,7 @@ impl Engine {
         fn_name: &str,
         mut hash: FnHash,
         target: &mut crate::engine::Target,
-        mut call_args: StaticVec<Dynamic>,
+        (call_args, call_arg_positions): &mut (StaticVec<Dynamic>, StaticVec<Position>),
         pos: Position,
         level: usize,
     ) -> Result<(Dynamic, bool), Box<EvalAltResult>> {
@@ -902,7 +911,7 @@ impl Engine {
                 let fn_name = fn_ptr.fn_name();
                 let args_len = call_args.len() + fn_ptr.curry().len();
                 // Recalculate hashes
-                let hash = FnHash::from_script(calc_fn_hash(empty(), fn_name, args_len));
+                let new_hash = FnHash::from_script(calc_fn_hash(empty(), fn_name, args_len));
                 // Arguments are passed as-is, adding the curried arguments
                 let mut curry = fn_ptr.curry().iter().cloned().collect::<StaticVec<_>>();
                 let mut arg_values = curry
@@ -913,17 +922,32 @@ impl Engine {
 
                 // Map it to name(args) in function-call style
                 self.exec_fn_call(
-                    mods, state, lib, fn_name, hash, args, false, false, pos, None, level,
+                    mods, state, lib, fn_name, new_hash, args, false, false, pos, None, level,
                 )
             }
-            KEYWORD_FN_PTR_CALL if call_args.len() > 0 && call_args[0].is::<FnPtr>() => {
+            KEYWORD_FN_PTR_CALL => {
+                if call_args.len() > 0 {
+                    if !call_args[0].is::<FnPtr>() {
+                        return Err(self.make_type_mismatch_err::<FnPtr>(
+                            self.map_type_name(obj.type_name()),
+                            call_arg_positions[0],
+                        ));
+                    }
+                } else {
+                    return Err(self.make_type_mismatch_err::<FnPtr>(
+                        self.map_type_name(obj.type_name()),
+                        pos,
+                    ));
+                }
+
                 // FnPtr call on object
                 let fn_ptr = call_args.remove(0).cast::<FnPtr>();
+                call_arg_positions.remove(0);
                 // Redirect function name
                 let fn_name = fn_ptr.fn_name();
                 let args_len = call_args.len() + fn_ptr.curry().len();
                 // Recalculate hash
-                let hash = FnHash::from_script_and_native(
+                let new_hash = FnHash::from_script_and_native(
                     calc_fn_hash(empty(), fn_name, args_len),
                     calc_fn_hash(empty(), fn_name, args_len + 1),
                 );
@@ -937,22 +961,34 @@ impl Engine {
 
                 // Map it to name(args) in function-call style
                 self.exec_fn_call(
-                    mods, state, lib, fn_name, hash, args, is_ref, true, pos, None, level,
+                    mods, state, lib, fn_name, new_hash, args, is_ref, true, pos, None, level,
                 )
             }
-            KEYWORD_FN_PTR_CURRY if obj.is::<FnPtr>() => {
-                // Curry call
+            KEYWORD_FN_PTR_CURRY => {
+                if !obj.is::<FnPtr>() {
+                    return Err(self.make_type_mismatch_err::<FnPtr>(
+                        self.map_type_name(obj.type_name()),
+                        pos,
+                    ));
+                }
+
                 let fn_ptr = obj.read_lock::<FnPtr>().unwrap();
+
+                // Curry call
                 Ok((
-                    FnPtr::new_unchecked(
-                        fn_ptr.get_fn_name().clone(),
-                        fn_ptr
-                            .curry()
-                            .iter()
-                            .cloned()
-                            .chain(call_args.into_iter())
-                            .collect(),
-                    )
+                    if call_args.is_empty() {
+                        fn_ptr.clone()
+                    } else {
+                        FnPtr::new_unchecked(
+                            fn_ptr.get_fn_name().clone(),
+                            fn_ptr
+                                .curry()
+                                .iter()
+                                .cloned()
+                                .chain(call_args.iter_mut().map(|v| mem::take(v)))
+                                .collect(),
+                        )
+                    }
                     .into(),
                     false,
                 ))
@@ -981,7 +1017,10 @@ impl Engine {
                                 .iter()
                                 .cloned()
                                 .enumerate()
-                                .for_each(|(i, v)| call_args.insert(i, v));
+                                .for_each(|(i, v)| {
+                                    call_args.insert(i, v);
+                                    call_arg_positions.insert(i, Position::NONE);
+                                });
                             // Recalculate the hash based on the new function name and new arguments
                             hash = FnHash::from_script_and_native(
                                 calc_fn_hash(empty(), fn_name, call_args.len()),
@@ -1065,7 +1104,6 @@ impl Engine {
                     FnHash::from_native(calc_fn_hash(empty(), name, args_len))
                 };
             }
-
             // Handle Fn()
             KEYWORD_FN_PTR if args_expr.len() == 1 => {
                 // Fn - only in function call style
@@ -1361,22 +1399,27 @@ impl Engine {
         match func {
             #[cfg(not(feature = "no_function"))]
             Some(f) if f.is_script() => {
-                let args = args.as_mut();
-                let new_scope = &mut Default::default();
-                let fn_def = f.get_fn_def().clone();
+                let fn_def = f.get_fn_def();
 
-                let mut source = module.id_raw().cloned();
-                mem::swap(&mut state.source, &mut source);
+                if fn_def.body.is_empty() {
+                    Ok(Dynamic::UNIT)
+                } else {
+                    let args = args.as_mut();
+                    let new_scope = &mut Default::default();
 
-                let level = level + 1;
+                    let mut source = module.id_raw().cloned();
+                    mem::swap(&mut state.source, &mut source);
 
-                let result = self.call_script_fn(
-                    new_scope, mods, state, lib, &mut None, &fn_def, args, pos, level,
-                );
+                    let level = level + 1;
 
-                state.source = source;
+                    let result = self.call_script_fn(
+                        new_scope, mods, state, lib, &mut None, fn_def, args, pos, level,
+                    );
 
-                result
+                    state.source = source;
+
+                    result
+                }
             }
 
             Some(f) if f.is_plugin_fn() => f
diff --git a/src/lib.rs b/src/lib.rs
index aa95de62..503890d5 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -124,7 +124,7 @@ pub type FLOAT = f32;
 
 pub use ast::{FnAccess, AST};
 pub use dynamic::Dynamic;
-pub use engine::{Engine, EvalContext};
+pub use engine::{Engine, EvalContext, OP_CONTAINS, OP_EQUALS};
 pub use fn_native::{FnPtr, NativeCallContext};
 pub use fn_register::{RegisterFn, RegisterResultFn};
 pub use module::{FnNamespace, Module};
@@ -192,7 +192,7 @@ pub use token::{get_next_token, parse_string_literal, InputStream, Token, Tokeni
 #[deprecated = "this type is volatile and may change"]
 pub use ast::{
     ASTNode, BinaryExpr, CustomExpr, Expr, FloatWrapper, FnCallExpr, FnHash, Ident, OpAssignment,
-    ReturnType, ScriptFnDef, Stmt,
+    ReturnType, ScriptFnDef, Stmt, StmtBlock,
 };
 
 #[cfg(feature = "internals")]
diff --git a/src/module/mod.rs b/src/module/mod.rs
index c79e2396..2a0c2edc 100644
--- a/src/module/mod.rs
+++ b/src/module/mod.rs
@@ -39,6 +39,7 @@ pub enum FnNamespace {
 }
 
 impl Default for FnNamespace {
+    #[inline(always)]
     fn default() -> Self {
         Self::Internal
     }
@@ -150,6 +151,7 @@ pub struct Module {
 }
 
 impl Default for Module {
+    #[inline(always)]
     fn default() -> Self {
         Self {
             id: None,
@@ -226,6 +228,7 @@ impl AsRef<Module> for Module {
 impl<M: AsRef<Module>> Add<M> for &Module {
     type Output = Module;
 
+    #[inline(always)]
     fn add(self, rhs: M) -> Self::Output {
         let mut module = self.clone();
         module.merge(rhs.as_ref());
@@ -236,6 +239,7 @@ impl<M: AsRef<Module>> Add<M> for &Module {
 impl<M: AsRef<Module>> Add<M> for Module {
     type Output = Self;
 
+    #[inline(always)]
     fn add(mut self, rhs: M) -> Self::Output {
         self.merge(rhs.as_ref());
         self
@@ -243,6 +247,7 @@ impl<M: AsRef<Module>> Add<M> for Module {
 }
 
 impl<M: Into<Module>> AddAssign<M> for Module {
+    #[inline(always)]
     fn add_assign(&mut self, rhs: M) {
         self.combine(rhs.into());
     }
@@ -1953,16 +1958,19 @@ impl Module {
     }
 
     /// Does a type iterator exist in the entire module tree?
+    #[inline(always)]
     pub fn contains_qualified_iter(&self, id: TypeId) -> bool {
         self.all_type_iterators.contains_key(&id)
     }
 
     /// Does a type iterator exist in the module?
+    #[inline(always)]
     pub fn contains_iter(&self, id: TypeId) -> bool {
         self.type_iterators.contains_key(&id)
     }
 
     /// Set a type iterator into the [`Module`].
+    #[inline(always)]
     pub fn set_iter(&mut self, typ: TypeId, func: IteratorFn) -> &mut Self {
         self.type_iterators.insert(typ, func);
         self.indexed = false;
@@ -1971,6 +1979,7 @@ impl Module {
     }
 
     /// Set a type iterator into the [`Module`].
+    #[inline(always)]
     pub fn set_iterable<T>(&mut self) -> &mut Self
     where
         T: Variant + Clone + IntoIterator,
@@ -1982,6 +1991,7 @@ impl Module {
     }
 
     /// Set an iterator type into the [`Module`] as a type iterator.
+    #[inline(always)]
     pub fn set_iterator<T>(&mut self) -> &mut Self
     where
         T: Variant + Clone + Iterator,
@@ -1993,11 +2003,13 @@ impl Module {
     }
 
     /// Get the specified type iterator.
+    #[inline(always)]
     pub(crate) fn get_qualified_iter(&self, id: TypeId) -> Option<IteratorFn> {
         self.all_type_iterators.get(&id).cloned()
     }
 
     /// Get the specified type iterator.
+    #[inline(always)]
     pub(crate) fn get_iter(&self, id: TypeId) -> Option<IteratorFn> {
         self.type_iterators.get(&id).cloned()
     }
@@ -2021,32 +2033,41 @@ pub struct NamespaceRef {
 }
 
 impl fmt::Debug for NamespaceRef {
+    #[inline(always)]
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt::Debug::fmt(&self.path, f)?;
-
         if let Some(index) = self.index {
-            write!(f, " -> {}", index)
-        } else {
-            Ok(())
+            write!(f, "{} -> ", index)?;
         }
+
+        f.write_str(
+            &self
+                .path
+                .iter()
+                .map(|Ident { name, .. }| name.as_str())
+                .collect::<Vec<_>>()
+                .join("::"),
+        )
     }
 }
 
 impl Deref for NamespaceRef {
     type Target = StaticVec<Ident>;
 
+    #[inline(always)]
     fn deref(&self) -> &Self::Target {
         &self.path
     }
 }
 
 impl DerefMut for NamespaceRef {
+    #[inline(always)]
     fn deref_mut(&mut self) -> &mut Self::Target {
         &mut self.path
     }
 }
 
 impl fmt::Display for NamespaceRef {
+    #[inline(always)]
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         for Ident { name, .. } in self.path.iter() {
             write!(f, "{}{}", name, Token::DoubleColon.syntax())?;
@@ -2056,6 +2077,7 @@ impl fmt::Display for NamespaceRef {
 }
 
 impl From<StaticVec<Ident>> for NamespaceRef {
+    #[inline(always)]
     fn from(path: StaticVec<Ident>) -> Self {
         Self { index: None, path }
     }
@@ -2063,11 +2085,13 @@ impl From<StaticVec<Ident>> for NamespaceRef {
 
 impl NamespaceRef {
     /// Get the [`Scope`][crate::Scope] index offset.
+    #[inline(always)]
     pub(crate) fn index(&self) -> Option<NonZeroUsize> {
         self.index
     }
     /// Set the [`Scope`][crate::Scope] index offset.
     #[cfg(not(feature = "no_module"))]
+    #[inline(always)]
     pub(crate) fn set_index(&mut self, index: Option<NonZeroUsize>) {
         self.index = index
     }
diff --git a/src/module/resolvers/file.rs b/src/module/resolvers/file.rs
index 5d71e0a2..e3a86634 100644
--- a/src/module/resolvers/file.rs
+++ b/src/module/resolvers/file.rs
@@ -41,6 +41,7 @@ use crate::{Engine, EvalAltResult, Module, ModuleResolver, Position, Shared};
 pub struct FileModuleResolver {
     base_path: PathBuf,
     extension: String,
+    cache_enabled: bool,
 
     #[cfg(not(feature = "sync"))]
     cache: crate::stdlib::cell::RefCell<HashMap<PathBuf, Shared<Module>>>,
@@ -101,6 +102,7 @@ impl FileModuleResolver {
         Self {
             base_path: path.into(),
             extension: extension.into(),
+            cache_enabled: true,
             cache: Default::default(),
         }
     }
@@ -152,9 +154,25 @@ impl FileModuleResolver {
         self
     }
 
+    /// Enable/disable the cache.
+    #[inline(always)]
+    pub fn enable_cache(&mut self, enable: bool) -> &mut Self {
+        self.cache_enabled = enable;
+        self
+    }
+    /// Is the cache enabled?
+    #[inline(always)]
+    pub fn is_cache_enabled(&self) -> bool {
+        self.cache_enabled
+    }
+
     /// Is a particular path cached?
     #[inline(always)]
     pub fn is_cached(&self, path: &str) -> bool {
+        if !self.cache_enabled {
+            return false;
+        }
+
         let file_path = self.get_file_path(path);
 
         #[cfg(not(feature = "sync"))]
@@ -211,7 +229,7 @@ impl ModuleResolver for FileModuleResolver {
         let file_path = self.get_file_path(path);
 
         // See if it is cached
-        {
+        if self.is_cache_enabled() {
             #[cfg(not(feature = "sync"))]
             let c = self.cache.borrow();
             #[cfg(feature = "sync")]
@@ -242,10 +260,12 @@ impl ModuleResolver for FileModuleResolver {
             .into();
 
         // Put it into the cache
-        #[cfg(not(feature = "sync"))]
-        self.cache.borrow_mut().insert(file_path, m.clone());
-        #[cfg(feature = "sync")]
-        self.cache.write().unwrap().insert(file_path, m.clone());
+        if self.is_cache_enabled() {
+            #[cfg(not(feature = "sync"))]
+            self.cache.borrow_mut().insert(file_path, m.clone());
+            #[cfg(feature = "sync")]
+            self.cache.write().unwrap().insert(file_path, m.clone());
+        }
 
         Ok(m)
     }
diff --git a/src/optimize.rs b/src/optimize.rs
index 20456ad2..06a06eaf 100644
--- a/src/optimize.rs
+++ b/src/optimize.rs
@@ -1,6 +1,6 @@
 //! Module implementing the [`AST`] optimizer.
 
-use crate::ast::{Expr, Stmt};
+use crate::ast::{Expr, Ident, Stmt, StmtBlock};
 use crate::dynamic::AccessMode;
 use crate::engine::{KEYWORD_DEBUG, KEYWORD_EVAL, KEYWORD_PRINT, KEYWORD_TYPE_OF};
 use crate::fn_builtin::get_builtin_binary_op_fn;
@@ -15,7 +15,6 @@ use crate::stdlib::{
     vec,
     vec::Vec,
 };
-use crate::token::is_valid_identifier;
 use crate::utils::get_hasher;
 use crate::{
     calc_fn_hash, calc_fn_params_hash, combine_hashes, Dynamic, Engine, Module, Position, Scope,
@@ -75,14 +74,18 @@ struct State<'a> {
 impl<'a> State<'a> {
     /// Create a new State.
     #[inline(always)]
-    pub fn new(engine: &'a Engine, lib: &'a [&'a Module], level: OptimizationLevel) -> Self {
+    pub fn new(
+        engine: &'a Engine,
+        lib: &'a [&'a Module],
+        optimization_level: OptimizationLevel,
+    ) -> Self {
         Self {
             changed: false,
             variables: vec![],
             propagate_constants: true,
             engine,
             lib,
-            optimization_level: level,
+            optimization_level,
         }
     }
     /// Reset the state from dirty to clean.
@@ -95,6 +98,11 @@ impl<'a> State<'a> {
     pub fn set_dirty(&mut self) {
         self.changed = true;
     }
+    /// Set the [`AST`] state to be not dirty (i.e. unchanged).
+    #[inline(always)]
+    pub fn clear_dirty(&mut self) {
+        self.changed = false;
+    }
     /// Is the [`AST`] dirty (i.e. changed)?
     #[inline(always)]
     pub fn is_dirty(&self) -> bool {
@@ -169,139 +177,112 @@ fn call_fn_with_constant_arguments(
 /// Optimize a block of [statements][Stmt].
 fn optimize_stmt_block(
     mut statements: Vec<Stmt>,
-    pos: Position,
     state: &mut State,
     preserve_result: bool,
-    count_promote_as_dirty: bool,
-) -> Stmt {
-    let orig_len = statements.len(); // Original number of statements in the block, for change detection
-    let orig_constants_len = state.variables.len(); // Original number of constants in the state, for restore later
-    let orig_propagate_constants = state.propagate_constants;
+) -> Vec<Stmt> {
+    if statements.is_empty() {
+        return statements;
+    }
 
-    // Optimize each statement in the block
-    statements.iter_mut().for_each(|stmt| {
-        match stmt {
-            // Add constant literals into the state
-            Stmt::Const(var_def, Some(value_expr), _, _) => {
-                optimize_expr(value_expr, state);
+    let mut is_dirty = state.is_dirty();
 
-                if value_expr.is_constant() {
-                    state.push_var(&var_def.name, AccessMode::ReadOnly, value_expr.clone());
-                }
+    loop {
+        state.clear_dirty();
+
+        let orig_constants_len = state.variables.len(); // Original number of constants in the state, for restore later
+        let orig_propagate_constants = state.propagate_constants;
+
+        // Remove everything following control flow breaking statements
+        let mut dead_code = false;
+
+        statements.retain(|stmt| {
+            if dead_code {
+                state.set_dirty();
+                false
+            } else if stmt.is_control_flow_break() {
+                dead_code = true;
+                true
+            } else {
+                true
             }
-            Stmt::Const(var_def, None, _, _) => {
-                state.push_var(&var_def.name, AccessMode::ReadOnly, Expr::Unit(var_def.pos));
-            }
-            // Add variables into the state
-            Stmt::Let(var_def, expr, _, _) => {
-                if let Some(value_expr) = expr {
+        });
+
+        // Optimize each statement in the block
+        statements.iter_mut().for_each(|stmt| {
+            match stmt {
+                // Add constant literals into the state
+                Stmt::Const(value_expr, Ident { name, .. }, _, _) => {
                     optimize_expr(value_expr, state);
+
+                    if value_expr.is_constant() {
+                        state.push_var(name, AccessMode::ReadOnly, value_expr.clone());
+                    }
                 }
-
-                state.push_var(
-                    &var_def.name,
-                    AccessMode::ReadWrite,
-                    Expr::Unit(var_def.pos),
-                );
+                // Add variables into the state
+                Stmt::Let(value_expr, Ident { name, pos, .. }, _, _) => {
+                    optimize_expr(value_expr, state);
+                    state.push_var(name, AccessMode::ReadWrite, Expr::Unit(*pos));
+                }
+                // Optimize the statement
+                _ => optimize_stmt(stmt, state, preserve_result),
             }
-            // Optimize the statement
-            _ => optimize_stmt(stmt, state, preserve_result),
-        }
-    });
+        });
 
-    // Remove all raw expression statements that are pure except for the very last statement
-    let last_stmt = if preserve_result {
-        statements.pop()
-    } else {
-        None
-    };
-
-    statements.retain(|stmt| !stmt.is_pure());
-
-    if let Some(stmt) = last_stmt {
-        statements.push(stmt);
-    }
-
-    // Remove all let/import statements at the end of a block - the new variables will go away anyway.
-    // But be careful only remove ones that have no initial values or have values that are pure expressions,
-    // otherwise there may be side effects.
-    let mut removed = false;
-
-    while let Some(expr) = statements.pop() {
-        match expr {
-            Stmt::Let(_, expr, _, _) | Stmt::Const(_, expr, _, _) => {
-                removed = expr.as_ref().map(Expr::is_pure).unwrap_or(true)
+        // Remove all pure statements that do not return values at the end of a block.
+        // We cannot remove anything for non-pure statements due to potential side-effects.
+        if preserve_result {
+            loop {
+                match &statements[..] {
+                    [stmt] if !stmt.returns_value() && stmt.is_internally_pure() => {
+                        state.set_dirty();
+                        statements.clear();
+                    }
+                    [.., second_last_stmt, Stmt::Noop(_)] if second_last_stmt.returns_value() => {}
+                    [.., second_last_stmt, last_stmt]
+                        if !last_stmt.returns_value() && last_stmt.is_internally_pure() =>
+                    {
+                        state.set_dirty();
+                        if second_last_stmt.returns_value() {
+                            *statements.last_mut().unwrap() = Stmt::Noop(last_stmt.position());
+                        } else {
+                            statements.pop().unwrap();
+                        }
+                    }
+                    _ => break,
+                }
             }
-            #[cfg(not(feature = "no_module"))]
-            Stmt::Import(expr, _, _) => removed = expr.is_pure(),
-            _ => {
-                statements.push(expr);
-                break;
+        } else {
+            loop {
+                match &statements[..] {
+                    [stmt] if stmt.is_internally_pure() => {
+                        state.set_dirty();
+                        statements.clear();
+                    }
+                    [.., last_stmt] if last_stmt.is_internally_pure() => {
+                        state.set_dirty();
+                        statements.pop().unwrap();
+                    }
+                    _ => break,
+                }
             }
         }
+
+        // Pop the stack and remove all the local constants
+        state.restore_var(orig_constants_len);
+        state.propagate_constants = orig_propagate_constants;
+
+        if !state.is_dirty() {
+            break;
+        }
+
+        is_dirty = true;
     }
 
-    if preserve_result {
-        if removed {
-            statements.push(Stmt::Noop(pos))
-        }
-
-        // Optimize all the statements again
-        let num_statements = statements.len();
-        statements
-            .iter_mut()
-            .enumerate()
-            .for_each(|(i, stmt)| optimize_stmt(stmt, state, i == num_statements));
-    }
-
-    // Remove everything following the the first return/throw
-    let mut dead_code = false;
-
-    statements.retain(|stmt| {
-        if dead_code {
-            return false;
-        }
-
-        match stmt {
-            Stmt::Return(_, _, _) | Stmt::Break(_) => dead_code = true,
-            _ => (),
-        }
-
-        true
-    });
-
-    // Change detection
-    if orig_len != statements.len() {
+    if is_dirty {
         state.set_dirty();
     }
 
-    // Pop the stack and remove all the local constants
-    state.restore_var(orig_constants_len);
-
-    state.propagate_constants = orig_propagate_constants;
-
-    match &statements[..] {
-        // No statements in block - change to No-op
-        [] => {
-            state.set_dirty();
-            Stmt::Noop(pos)
-        }
-        // Only one let statement - leave it alone
-        [x] if matches!(x, Stmt::Let(_, _, _, _)) => Stmt::Block(statements, pos),
-        // Only one const statement - leave it alone
-        [x] if matches!(x, Stmt::Const(_, _, _, _)) => Stmt::Block(statements, pos),
-        // Only one import statement - leave it alone
-        #[cfg(not(feature = "no_module"))]
-        [x] if matches!(x, Stmt::Import(_, _, _)) => Stmt::Block(statements, pos),
-        // Only one statement - promote
-        [_] => {
-            if count_promote_as_dirty {
-                state.set_dirty();
-            }
-            statements.remove(0)
-        }
-        _ => Stmt::Block(statements, pos),
-    }
+    statements
 }
 
 /// Optimize a [statement][Stmt].
@@ -316,18 +297,8 @@ fn optimize_stmt(stmt: &mut Stmt, state: &mut State, preserve_result: bool) {
             }
         },
 
-        // if false { if_block } -> Noop
-        Stmt::If(Expr::BoolConstant(false, pos), x, _) if x.1.is_none() => {
-            state.set_dirty();
-            *stmt = Stmt::Noop(*pos);
-        }
-        // if true { if_block } -> if_block
-        Stmt::If(Expr::BoolConstant(true, _), x, _) if x.1.is_none() => {
-            *stmt = mem::take(&mut x.0);
-            optimize_stmt(stmt, state, true);
-        }
-        // if expr { Noop }
-        Stmt::If(condition, x, _) if x.1.is_none() && matches!(x.0, Stmt::Noop(_)) => {
+        // if expr {}
+        Stmt::If(condition, x, _) if x.0.is_empty() && x.1.is_empty() => {
             state.set_dirty();
 
             let pos = condition.position();
@@ -336,38 +307,56 @@ fn optimize_stmt(stmt: &mut Stmt, state: &mut State, preserve_result: bool) {
 
             *stmt = if preserve_result {
                 // -> { expr, Noop }
-                Stmt::Block(vec![Stmt::Expr(expr), mem::take(&mut x.0)], pos)
+                Stmt::Block(vec![Stmt::Expr(expr), Stmt::Noop(pos)], pos)
             } else {
                 // -> expr
                 Stmt::Expr(expr)
             };
         }
-        // if expr { if_block }
-        Stmt::If(condition, x, _) if x.1.is_none() => {
-            optimize_expr(condition, state);
-            optimize_stmt(&mut x.0, state, true);
+        // if false { if_block } -> Noop
+        Stmt::If(Expr::BoolConstant(false, pos), x, _) if x.1.is_empty() => {
+            state.set_dirty();
+            *stmt = Stmt::Noop(*pos);
         }
         // if false { if_block } else { else_block } -> else_block
-        Stmt::If(Expr::BoolConstant(false, _), x, _) if x.1.is_some() => {
-            *stmt = mem::take(x.1.as_mut().unwrap());
-            optimize_stmt(stmt, state, true);
+        Stmt::If(Expr::BoolConstant(false, _), x, _) => {
+            state.set_dirty();
+            *stmt = match optimize_stmt_block(
+                mem::take(&mut x.1.statements).into_vec(),
+                state,
+                preserve_result,
+            ) {
+                statements if statements.is_empty() => Stmt::Noop(x.1.pos),
+                statements => Stmt::Block(statements, x.1.pos),
+            }
         }
         // if true { if_block } else { else_block } -> if_block
         Stmt::If(Expr::BoolConstant(true, _), x, _) => {
-            *stmt = mem::take(&mut x.0);
-            optimize_stmt(stmt, state, true);
+            state.set_dirty();
+            *stmt = match optimize_stmt_block(
+                mem::take(&mut x.0.statements).into_vec(),
+                state,
+                preserve_result,
+            ) {
+                statements if statements.is_empty() => Stmt::Noop(x.0.pos),
+                statements => Stmt::Block(statements, x.0.pos),
+            }
         }
         // if expr { if_block } else { else_block }
         Stmt::If(condition, x, _) => {
             optimize_expr(condition, state);
-            optimize_stmt(&mut x.0, state, true);
-            if let Some(else_block) = x.1.as_mut() {
-                optimize_stmt(else_block, state, true);
-                match else_block {
-                    Stmt::Noop(_) => x.1 = None, // Noop -> no else block
-                    _ => (),
-                }
-            }
+            x.0.statements = optimize_stmt_block(
+                mem::take(&mut x.0.statements).into_vec(),
+                state,
+                preserve_result,
+            )
+            .into();
+            x.1.statements = optimize_stmt_block(
+                mem::take(&mut x.1.statements).into_vec(),
+                state,
+                preserve_result,
+            )
+            .into();
         }
 
         // switch const { ... }
@@ -381,117 +370,144 @@ fn optimize_stmt(stmt: &mut Stmt, state: &mut State, preserve_result: bool) {
 
             let table = &mut x.0;
 
-            if let Some(stmt) = table.get_mut(&hash) {
-                optimize_stmt(stmt, state, true);
-                *expr = Expr::Stmt(Box::new(vec![mem::take(stmt)].into()), *pos);
-            } else if let Some(def_stmt) = x.1.as_mut() {
-                optimize_stmt(def_stmt, state, true);
-                *expr = Expr::Stmt(Box::new(vec![mem::take(def_stmt)].into()), *pos);
+            let (statements, new_pos) = if let Some(block) = table.get_mut(&hash) {
+                (
+                    optimize_stmt_block(mem::take(&mut block.statements).into_vec(), state, true)
+                        .into(),
+                    block.pos,
+                )
             } else {
-                *expr = Expr::Unit(*pos);
-            }
+                (
+                    optimize_stmt_block(mem::take(&mut x.1.statements).into_vec(), state, true)
+                        .into(),
+                    if x.1.pos.is_none() { *pos } else { x.1.pos },
+                )
+            };
+
+            *expr = Expr::Stmt(Box::new(StmtBlock {
+                statements,
+                pos: new_pos,
+            }));
         }
         // switch
         Stmt::Switch(expr, x, _) => {
             optimize_expr(expr, state);
-            x.0.values_mut()
-                .for_each(|stmt| optimize_stmt(stmt, state, true));
-            if let Some(def_stmt) = x.1.as_mut() {
-                optimize_stmt(def_stmt, state, true);
-
-                match def_stmt {
-                    Stmt::Noop(_) | Stmt::Expr(Expr::Unit(_)) => x.1 = None,
-                    _ => (),
-                }
-            }
+            x.0.values_mut().for_each(|block| {
+                block.statements = optimize_stmt_block(
+                    mem::take(&mut block.statements).into_vec(),
+                    state,
+                    preserve_result,
+                )
+                .into()
+            });
+            x.1.statements = optimize_stmt_block(
+                mem::take(&mut x.1.statements).into_vec(),
+                state,
+                preserve_result,
+            )
+            .into()
         }
 
         // while false { block } -> Noop
-        Stmt::While(Some(Expr::BoolConstant(false, pos)), _, _) => {
+        Stmt::While(Expr::BoolConstant(false, pos), _, _) => {
             state.set_dirty();
             *stmt = Stmt::Noop(*pos)
         }
         // while expr { block }
         Stmt::While(condition, block, _) => {
-            optimize_stmt(block, state, false);
+            optimize_expr(condition, state);
 
-            if let Some(condition) = condition {
-                optimize_expr(condition, state);
-            }
+            block.statements =
+                optimize_stmt_block(mem::take(&mut block.statements).into_vec(), state, false)
+                    .into();
 
-            match **block {
-                // while expr { break; } -> { expr; }
-                Stmt::Break(pos) => {
-                    // Only a single break statement - turn into running the guard expression once
-                    state.set_dirty();
-                    if let Some(condition) = condition {
-                        let mut statements = vec![Stmt::Expr(mem::take(condition))];
-                        if preserve_result {
-                            statements.push(Stmt::Noop(pos))
-                        }
-                        *stmt = Stmt::Block(statements, pos);
-                    } else {
-                        *stmt = Stmt::Noop(pos);
-                    };
+            if block.len() == 1 {
+                match block.statements[0] {
+                    // while expr { break; } -> { expr; }
+                    Stmt::Break(pos) => {
+                        // Only a single break statement - turn into running the guard expression once
+                        state.set_dirty();
+                        if !condition.is_unit() {
+                            let mut statements = vec![Stmt::Expr(mem::take(condition))];
+                            if preserve_result {
+                                statements.push(Stmt::Noop(pos))
+                            }
+                            *stmt = Stmt::Block(statements, pos);
+                        } else {
+                            *stmt = Stmt::Noop(pos);
+                        };
+                    }
+                    _ => (),
                 }
-                _ => (),
             }
         }
         // do { block } while false | do { block } until true -> { block }
         Stmt::Do(block, Expr::BoolConstant(true, _), false, _)
         | Stmt::Do(block, Expr::BoolConstant(false, _), true, _) => {
             state.set_dirty();
-            optimize_stmt(block.as_mut(), state, false);
-            *stmt = mem::take(block.as_mut());
+            *stmt = Stmt::Block(
+                optimize_stmt_block(mem::take(&mut block.statements).into_vec(), state, false),
+                block.pos,
+            );
         }
         // do { block } while|until expr
         Stmt::Do(block, condition, _, _) => {
-            optimize_stmt(block.as_mut(), state, false);
             optimize_expr(condition, state);
+            block.statements =
+                optimize_stmt_block(mem::take(&mut block.statements).into_vec(), state, false)
+                    .into();
         }
         // for id in expr { block }
         Stmt::For(iterable, x, _) => {
             optimize_expr(iterable, state);
-            optimize_stmt(&mut x.1, state, false);
+            x.1.statements =
+                optimize_stmt_block(mem::take(&mut x.1.statements).into_vec(), state, false).into();
         }
         // let id = expr;
-        Stmt::Let(_, Some(expr), _, _) => optimize_expr(expr, state),
-        // let id;
-        Stmt::Let(_, None, _, _) => (),
+        Stmt::Let(expr, _, _, _) => optimize_expr(expr, state),
         // import expr as var;
         #[cfg(not(feature = "no_module"))]
         Stmt::Import(expr, _, _) => optimize_expr(expr, state),
         // { block }
         Stmt::Block(statements, pos) => {
-            *stmt = optimize_stmt_block(mem::take(statements), *pos, state, preserve_result, true);
+            *stmt = match optimize_stmt_block(mem::take(statements), state, preserve_result) {
+                statements if statements.is_empty() => {
+                    state.set_dirty();
+                    Stmt::Noop(*pos)
+                }
+                // Only one statement - promote
+                mut statements if statements.len() == 1 => {
+                    state.set_dirty();
+                    statements.pop().unwrap()
+                }
+                statements => Stmt::Block(statements, *pos),
+            };
         }
-        // try { block } catch ( var ) { block }
-        Stmt::TryCatch(x, _, _) if x.0.is_pure() => {
+        // try { pure block } catch ( var ) { block }
+        Stmt::TryCatch(x, _, _) if x.0.statements.iter().all(Stmt::is_pure) => {
             // If try block is pure, there will never be any exceptions
             state.set_dirty();
-            let pos = x.0.position();
-            optimize_stmt(&mut x.0, state, preserve_result);
-            let mut statements = match mem::take(&mut x.0) {
-                Stmt::Block(statements, _) => statements,
-                stmt => vec![stmt],
-            };
-            statements.push(Stmt::Noop(pos));
-            *stmt = Stmt::Block(statements, pos);
+            *stmt = Stmt::Block(
+                optimize_stmt_block(mem::take(&mut x.0.statements).into_vec(), state, false),
+                x.0.pos,
+            );
         }
         // try { block } catch ( var ) { block }
         Stmt::TryCatch(x, _, _) => {
-            optimize_stmt(&mut x.0, state, false);
-            optimize_stmt(&mut x.2, state, false);
+            x.0.statements =
+                optimize_stmt_block(mem::take(&mut x.0.statements).into_vec(), state, false).into();
+            x.2.statements =
+                optimize_stmt_block(mem::take(&mut x.2.statements).into_vec(), state, false).into();
         }
         // {}
-        Stmt::Expr(Expr::Stmt(x, pos)) if x.is_empty() => {
+        Stmt::Expr(Expr::Stmt(x)) if x.statements.is_empty() => {
             state.set_dirty();
-            *stmt = Stmt::Noop(*pos);
+            *stmt = Stmt::Noop(x.pos);
         }
         // {...};
-        Stmt::Expr(Expr::Stmt(x, pos)) => {
+        Stmt::Expr(Expr::Stmt(x)) => {
             state.set_dirty();
-            *stmt = Stmt::Block(mem::take(x).into_vec(), *pos);
+            *stmt = Stmt::Block(mem::take(&mut x.statements).into_vec(), x.pos);
         }
         // expr;
         Stmt::Expr(expr) => optimize_expr(expr, state),
@@ -514,25 +530,15 @@ fn optimize_expr(expr: &mut Expr, state: &mut State) {
 
     match expr {
         // {}
-        Expr::Stmt(x, pos) if x.is_empty() => { state.set_dirty(); *expr = Expr::Unit(*pos) }
+        Expr::Stmt(x) if x.statements.is_empty() => { state.set_dirty(); *expr = Expr::Unit(x.pos) }
         // { stmt; ... } - do not count promotion as dirty because it gets turned back into an array
-        Expr::Stmt(x, pos) => match optimize_stmt_block(mem::take(x).into_vec(), *pos, state, true, false) {
-            // {}
-            Stmt::Noop(_) => { state.set_dirty(); *expr = Expr::Unit(*pos); }
-            // { stmt, .. }
-            Stmt::Block(statements, _) => *x = Box::new(statements.into()),
-            // { expr }
-            Stmt::Expr(inner) => { state.set_dirty(); *expr = inner; }
-            // { stmt }
-            stmt => x.push(stmt),
-        }
-
+        Expr::Stmt(x) => x.statements = optimize_stmt_block(mem::take(&mut x.statements).into_vec(), state, true).into(),
         // lhs.rhs
         #[cfg(not(feature = "no_object"))]
         Expr::Dot(x, _) => match (&mut x.lhs, &mut x.rhs) {
             // map.string
             (Expr::Map(m, pos), Expr::Property(p)) if m.iter().all(|(_, x)| x.is_pure()) => {
-                let prop = &p.4.name;
+                let prop = &p.2.name;
                 // Map literal where everything is pure - promote the indexed item.
                 // All other items can be thrown away.
                 state.set_dirty();
@@ -598,32 +604,6 @@ fn optimize_expr(expr: &mut Expr, state: &mut State) {
         // #{ key:value, .. }
         #[cfg(not(feature = "no_object"))]
         Expr::Map(x, _) => x.iter_mut().for_each(|(_, expr)| optimize_expr(expr, state)),
-        // lhs in rhs
-        Expr::In(x, _) => match (&mut x.lhs, &mut x.rhs) {
-            // "xxx" in "xxxxx"
-            (Expr::StringConstant(a, pos), Expr::StringConstant(b, _)) => {
-                state.set_dirty();
-                *expr = Expr::BoolConstant( b.contains(a.as_str()), *pos);
-            }
-            // 'x' in "xxxxx"
-            (Expr::CharConstant(a, pos), Expr::StringConstant(b, _)) => {
-                state.set_dirty();
-                *expr = Expr::BoolConstant(b.contains(*a), *pos);
-            }
-            // "xxx" in #{...}
-            (Expr::StringConstant(a, pos), Expr::Map(b, _)) => {
-                state.set_dirty();
-                *expr = Expr::BoolConstant(b.iter().find(|(x, _)| x.name == *a).is_some(), *pos);
-            }
-            // 'x' in #{...}
-            (Expr::CharConstant(a, pos), Expr::Map(b, _)) => {
-                state.set_dirty();
-                let ch = a.to_string();
-                *expr = Expr::BoolConstant(b.iter().find(|(x, _)| x.name == &ch).is_some(), *pos);
-            }
-            // lhs in rhs
-            (lhs, rhs) => { optimize_expr(lhs, state); optimize_expr(rhs, state); }
-        },
         // lhs && rhs
         Expr::And(x, _) => match (&mut x.lhs, &mut x.rhs) {
             // true && rhs -> rhs
@@ -684,7 +664,7 @@ fn optimize_expr(expr: &mut Expr, state: &mut State) {
                 && state.optimization_level == OptimizationLevel::Simple // simple optimizations
                 && x.args.len() == 2 // binary call
                 && x.args.iter().all(Expr::is_constant) // all arguments are constants
-                && !is_valid_identifier(x.name.chars()) // cannot be scripted
+                //&& !is_valid_identifier(x.name.chars()) // cannot be scripted
         => {
             let mut arg_values: StaticVec<_> = x.args.iter().map(|e| e.get_constant_value().unwrap()).collect();
             let arg_types: StaticVec<_> = arg_values.iter().map(Dynamic::type_id).collect();
@@ -783,16 +763,16 @@ fn optimize_top_level(
     engine: &Engine,
     scope: &Scope,
     lib: &[&Module],
-    level: OptimizationLevel,
+    optimization_level: OptimizationLevel,
 ) -> Vec<Stmt> {
     // If optimization level is None then skip optimizing
-    if level == OptimizationLevel::None {
+    if optimization_level == OptimizationLevel::None {
         statements.shrink_to_fit();
         return statements;
     }
 
     // Set up the state
-    let mut state = State::new(engine, lib, level);
+    let mut state = State::new(engine, lib, optimization_level);
 
     // Add constants and variables from the scope
     scope.iter().for_each(|(name, constant, value)| {
@@ -816,34 +796,17 @@ fn optimize_top_level(
 
         statements.iter_mut().enumerate().for_each(|(i, stmt)| {
             match stmt {
-                Stmt::Const(var_def, expr, _, _) if expr.is_some() => {
+                Stmt::Const(value_expr, Ident { name, .. }, _, _) => {
                     // Load constants
-                    let value_expr = expr.as_mut().unwrap();
                     optimize_expr(value_expr, &mut state);
 
                     if value_expr.is_constant() {
-                        state.push_var(&var_def.name, AccessMode::ReadOnly, value_expr.clone());
-                    }
-
-                    // Keep it in the global scope
-                    if value_expr.is_unit() {
-                        state.set_dirty();
-                        *expr = None;
+                        state.push_var(name, AccessMode::ReadOnly, value_expr.clone());
                     }
                 }
-                Stmt::Const(var_def, None, _, _) => {
-                    state.push_var(&var_def.name, AccessMode::ReadOnly, Expr::Unit(var_def.pos));
-                }
-                Stmt::Let(var_def, expr, _, _) => {
-                    if let Some(value_expr) = expr {
-                        optimize_expr(value_expr, &mut state);
-                    }
-
-                    state.push_var(
-                        &var_def.name,
-                        AccessMode::ReadWrite,
-                        Expr::Unit(var_def.pos),
-                    );
+                Stmt::Let(value_expr, Ident { name, pos, .. }, _, _) => {
+                    optimize_expr(value_expr, &mut state);
+                    state.push_var(name, AccessMode::ReadWrite, Expr::Unit(*pos));
                 }
                 _ => {
                     // Keep all variable declarations at this level
@@ -852,7 +815,7 @@ fn optimize_top_level(
                         Stmt::Let(_, _, _, _) | Stmt::Const(_, _, _, _) => true,
                         #[cfg(not(feature = "no_module"))]
                         Stmt::Import(_, _, _) => true,
-                        _ => i == num_statements - 1,
+                        _ => i >= num_statements - 1,
                     };
                     optimize_stmt(stmt, &mut state, keep);
                 }
@@ -887,12 +850,12 @@ pub fn optimize_into_ast(
     scope: &Scope,
     mut statements: Vec<Stmt>,
     _functions: Vec<crate::ast::ScriptFnDef>,
-    level: OptimizationLevel,
+    optimization_level: OptimizationLevel,
 ) -> AST {
     let level = if cfg!(feature = "no_optimize") {
         OptimizationLevel::None
     } else {
-        level
+        optimization_level
     };
 
     #[cfg(not(feature = "no_function"))]
@@ -921,33 +884,49 @@ pub fn optimize_into_ast(
                     lib2.set_script_fn(fn_def);
                 });
 
+            let lib2 = &[&lib2];
+
             _functions
                 .into_iter()
                 .map(|mut fn_def| {
-                    let pos = fn_def.body.position();
+                    let pos = fn_def.body.pos;
 
-                    // Optimize the function body
-                    let mut body = optimize_top_level(
-                        vec![fn_def.body],
-                        engine,
-                        &Scope::new(),
-                        &[&lib2],
-                        level,
-                    );
+                    let mut body = fn_def.body.statements.into_vec();
 
-                    // {} -> Noop
-                    fn_def.body = match body.pop().unwrap_or_else(|| Stmt::Noop(pos)) {
-                        // { return val; } -> val
-                        Stmt::Return((crate::ast::ReturnType::Return, _), Some(expr), _) => {
-                            Stmt::Expr(expr)
+                    loop {
+                        // Optimize the function body
+                        let state = &mut State::new(engine, lib2, level);
+
+                        body = optimize_stmt_block(body, state, true);
+
+                        match &mut body[..] {
+                            // { return; } -> {}
+                            [Stmt::Return(crate::ast::ReturnType::Return, None, _)] => {
+                                body.clear();
+                            }
+                            // { ...; return; } -> { ... }
+                            [.., last_stmt, Stmt::Return(crate::ast::ReturnType::Return, None, _)]
+                                if !last_stmt.returns_value() =>
+                            {
+                                body.pop().unwrap();
+                            }
+                            // { ...; return val; } -> { ...; val }
+                            [.., Stmt::Return(crate::ast::ReturnType::Return, expr, pos)] => {
+                                *body.last_mut().unwrap() = if let Some(expr) = expr {
+                                    Stmt::Expr(mem::take(expr))
+                                } else {
+                                    Stmt::Noop(*pos)
+                                };
+                            }
+                            _ => break,
                         }
-                        // { return; } -> ()
-                        Stmt::Return((crate::ast::ReturnType::Return, pos), None, _) => {
-                            Stmt::Expr(Expr::Unit(pos))
-                        }
-                        // All others
-                        stmt => stmt,
+                    }
+
+                    fn_def.body = StmtBlock {
+                        statements: body.into(),
+                        pos,
                     };
+
                     fn_def
                 })
                 .for_each(|fn_def| {
diff --git a/src/packages/array_basic.rs b/src/packages/array_basic.rs
index dc66d207..7e3012f9 100644
--- a/src/packages/array_basic.rs
+++ b/src/packages/array_basic.rs
@@ -164,7 +164,7 @@ mod array_functions {
             result
         }
     }
-    #[rhai_fn(return_raw)]
+    #[rhai_fn(return_raw, pure)]
     pub fn map(
         ctx: NativeCallContext,
         array: &mut Array,
@@ -197,7 +197,7 @@ mod array_functions {
 
         Ok(ar.into())
     }
-    #[rhai_fn(return_raw)]
+    #[rhai_fn(return_raw, pure)]
     pub fn filter(
         ctx: NativeCallContext,
         array: &mut Array,
@@ -233,8 +233,70 @@ mod array_functions {
 
         Ok(ar.into())
     }
-    #[rhai_fn(return_raw)]
+    #[rhai_fn(return_raw, pure)]
+    pub fn contains(
+        ctx: NativeCallContext,
+        array: &mut Array,
+        value: Dynamic,
+    ) -> Result<Dynamic, Box<EvalAltResult>> {
+        for item in array.iter_mut() {
+            if ctx
+                .call_fn_dynamic_raw(OP_EQUALS, true, &mut [item, &mut value.clone()])
+                .or_else(|err| match *err {
+                    EvalAltResult::ErrorFunctionNotFound(ref fn_sig, _)
+                        if fn_sig.starts_with(OP_EQUALS) =>
+                    {
+                        if item.type_id() == value.type_id() {
+                            // No default when comparing same type
+                            Err(err)
+                        } else {
+                            Ok(Dynamic::FALSE)
+                        }
+                    }
+                    _ => Err(err),
+                })?
+                .as_bool()
+                .unwrap_or(false)
+            {
+                return Ok(Dynamic::TRUE);
+            }
+        }
+
+        Ok(Dynamic::FALSE)
+    }
+    #[rhai_fn(return_raw, pure)]
     pub fn index_of(
+        ctx: NativeCallContext,
+        array: &mut Array,
+        value: Dynamic,
+    ) -> Result<Dynamic, Box<EvalAltResult>> {
+        for (i, item) in array.iter_mut().enumerate() {
+            if ctx
+                .call_fn_dynamic_raw(OP_EQUALS, true, &mut [item, &mut value.clone()])
+                .or_else(|err| match *err {
+                    EvalAltResult::ErrorFunctionNotFound(ref fn_sig, _)
+                        if fn_sig.starts_with(OP_EQUALS) =>
+                    {
+                        if item.type_id() == value.type_id() {
+                            // No default when comparing same type
+                            Err(err)
+                        } else {
+                            Ok(Dynamic::FALSE)
+                        }
+                    }
+                    _ => Err(err),
+                })?
+                .as_bool()
+                .unwrap_or(false)
+            {
+                return Ok((i as INT).into());
+            }
+        }
+
+        Ok((-1 as INT).into())
+    }
+    #[rhai_fn(name = "index_of", return_raw, pure)]
+    pub fn index_of_filter(
         ctx: NativeCallContext,
         array: &mut Array,
         filter: FnPtr,
@@ -267,7 +329,7 @@ mod array_functions {
 
         Ok((-1 as INT).into())
     }
-    #[rhai_fn(return_raw)]
+    #[rhai_fn(return_raw, pure)]
     pub fn some(
         ctx: NativeCallContext,
         array: &mut Array,
@@ -301,7 +363,7 @@ mod array_functions {
 
         Ok(false.into())
     }
-    #[rhai_fn(return_raw)]
+    #[rhai_fn(return_raw, pure)]
     pub fn all(
         ctx: NativeCallContext,
         array: &mut Array,
@@ -335,7 +397,7 @@ mod array_functions {
 
         Ok(true.into())
     }
-    #[rhai_fn(return_raw)]
+    #[rhai_fn(return_raw, pure)]
     pub fn reduce(
         ctx: NativeCallContext,
         array: &mut Array,
@@ -366,7 +428,7 @@ mod array_functions {
 
         Ok(result)
     }
-    #[rhai_fn(name = "reduce", return_raw)]
+    #[rhai_fn(name = "reduce", return_raw, pure)]
     pub fn reduce_with_initial(
         ctx: NativeCallContext,
         array: &mut Array,
@@ -405,7 +467,7 @@ mod array_functions {
 
         Ok(result)
     }
-    #[rhai_fn(return_raw)]
+    #[rhai_fn(return_raw, pure)]
     pub fn reduce_rev(
         ctx: NativeCallContext,
         array: &mut Array,
@@ -436,7 +498,7 @@ mod array_functions {
 
         Ok(result)
     }
-    #[rhai_fn(name = "reduce_rev", return_raw)]
+    #[rhai_fn(name = "reduce_rev", return_raw, pure)]
     pub fn reduce_rev_with_initial(
         ctx: NativeCallContext,
         array: &mut Array,
@@ -634,7 +696,7 @@ mod array_functions {
 
         drained
     }
-    #[rhai_fn(name = "==", return_raw)]
+    #[rhai_fn(name = "==", return_raw, pure)]
     pub fn equals(
         ctx: NativeCallContext,
         array: &mut Array,
@@ -648,18 +710,31 @@ mod array_functions {
         }
 
         for (a1, a2) in array.iter_mut().zip(array2.iter_mut()) {
-            let equals = ctx
+            if !ctx
                 .call_fn_dynamic_raw(OP_EQUALS, true, &mut [a1, a2])
-                .map(|v| v.as_bool().unwrap_or(false))?;
-
-            if !equals {
-                return Ok(false.into());
+                .or_else(|err| match *err {
+                    EvalAltResult::ErrorFunctionNotFound(ref fn_sig, _)
+                        if fn_sig.starts_with(OP_EQUALS) =>
+                    {
+                        if a1.type_id() == a2.type_id() {
+                            // No default when comparing same type
+                            Err(err)
+                        } else {
+                            Ok(Dynamic::FALSE)
+                        }
+                    }
+                    _ => Err(err),
+                })?
+                .as_bool()
+                .unwrap_or(false)
+            {
+                return Ok(Dynamic::FALSE);
             }
         }
 
-        Ok(true.into())
+        Ok(Dynamic::TRUE)
     }
-    #[rhai_fn(name = "!=", return_raw)]
+    #[rhai_fn(name = "!=", return_raw, pure)]
     pub fn not_equals(
         ctx: NativeCallContext,
         array: &mut Array,
diff --git a/src/packages/map_basic.rs b/src/packages/map_basic.rs
index e1c0558b..f54af86c 100644
--- a/src/packages/map_basic.rs
+++ b/src/packages/map_basic.rs
@@ -13,8 +13,8 @@ def_package!(crate:BasicMapPackage:"Basic object map utilities.", lib, {
 
 #[export_module]
 mod map_functions {
-    #[rhai_fn(pure)]
-    pub fn has(map: &mut Map, prop: ImmutableString) -> bool {
+    #[rhai_fn(name = "has", pure)]
+    pub fn contains(map: &mut Map, prop: ImmutableString) -> bool {
         map.contains_key(&prop)
     }
     #[rhai_fn(pure)]
diff --git a/src/packages/string_more.rs b/src/packages/string_more.rs
index 783dc2c4..ce3d819e 100644
--- a/src/packages/string_more.rs
+++ b/src/packages/string_more.rs
@@ -74,14 +74,6 @@ mod string_functions {
         }
     }
 
-    #[rhai_fn(name = "contains")]
-    pub fn contains_char(string: &str, character: char) -> bool {
-        string.contains(character)
-    }
-    pub fn contains(string: &str, find_string: &str) -> bool {
-        string.contains(find_string)
-    }
-
     #[rhai_fn(name = "index_of")]
     pub fn index_of_char_starting_from(string: &str, character: char, start: INT) -> INT {
         let start = if start < 0 {
diff --git a/src/parse_error.rs b/src/parse_error.rs
index a965af0b..b31608b6 100644
--- a/src/parse_error.rs
+++ b/src/parse_error.rs
@@ -38,6 +38,7 @@ pub enum LexError {
 impl Error for LexError {}
 
 impl fmt::Display for LexError {
+    #[inline(always)]
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
             Self::UnexpectedInput(s) => write!(f, "Unexpected '{}'", s),
@@ -293,7 +294,7 @@ pub struct ParseError(pub Box<ParseErrorType>, pub Position);
 impl Error for ParseError {}
 
 impl fmt::Display for ParseError {
-    #[inline]
+    #[inline(always)]
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         fmt::Display::fmt(&self.0, f)?;
 
diff --git a/src/parser.rs b/src/parser.rs
index 3361921b..db15c06a 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -2,10 +2,10 @@
 
 use crate::ast::{
     BinaryExpr, CustomExpr, Expr, FnCallExpr, FnHash, Ident, OpAssignment, ReturnType, ScriptFnDef,
-    Stmt,
+    Stmt, StmtBlock,
 };
 use crate::dynamic::{AccessMode, Union};
-use crate::engine::KEYWORD_THIS;
+use crate::engine::{KEYWORD_THIS, OP_CONTAINS};
 use crate::module::NamespaceRef;
 use crate::optimize::optimize_into_ast;
 use crate::optimize::OptimizationLevel;
@@ -243,7 +243,11 @@ impl Expr {
                 let setter = state.get_interned_string(crate::engine::make_setter(&ident.name));
                 let hash_set = calc_fn_hash(empty(), &setter, 2);
 
-                Self::Property(Box::new((getter, hash_get, setter, hash_set, ident.into())))
+                Self::Property(Box::new((
+                    (getter, hash_get),
+                    (setter, hash_set),
+                    ident.into(),
+                )))
             }
             _ => self,
         }
@@ -480,7 +484,6 @@ fn parse_index_chain(
             Expr::CharConstant(_, _)
             | Expr::And(_, _)
             | Expr::Or(_, _)
-            | Expr::In(_, _)
             | Expr::BoolConstant(_, _)
             | Expr::Unit(_) => {
                 return Err(PERR::MalformedIndexExpr(
@@ -514,7 +517,6 @@ fn parse_index_chain(
             Expr::CharConstant(_, _)
             | Expr::And(_, _)
             | Expr::Or(_, _)
-            | Expr::In(_, _)
             | Expr::BoolConstant(_, _)
             | Expr::Unit(_) => {
                 return Err(PERR::MalformedIndexExpr(
@@ -548,8 +550,8 @@ fn parse_index_chain(
             )
             .into_err(x.position()))
         }
-        // lhs[??? && ???], lhs[??? || ???], lhs[??? in ???]
-        x @ Expr::And(_, _) | x @ Expr::Or(_, _) | x @ Expr::In(_, _) => {
+        // lhs[??? && ???], lhs[??? || ???]
+        x @ Expr::And(_, _) | x @ Expr::Or(_, _) => {
             return Err(PERR::MalformedIndexExpr(
                 "Array access expects integer index, not a boolean".into(),
             )
@@ -811,7 +813,7 @@ fn parse_switch(
         }
     }
 
-    let mut table = HashMap::new();
+    let mut table = HashMap::<u64, StmtBlock>::new();
     let mut def_stmt = None;
 
     loop {
@@ -871,10 +873,10 @@ fn parse_switch(
         let need_comma = !stmt.is_self_terminated();
 
         def_stmt = if let Some(hash) = hash {
-            table.insert(hash, stmt);
+            table.insert(hash, stmt.into());
             None
         } else {
-            Some(stmt)
+            Some(stmt.into())
         };
 
         match input.peek().unwrap() {
@@ -905,7 +907,10 @@ fn parse_switch(
 
     Ok(Stmt::Switch(
         item,
-        Box::new((final_table.into(), def_stmt)),
+        Box::new((
+            final_table.into(),
+            def_stmt.unwrap_or_else(|| Stmt::Noop(Position::NONE).into()),
+        )),
         settings.pos,
     ))
 }
@@ -956,7 +961,7 @@ fn parse_primary(
         // { - block statement as expression
         Token::LeftBrace if settings.allow_stmt_expr => {
             match parse_block(input, state, lib, settings.level_up())? {
-                Stmt::Block(statements, pos) => Expr::Stmt(Box::new(statements.into()), pos),
+                block @ Stmt::Block(_, _) => Expr::Stmt(Box::new(block.into())),
                 stmt => unreachable!("expecting Stmt::Block, but gets {:?}", stmt),
             }
         }
@@ -964,15 +969,14 @@ fn parse_primary(
         Token::LeftParen => parse_paren_expr(input, state, lib, settings.level_up())?,
 
         // If statement is allowed to act as expressions
-        Token::If if settings.allow_if_expr => Expr::Stmt(
-            Box::new(vec![parse_if(input, state, lib, settings.level_up())?].into()),
-            settings.pos,
-        ),
+        Token::If if settings.allow_if_expr => Expr::Stmt(Box::new(
+            parse_if(input, state, lib, settings.level_up())?.into(),
+        )),
         // Switch statement is allowed to act as expressions
-        Token::Switch if settings.allow_switch_expr => Expr::Stmt(
-            Box::new(vec![parse_switch(input, state, lib, settings.level_up())?].into()),
-            settings.pos,
-        ),
+        Token::Switch if settings.allow_switch_expr => Expr::Stmt(Box::new(
+            parse_switch(input, state, lib, settings.level_up())?.into(),
+        )),
+
         // | ...
         #[cfg(not(feature = "no_function"))]
         Token::Pipe | Token::Or if settings.allow_anonymous_fn => {
@@ -1149,14 +1153,14 @@ fn parse_primary(
                     .into_err(pos));
                 }
 
-                let (_, namespace, Ident { name, pos }) = *x;
+                let (_, namespace, Ident { name, pos, .. }) = *x;
                 settings.pos = pos;
                 let ns = namespace.map(|(_, ns)| ns);
                 parse_fn_call(input, state, lib, name, true, ns, settings.level_up())?
             }
             // Function call
             (Expr::Variable(x), Token::LeftParen) => {
-                let (_, namespace, Ident { name, pos }) = *x;
+                let (_, namespace, Ident { name, pos, .. }) = *x;
                 settings.pos = pos;
                 let ns = namespace.map(|(_, ns)| ns);
                 parse_fn_call(input, state, lib, name, false, ns, settings.level_up())?
@@ -1397,7 +1401,7 @@ fn make_assignment_stmt<'a>(
         }
         // var (indexed) = rhs
         Expr::Variable(x) => {
-            let (index, _, Ident { name, pos }) = x.as_ref();
+            let (index, _, Ident { name, pos, .. }) = x.as_ref();
             match state.stack[(state.stack.len() - index.unwrap().get())].1 {
                 AccessMode::ReadWrite => {
                     Ok(Stmt::Assignment(Box::new((lhs, rhs, op_info)), op_pos))
@@ -1418,7 +1422,7 @@ fn make_assignment_stmt<'a>(
                     }
                     // var[???] (indexed) = rhs, var.??? (indexed) = rhs
                     Expr::Variable(x) => {
-                        let (index, _, Ident { name, pos }) = x.as_ref();
+                        let (index, _, Ident { name, pos, .. }) = x.as_ref();
                         match state.stack[(state.stack.len() - index.unwrap().get())].1 {
                             AccessMode::ReadWrite => {
                                 Ok(Stmt::Assignment(Box::new((lhs, rhs, op_info)), op_pos))
@@ -1508,7 +1512,7 @@ fn make_dot_expr(
             let setter = state.get_interned_string(crate::engine::make_setter(&ident.name));
             let hash_set = calc_fn_hash(empty(), &setter, 2);
 
-            let rhs = Expr::Property(Box::new((getter, hash_get, setter, hash_set, ident)));
+            let rhs = Expr::Property(Box::new(((getter, hash_get), (setter, hash_set), ident)));
 
             Expr::Dot(Box::new(BinaryExpr { lhs, rhs }), op_pos)
         }
@@ -1602,139 +1606,6 @@ fn make_dot_expr(
     })
 }
 
-/// Make an 'in' expression.
-fn make_in_expr(lhs: Expr, rhs: Expr, op_pos: Position) -> Result<Expr, ParseError> {
-    match (&lhs, &rhs) {
-        (_, x @ Expr::IntegerConstant(_, _))
-        | (_, x @ Expr::And(_, _))
-        | (_, x @ Expr::Or(_, _))
-        | (_, x @ Expr::In(_, _))
-        | (_, x @ Expr::BoolConstant(_, _))
-        | (_, x @ Expr::Unit(_)) => {
-            return Err(PERR::MalformedInExpr(
-                "'in' expression expects a string, array or object map".into(),
-            )
-            .into_err(x.position()))
-        }
-
-        #[cfg(not(feature = "no_float"))]
-        (_, x @ Expr::FloatConstant(_, _)) => {
-            return Err(PERR::MalformedInExpr(
-                "'in' expression expects a string, array or object map".into(),
-            )
-            .into_err(x.position()))
-        }
-
-        // "xxx" in "xxxx", 'x' in "xxxx" - OK!
-        (Expr::StringConstant(_, _), Expr::StringConstant(_, _))
-        | (Expr::CharConstant(_, _), Expr::StringConstant(_, _)) => (),
-
-        // 123.456 in "xxxx"
-        #[cfg(not(feature = "no_float"))]
-        (x @ Expr::FloatConstant(_, _), Expr::StringConstant(_, _)) => {
-            return Err(PERR::MalformedInExpr(
-                "'in' expression for a string expects a string, not a float".into(),
-            )
-            .into_err(x.position()))
-        }
-        // 123 in "xxxx"
-        (x @ Expr::IntegerConstant(_, _), Expr::StringConstant(_, _)) => {
-            return Err(PERR::MalformedInExpr(
-                "'in' expression for a string expects a string, not a number".into(),
-            )
-            .into_err(x.position()))
-        }
-        // (??? && ???) in "xxxx", (??? || ???) in "xxxx", (??? in ???) in "xxxx",
-        //  true in "xxxx", false in "xxxx"
-        (x @ Expr::And(_, _), Expr::StringConstant(_, _))
-        | (x @ Expr::Or(_, _), Expr::StringConstant(_, _))
-        | (x @ Expr::In(_, _), Expr::StringConstant(_, _))
-        | (x @ Expr::BoolConstant(_, _), Expr::StringConstant(_, _)) => {
-            return Err(PERR::MalformedInExpr(
-                "'in' expression for a string expects a string, not a boolean".into(),
-            )
-            .into_err(x.position()))
-        }
-        // [???, ???, ???] in "xxxx"
-        (x @ Expr::Array(_, _), Expr::StringConstant(_, _)) => {
-            return Err(PERR::MalformedInExpr(
-                "'in' expression for a string expects a string, not an array".into(),
-            )
-            .into_err(x.position()))
-        }
-        // #{...} in "xxxx"
-        (x @ Expr::Map(_, _), Expr::StringConstant(_, _)) => {
-            return Err(PERR::MalformedInExpr(
-                "'in' expression for a string expects a string, not an object map".into(),
-            )
-            .into_err(x.position()))
-        }
-        // () in "xxxx"
-        (x @ Expr::Unit(_), Expr::StringConstant(_, _)) => {
-            return Err(PERR::MalformedInExpr(
-                "'in' expression for a string expects a string, not ()".into(),
-            )
-            .into_err(x.position()))
-        }
-
-        // "xxx" in #{...}, 'x' in #{...} - OK!
-        (Expr::StringConstant(_, _), Expr::Map(_, _))
-        | (Expr::CharConstant(_, _), Expr::Map(_, _)) => (),
-
-        // 123.456 in #{...}
-        #[cfg(not(feature = "no_float"))]
-        (x @ Expr::FloatConstant(_, _), Expr::Map(_, _)) => {
-            return Err(PERR::MalformedInExpr(
-                "'in' expression for an object map expects a string, not a float".into(),
-            )
-            .into_err(x.position()))
-        }
-        // 123 in #{...}
-        (x @ Expr::IntegerConstant(_, _), Expr::Map(_, _)) => {
-            return Err(PERR::MalformedInExpr(
-                "'in' expression for an object map expects a string, not a number".into(),
-            )
-            .into_err(x.position()))
-        }
-        // (??? && ???) in #{...}, (??? || ???) in #{...}, (??? in ???) in #{...},
-        // true in #{...}, false in #{...}
-        (x @ Expr::And(_, _), Expr::Map(_, _))
-        | (x @ Expr::Or(_, _), Expr::Map(_, _))
-        | (x @ Expr::In(_, _), Expr::Map(_, _))
-        | (x @ Expr::BoolConstant(_, _), Expr::Map(_, _)) => {
-            return Err(PERR::MalformedInExpr(
-                "'in' expression for an object map expects a string, not a boolean".into(),
-            )
-            .into_err(x.position()))
-        }
-        // [???, ???, ???] in #{..}
-        (x @ Expr::Array(_, _), Expr::Map(_, _)) => {
-            return Err(PERR::MalformedInExpr(
-                "'in' expression for an object map expects a string, not an array".into(),
-            )
-            .into_err(x.position()))
-        }
-        // #{...} in #{..}
-        (x @ Expr::Map(_, _), Expr::Map(_, _)) => {
-            return Err(PERR::MalformedInExpr(
-                "'in' expression for an object map expects a string, not an object map".into(),
-            )
-            .into_err(x.position()))
-        }
-        // () in #{...}
-        (x @ Expr::Unit(_), Expr::Map(_, _)) => {
-            return Err(PERR::MalformedInExpr(
-                "'in' expression for an object map expects a string, not ()".into(),
-            )
-            .into_err(x.position()))
-        }
-
-        _ => (),
-    }
-
-    Ok(Expr::In(Box::new(BinaryExpr { lhs, rhs }), op_pos))
-}
-
 /// Parse a binary expression.
 fn parse_binary_op(
     input: &mut TokenStream,
@@ -1880,9 +1751,21 @@ fn parse_binary_op(
                 )
             }
             Token::In => {
-                let rhs = args.pop().unwrap();
-                let current_lhs = args.pop().unwrap();
-                make_in_expr(current_lhs, rhs, pos)?
+                // Swap the arguments
+                let current_lhs = args.remove(0);
+                args.push(current_lhs);
+
+                // Convert into a call to `contains`
+                let hash = calc_fn_hash(empty(), OP_CONTAINS, 2);
+                Expr::FnCall(
+                    Box::new(FnCallExpr {
+                        hash: FnHash::from_script(hash),
+                        args,
+                        name: OP_CONTAINS.into(),
+                        ..op_base
+                    }),
+                    pos,
+                )
             }
 
             Token::Custom(s)
@@ -1984,8 +1867,8 @@ fn parse_custom_syntax(
                 tokens.push(keyword);
             }
             MARKER_BLOCK => match parse_block(input, state, lib, settings)? {
-                Stmt::Block(statements, pos) => {
-                    keywords.push(Expr::Stmt(Box::new(statements.into()), pos));
+                block @ Stmt::Block(_, _) => {
+                    keywords.push(Expr::Stmt(Box::new(block.into())));
                     let keyword = state.get_interned_string(MARKER_BLOCK);
                     segments.push(keyword.clone());
                     tokens.push(keyword);
@@ -2118,20 +2001,20 @@ fn parse_if(
 
     // if guard { if_body } else ...
     let else_body = if match_token(input, Token::Else).0 {
-        Some(if let (Token::If, _) = input.peek().unwrap() {
+        if let (Token::If, _) = input.peek().unwrap() {
             // if guard { if_body } else if ...
             parse_if(input, state, lib, settings.level_up())?
         } else {
             // if guard { if_body } else { else-body }
             parse_block(input, state, lib, settings.level_up())?
-        })
+        }
     } else {
-        None
+        Stmt::Noop(Position::NONE)
     };
 
     Ok(Stmt::If(
         guard,
-        Box::new((if_body, else_body)),
+        Box::new((if_body.into(), else_body.into())),
         settings.pos,
     ))
 }
@@ -2151,18 +2034,18 @@ fn parse_while_loop(
         (Token::While, pos) => {
             ensure_not_statement_expr(input, "a boolean")?;
             let expr = parse_expr(input, state, lib, settings.level_up())?;
-            (Some(expr), pos)
+            (expr, pos)
         }
-        (Token::Loop, pos) => (None, pos),
+        (Token::Loop, pos) => (Expr::Unit(Position::NONE), pos),
         _ => unreachable!(),
     };
     settings.pos = token_pos;
 
     ensure_not_assignment(input)?;
     settings.is_breakable = true;
-    let body = Box::new(parse_block(input, state, lib, settings.level_up())?);
+    let body = parse_block(input, state, lib, settings.level_up())?;
 
-    Ok(Stmt::While(guard, body, settings.pos))
+    Ok(Stmt::While(guard, Box::new(body.into()), settings.pos))
 }
 
 /// Parse a do loop.
@@ -2180,7 +2063,7 @@ fn parse_do(
 
     // do { body } [while|until] guard
     settings.is_breakable = true;
-    let body = Box::new(parse_block(input, state, lib, settings.level_up())?);
+    let body = parse_block(input, state, lib, settings.level_up())?;
 
     let is_while = match input.next().unwrap() {
         (Token::While, _) => true,
@@ -2198,7 +2081,12 @@ fn parse_do(
     let guard = parse_expr(input, state, lib, settings.level_up())?;
     ensure_not_assignment(input)?;
 
-    Ok(Stmt::Do(body, guard, is_while, settings.pos))
+    Ok(Stmt::Do(
+        Box::new(body.into()),
+        guard,
+        is_while,
+        settings.pos,
+    ))
 }
 
 /// Parse a for loop.
@@ -2253,7 +2141,7 @@ fn parse_for(
 
     state.stack.truncate(prev_stack_len);
 
-    Ok(Stmt::For(expr, Box::new((name, body)), settings.pos))
+    Ok(Stmt::For(expr, Box::new((name, body.into())), settings.pos))
 }
 
 /// Parse a variable definition statement.
@@ -2290,18 +2178,18 @@ fn parse_let(
     // let name = ...
     let expr = if match_token(input, Token::Equals).0 {
         // let name = expr
-        Some(parse_expr(input, state, lib, settings.level_up())?)
+        parse_expr(input, state, lib, settings.level_up())?
     } else {
-        None
+        Expr::Unit(Position::NONE)
     };
 
     state.stack.push((name, var_type));
 
     match var_type {
         // let name = expr
-        AccessMode::ReadWrite => Ok(Stmt::Let(Box::new(var_def), expr, export, settings.pos)),
+        AccessMode::ReadWrite => Ok(Stmt::Let(expr, var_def, export, settings.pos)),
         // const name = { expr:constant }
-        AccessMode::ReadOnly => Ok(Stmt::Const(Box::new(var_def), expr, export, settings.pos)),
+        AccessMode::ReadOnly => Ok(Stmt::Const(expr, var_def, export, settings.pos)),
     }
 }
 
@@ -2342,10 +2230,10 @@ fn parse_import(
 
     Ok(Stmt::Import(
         expr,
-        Some(Box::new(Ident {
+        Some(Ident {
             name,
             pos: name_pos,
-        })),
+        }),
         settings.pos,
     ))
 }
@@ -2542,7 +2430,7 @@ fn parse_stmt(
 ) -> Result<Stmt, ParseError> {
     use AccessMode::{ReadOnly, ReadWrite};
 
-    let mut _comments: Vec<String> = Default::default();
+    let mut _comments: StaticVec<String> = Default::default();
 
     #[cfg(not(feature = "no_function"))]
     {
@@ -2688,16 +2576,13 @@ fn parse_stmt(
 
             match input.peek().unwrap() {
                 // `return`/`throw` at <EOF>
-                (Token::EOF, pos) => Ok(Stmt::Return((return_type, token_pos), None, *pos)),
+                (Token::EOF, _) => Ok(Stmt::Return(return_type, None, token_pos)),
                 // `return;` or `throw;`
-                (Token::SemiColon, _) => {
-                    Ok(Stmt::Return((return_type, token_pos), None, settings.pos))
-                }
+                (Token::SemiColon, _) => Ok(Stmt::Return(return_type, None, token_pos)),
                 // `return` or `throw` with expression
                 (_, _) => {
                     let expr = parse_expr(input, state, lib, settings.level_up())?;
-                    let pos = expr.position();
-                    Ok(Stmt::Return((return_type, token_pos), Some(expr), pos))
+                    Ok(Stmt::Return(return_type, Some(expr), token_pos))
                 }
             }
         }
@@ -2775,7 +2660,7 @@ fn parse_try_catch(
     let catch_body = parse_block(input, state, lib, settings.level_up())?;
 
     Ok(Stmt::TryCatch(
-        Box::new((body, var_def, catch_body)),
+        Box::new((body.into(), var_def, catch_body.into())),
         settings.pos,
         catch_pos,
     ))
@@ -2789,7 +2674,7 @@ fn parse_fn(
     lib: &mut FunctionsLib,
     access: FnAccess,
     mut settings: ParseSettings,
-    comments: Vec<String>,
+    comments: StaticVec<String>,
 ) -> Result<ScriptFnDef, ParseError> {
     #[cfg(not(feature = "unchecked"))]
     settings.ensure_level_within_max_limit(state.max_expr_depth)?;
@@ -2852,7 +2737,8 @@ fn parse_fn(
             parse_block(input, state, lib, settings.level_up())?
         }
         (_, pos) => return Err(PERR::FnMissingBody(name).into_err(*pos)),
-    };
+    }
+    .into();
 
     let params: StaticVec<_> = params.into_iter().map(|(p, _)| p).collect();
 
@@ -2914,7 +2800,7 @@ fn make_curry_from_externals(fn_expr: Expr, externals: StaticVec<Ident>, pos: Po
     let mut statements: StaticVec<_> = Default::default();
     statements.extend(externals.into_iter().map(Stmt::Share));
     statements.push(Stmt::Expr(expr));
-    Expr::Stmt(Box::new(statements), pos)
+    Expr::Stmt(Box::new(StmtBlock { statements, pos }))
 }
 
 /// Parse an anonymous function definition.
@@ -3016,7 +2902,7 @@ fn parse_anon_fn(
         params,
         #[cfg(not(feature = "no_closure"))]
         externals: Default::default(),
-        body,
+        body: body.into(),
         lib: None,
         #[cfg(not(feature = "no_module"))]
         mods: Default::default(),
diff --git a/src/result.rs b/src/result.rs
index 49743d1d..0e008cad 100644
--- a/src/result.rs
+++ b/src/result.rs
@@ -58,8 +58,6 @@ pub enum EvalAltResult {
     /// Trying to index into a type that is not an array, an object map, or a string, and has no
     /// indexer function defined. Wrapped value is the type name.
     ErrorIndexingType(String, Position),
-    /// Invalid arguments for `in` operator.
-    ErrorInExpr(Position),
     /// The `for` statement encounters a type that is not an iterator.
     ErrorFor(Position),
     /// Data race detected when accessing a variable. Wrapped value is the variable name.
@@ -122,7 +120,6 @@ impl EvalAltResult {
             Self::ErrorDataRace(_, _) => "Data race detected when accessing variable",
             Self::ErrorAssignmentToConstant(_, _) => "Cannot modify a constant",
             Self::ErrorMismatchOutputType(_, _, _) => "Output type is incorrect",
-            Self::ErrorInExpr(_) => "Malformed 'in' expression",
             Self::ErrorDotExpr(_, _) => "Malformed dot expression",
             Self::ErrorArithmetic(_, _) => "Arithmetic error",
             Self::ErrorTooManyOperations(_) => "Too many operations",
@@ -182,7 +179,6 @@ impl fmt::Display for EvalAltResult {
 
             Self::ErrorUnboundThis(_)
             | Self::ErrorFor(_)
-            | Self::ErrorInExpr(_)
             | Self::ErrorDotExpr(_, _)
             | Self::ErrorTooManyOperations(_)
             | Self::ErrorTooManyModules(_)
@@ -305,7 +301,6 @@ impl EvalAltResult {
             | Self::ErrorDataRace(_, _)
             | Self::ErrorAssignmentToConstant(_, _)
             | Self::ErrorMismatchOutputType(_, _, _)
-            | Self::ErrorInExpr(_)
             | Self::ErrorDotExpr(_, _)
             | Self::ErrorArithmetic(_, _)
             | Self::ErrorRuntime(_, _) => true,
@@ -362,7 +357,6 @@ impl EvalAltResult {
             | Self::ErrorParsing(_, _)
             | Self::ErrorUnboundThis(_)
             | Self::ErrorFor(_)
-            | Self::ErrorInExpr(_)
             | Self::ErrorArithmetic(_, _)
             | Self::ErrorTooManyOperations(_)
             | Self::ErrorTooManyModules(_)
@@ -430,7 +424,6 @@ impl EvalAltResult {
             | Self::ErrorDataRace(_, pos)
             | Self::ErrorAssignmentToConstant(_, pos)
             | Self::ErrorMismatchOutputType(_, _, pos)
-            | Self::ErrorInExpr(pos)
             | Self::ErrorDotExpr(_, pos)
             | Self::ErrorArithmetic(_, pos)
             | Self::ErrorTooManyOperations(pos)
@@ -471,7 +464,6 @@ impl EvalAltResult {
             | Self::ErrorDataRace(_, pos)
             | Self::ErrorAssignmentToConstant(_, pos)
             | Self::ErrorMismatchOutputType(_, _, pos)
-            | Self::ErrorInExpr(pos)
             | Self::ErrorDotExpr(_, pos)
             | Self::ErrorArithmetic(_, pos)
             | Self::ErrorTooManyOperations(pos)
diff --git a/src/serde/metadata.rs b/src/serde/metadata.rs
index 7f4edb25..05a709a9 100644
--- a/src/serde/metadata.rs
+++ b/src/serde/metadata.rs
@@ -151,7 +151,7 @@ impl From<&crate::module::FuncInfo> for FnMetadata {
                 }
                 #[cfg(not(feature = "no_function"))]
                 {
-                    info.func.get_fn_def().comments.clone()
+                    info.func.get_fn_def().comments.to_vec()
                 }
             } else {
                 Default::default()
diff --git a/src/utils.rs b/src/utils.rs
index a51501e9..dd768752 100644
--- a/src/utils.rs
+++ b/src/utils.rs
@@ -116,16 +116,19 @@ pub(crate) fn combine_hashes(a: u64, b: u64) -> u64 {
 pub struct HashableHashMap<K, T, H: BuildHasher>(HashMap<K, T, H>);
 
 impl<K, T, H: BuildHasher> From<HashMap<K, T, H>> for HashableHashMap<K, T, H> {
+    #[inline(always)]
     fn from(value: HashMap<K, T, H>) -> Self {
         Self(value)
     }
 }
 impl<K, T, H: BuildHasher> AsRef<HashMap<K, T, H>> for HashableHashMap<K, T, H> {
+    #[inline(always)]
     fn as_ref(&self) -> &HashMap<K, T, H> {
         &self.0
     }
 }
 impl<K, T, H: BuildHasher> AsMut<HashMap<K, T, H>> for HashableHashMap<K, T, H> {
+    #[inline(always)]
     fn as_mut(&mut self) -> &mut HashMap<K, T, H> {
         &mut self.0
     }
@@ -133,21 +136,25 @@ impl<K, T, H: BuildHasher> AsMut<HashMap<K, T, H>> for HashableHashMap<K, T, H>
 impl<K, T, H: BuildHasher> Deref for HashableHashMap<K, T, H> {
     type Target = HashMap<K, T, H>;
 
+    #[inline(always)]
     fn deref(&self) -> &Self::Target {
         &self.0
     }
 }
 impl<K, T, H: BuildHasher> DerefMut for HashableHashMap<K, T, H> {
+    #[inline(always)]
     fn deref_mut(&mut self) -> &mut Self::Target {
         &mut self.0
     }
 }
 impl<K: Debug, T: Debug, H: BuildHasher> Debug for HashableHashMap<K, T, H> {
+    #[inline(always)]
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         self.0.fmt(f)
     }
 }
 impl<K: Hash + Ord, T: Hash, H: BuildHasher> Hash for HashableHashMap<K, T, H> {
+    #[inline(always)]
     fn hash<B: Hasher>(&self, state: &mut B) {
         let mut keys: Vec<_> = self.0.keys().collect();
         keys.sort();
diff --git a/tests/maps.rs b/tests/maps.rs
index 988c51a3..5892c29d 100644
--- a/tests/maps.rs
+++ b/tests/maps.rs
@@ -38,7 +38,7 @@ fn test_map_indexing() -> Result<(), Box<EvalAltResult>> {
     engine.eval::<()>("let y = #{a: 1, b: 2, c: 3}; y.z")?;
 
     assert!(engine.eval::<bool>(r#"let y = #{a: 1, b: 2, c: 3}; "c" in y"#)?);
-    assert!(engine.eval::<bool>("let y = #{a: 1, b: 2, c: 3}; 'b' in y")?);
+    assert!(engine.eval::<bool>(r#"let y = #{a: 1, b: 2, c: 3}; "b" in y"#)?);
     assert!(!engine.eval::<bool>(r#"let y = #{a: 1, b: 2, c: 3}; "z" in y"#)?);
 
     assert_eq!(
diff --git a/tests/optimizer.rs b/tests/optimizer.rs
index d062ec5f..4c7cfb49 100644
--- a/tests/optimizer.rs
+++ b/tests/optimizer.rs
@@ -55,20 +55,18 @@ fn test_optimizer_parse() -> Result<(), Box<EvalAltResult>> {
 
     let ast = engine.compile("{ const DECISION = false; if DECISION { 42 } else { 123 } }")?;
 
-    assert!(format!("{:?}", ast).starts_with(r#"AST { source: None, statements: [Block([Const(Ident("DECISION" @ 1:9), Some(BoolConstant(false, 1:20)), false, 1:3), Expr(IntegerConstant(123, 1:53))], 1:1)]"#));
+    assert!(format!("{:?}", ast).starts_with(r#"AST { source: None, body: [Block([Const(BoolConstant(false, 1:20), Ident("DECISION" @ 1:9), false, 1:3), Expr(IntegerConstant(123, 1:53))], 1:1)], functions: Module("#));
 
     let ast = engine.compile("if 1 == 2 { 42 }")?;
 
-    assert!(
-        format!("{:?}", ast).starts_with("AST { source: None, statements: [], functions: Module(")
-    );
+    assert!(format!("{:?}", ast).starts_with("AST { source: None, body: [], functions: Module("));
 
     engine.set_optimization_level(OptimizationLevel::Full);
 
     let ast = engine.compile("abs(-42)")?;
 
     assert!(format!("{:?}", ast)
-        .starts_with(r"AST { source: None, statements: [Expr(IntegerConstant(42, 1:1))]"));
+        .starts_with(r"AST { source: None, body: [Expr(IntegerConstant(42, 1:1))]"));
 
     Ok(())
 }