diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3680c038..9457c558 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,8 @@ Bug fixes
 ---------
 
 * Fixed infinite loop in certain script optimizations.
+* Building for `no-std` no longer requires patching `smartstring`.
+* Parsing a lone `return` or `throw` without a semicolon at the end of a block no longer raises an error.
 
 Breaking changes
 ----------------
diff --git a/codegen/src/module.rs b/codegen/src/module.rs
index df065797..f78dad1f 100644
--- a/codegen/src/module.rs
+++ b/codegen/src/module.rs
@@ -205,7 +205,6 @@ impl Parse for Module {
     }
 }
 
-#[allow(dead_code)]
 impl Module {
     pub fn attrs(&self) -> &Vec<syn::Attribute> {
         &self.mod_all.attrs
@@ -300,22 +299,27 @@ impl Module {
         }
     }
 
+    #[allow(dead_code)]
     pub fn name(&self) -> &syn::Ident {
         &self.mod_all.ident
     }
 
+    #[allow(dead_code)]
     pub fn consts(&self) -> &[ExportedConst] {
         &self.consts
     }
 
+    #[allow(dead_code)]
     pub fn fns(&self) -> &[ExportedFn] {
         &self.fns
     }
 
+    #[allow(dead_code)]
     pub fn sub_modules(&self) -> &[Module] {
         &self.sub_modules
     }
 
+    #[allow(dead_code)]
     pub fn content(&self) -> Option<&Vec<syn::Item>> {
         match self.mod_all {
             syn::ItemMod {
diff --git a/no_std/no_std_test/Cargo.toml b/no_std/no_std_test/Cargo.toml
index 94087a70..6a85b79d 100644
--- a/no_std/no_std_test/Cargo.toml
+++ b/no_std/no_std_test/Cargo.toml
@@ -36,7 +36,3 @@ inherits = "release"
 [profile.macos]
 inherits = "release"
 lto = "fat"
-
-[patch.crates-io]
-# Patch smartstring wth a PR fix because it doesn't properly handle no-std builds.
-smartstring = { git = "https://github.com/rhaiscript/smartstring" }
diff --git a/no_std/no_std_test/README.md b/no_std/no_std_test/README.md
index 0a12e48b..1c89b11f 100644
--- a/no_std/no_std_test/README.md
+++ b/no_std/no_std_test/README.md
@@ -12,7 +12,7 @@ To Compile
 The nightly compiler is required:
 
 ```bash
-cargo +nightly build --release --profile unix -Z unstable-features
+cargo +nightly build --profile unix -Z unstable-options
 ```
 
 Available profiles are: `unix`, `windows` and `macos`.
diff --git a/src/ast.rs b/src/ast.rs
index 87af9821..3bd1d9b9 100644
--- a/src/ast.rs
+++ b/src/ast.rs
@@ -1394,20 +1394,11 @@ pub struct CustomExpr {
     /// List of keywords.
     pub keywords: StaticVec<Expr>,
     /// Is the current [`Scope`][crate::Scope] modified?
-    pub scope_changed: bool,
+    pub scope_may_be_changed: bool,
     /// List of tokens actually parsed.
     pub tokens: StaticVec<Identifier>,
 }
 
-impl CustomExpr {
-    /// Convert this into a [`Expr::Custom`].
-    #[inline(always)]
-    #[must_use]
-    pub fn into_custom_syntax_expr(self, pos: Position) -> Expr {
-        Expr::Custom(self.into(), pos)
-    }
-}
-
 /// _(INTERNALS)_ A binary expression.
 /// Exported under the `internals` feature only.
 ///
diff --git a/src/custom_syntax.rs b/src/custom_syntax.rs
index 36c97003..7e1aa614 100644
--- a/src/custom_syntax.rs
+++ b/src/custom_syntax.rs
@@ -156,33 +156,34 @@ pub struct CustomSyntax {
     /// Custom syntax implementation function.
     pub func: Shared<FnCustomSyntaxEval>,
     /// Any variables added/removed in the scope?
-    pub scope_changed: bool,
+    pub scope_may_be_changed: bool,
 }
 
 impl Engine {
     /// Register a custom syntax with the [`Engine`].
     ///
     /// * `keywords` holds a slice of strings that define the custom syntax.  
-    /// * `scope_changed` specifies variables have been added/removed by this custom syntax.
+    /// * `scope_may_be_changed` specifies variables _may_ be added/removed by this custom syntax.
     /// * `func` is the implementation function.
     ///
-    /// # Caveat - Do not change beyond block scope
+    /// ## Note on `scope_may_be_changed`
     ///
-    /// If `scope_changed` is `true`, then the current [`Scope`][crate::Scope] is assumed to be
-    /// modified by this custom syntax.
+    /// If `scope_may_be_changed` is `true`, then _size_ of the current [`Scope`][crate::Scope]
+    /// _may_ be modified by this custom syntax.
     ///
-    /// Adding new variables and/or removing variables are allowed. Simply modifying the values of
-    /// variables does NOT count, so `false` should be passed in this case.
+    /// Adding new variables and/or removing variables count.
     ///
-    /// However, only variables declared within the current _block scope_ should be touched,
-    /// since they all go away at the end of the block.
+    /// Simply modifying the values of existing variables does NOT count, as the _size_ of the
+    /// current [`Scope`][crate::Scope] is unchanged, so `false` should be passed.
     ///
-    /// Variables in parent blocks should be left untouched as they persist beyond the current block.
+    /// Replacing one variable with another (i.e. adding a new variable and removing one variable at
+    /// the same time so that the total _size_ of the [`Scope`][crate::Scope] is unchanged) also
+    /// does NOT count, so `false` should be passed.
     #[must_use]
     pub fn register_custom_syntax<S: AsRef<str> + Into<Identifier>>(
         &mut self,
         keywords: &[S],
-        scope_changed: bool,
+        scope_may_be_changed: bool,
         func: impl Fn(&mut EvalContext, &[Expression]) -> RhaiResult + SendSync + 'static,
     ) -> Result<&mut Self, ParseError> {
         let keywords = keywords.as_ref();
@@ -289,7 +290,7 @@ impl Engine {
                     Ok(Some(segments[stream.len()].clone()))
                 }
             },
-            scope_changed,
+            scope_may_be_changed,
             func,
         );
 
@@ -301,7 +302,7 @@ impl Engine {
     ///
     /// This function is very low level.
     ///
-    /// * `scope_changed` specifies variables have been added/removed by this custom syntax.
+    /// * `scope_may_be_changed` specifies variables have been added/removed by this custom syntax.
     /// * `parse` is the parsing function.
     /// * `func` is the implementation function.
     ///
@@ -313,7 +314,7 @@ impl Engine {
         parse: impl Fn(&[ImmutableString], &str) -> Result<Option<ImmutableString>, ParseError>
             + SendSync
             + 'static,
-        scope_changed: bool,
+        scope_may_be_changed: bool,
         func: impl Fn(&mut EvalContext, &[Expression]) -> RhaiResult + SendSync + 'static,
     ) -> &mut Self {
         self.custom_syntax.insert(
@@ -321,7 +322,7 @@ impl Engine {
             CustomSyntax {
                 parse: Box::new(parse),
                 func: (Box::new(func) as Box<FnCustomSyntaxEval>).into(),
-                scope_changed,
+                scope_may_be_changed,
             }
             .into(),
         );
diff --git a/src/dynamic.rs b/src/dynamic.rs
index 81dde6f6..d4c0732e 100644
--- a/src/dynamic.rs
+++ b/src/dynamic.rs
@@ -508,6 +508,8 @@ impl Hash for Dynamic {
             Union::Int(ref i, _, _) => i.hash(state),
             #[cfg(not(feature = "no_float"))]
             Union::Float(ref f, _, _) => f.hash(state),
+            #[cfg(feature = "decimal")]
+            Union::Decimal(ref d, _, _) => d.hash(state),
             #[cfg(not(feature = "no_index"))]
             Union::Array(ref a, _, _) => a.as_ref().hash(state),
             #[cfg(not(feature = "no_object"))]
@@ -522,49 +524,54 @@ impl Hash for Dynamic {
             #[cfg(feature = "sync")]
             Union::Shared(ref cell, _, _) => (*cell.read().unwrap()).hash(state),
 
-            #[cfg(not(feature = "only_i32"))]
-            #[cfg(not(feature = "only_i64"))]
-            Union::Variant(ref value, _, _) => {
-                let value_any = (***value).as_any();
-                let type_id = value_any.type_id();
+            Union::Variant(ref _value, _, _) => {
+                #[cfg(not(feature = "only_i32"))]
+                #[cfg(not(feature = "only_i64"))]
+                {
+                    let value_any = (***_value).as_any();
+                    let type_id = value_any.type_id();
 
-                if type_id == TypeId::of::<u8>() {
-                    TypeId::of::<u8>().hash(state);
-                    value_any.downcast_ref::<u8>().expect(CHECKED).hash(state);
-                } else if type_id == TypeId::of::<u16>() {
-                    TypeId::of::<u16>().hash(state);
-                    value_any.downcast_ref::<u16>().expect(CHECKED).hash(state);
-                } else if type_id == TypeId::of::<u32>() {
-                    TypeId::of::<u32>().hash(state);
-                    value_any.downcast_ref::<u32>().expect(CHECKED).hash(state);
-                } else if type_id == TypeId::of::<u64>() {
-                    TypeId::of::<u64>().hash(state);
-                    value_any.downcast_ref::<u64>().expect(CHECKED).hash(state);
-                } else if type_id == TypeId::of::<i8>() {
-                    TypeId::of::<i8>().hash(state);
-                    value_any.downcast_ref::<i8>().expect(CHECKED).hash(state);
-                } else if type_id == TypeId::of::<i16>() {
-                    TypeId::of::<i16>().hash(state);
-                    value_any.downcast_ref::<i16>().expect(CHECKED).hash(state);
-                } else if type_id == TypeId::of::<i32>() {
-                    TypeId::of::<i32>().hash(state);
-                    value_any.downcast_ref::<i32>().expect(CHECKED).hash(state);
-                } else if type_id == TypeId::of::<i64>() {
-                    TypeId::of::<i64>().hash(state);
-                    value_any.downcast_ref::<i64>().expect(CHECKED).hash(state);
+                    if type_id == TypeId::of::<u8>() {
+                        TypeId::of::<u8>().hash(state);
+                        value_any.downcast_ref::<u8>().expect(CHECKED).hash(state);
+                    } else if type_id == TypeId::of::<u16>() {
+                        TypeId::of::<u16>().hash(state);
+                        value_any.downcast_ref::<u16>().expect(CHECKED).hash(state);
+                    } else if type_id == TypeId::of::<u32>() {
+                        TypeId::of::<u32>().hash(state);
+                        value_any.downcast_ref::<u32>().expect(CHECKED).hash(state);
+                    } else if type_id == TypeId::of::<u64>() {
+                        TypeId::of::<u64>().hash(state);
+                        value_any.downcast_ref::<u64>().expect(CHECKED).hash(state);
+                    } else if type_id == TypeId::of::<i8>() {
+                        TypeId::of::<i8>().hash(state);
+                        value_any.downcast_ref::<i8>().expect(CHECKED).hash(state);
+                    } else if type_id == TypeId::of::<i16>() {
+                        TypeId::of::<i16>().hash(state);
+                        value_any.downcast_ref::<i16>().expect(CHECKED).hash(state);
+                    } else if type_id == TypeId::of::<i32>() {
+                        TypeId::of::<i32>().hash(state);
+                        value_any.downcast_ref::<i32>().expect(CHECKED).hash(state);
+                    } else if type_id == TypeId::of::<i64>() {
+                        TypeId::of::<i64>().hash(state);
+                        value_any.downcast_ref::<i64>().expect(CHECKED).hash(state);
+                    }
+
+                    #[cfg(not(any(target_arch = "wasm32", target_arch = "wasm64")))]
+                    if type_id == TypeId::of::<u128>() {
+                        TypeId::of::<u128>().hash(state);
+                        value_any.downcast_ref::<u128>().expect(CHECKED).hash(state);
+                    } else if type_id == TypeId::of::<i128>() {
+                        TypeId::of::<i128>().hash(state);
+                        value_any.downcast_ref::<i128>().expect(CHECKED).hash(state);
+                    }
                 }
 
-                #[cfg(not(any(target_arch = "wasm32", target_arch = "wasm64")))]
-                if type_id == TypeId::of::<u128>() {
-                    TypeId::of::<u128>().hash(state);
-                    value_any.downcast_ref::<u128>().expect(CHECKED).hash(state);
-                } else if type_id == TypeId::of::<i128>() {
-                    TypeId::of::<i128>().hash(state);
-                    value_any.downcast_ref::<i128>().expect(CHECKED).hash(state);
-                }
+                unimplemented!("a custom type cannot be hashed")
             }
 
-            _ => unimplemented!("{} cannot be hashed", self.type_name()),
+            #[cfg(not(feature = "no_std"))]
+            Union::TimeStamp(_, _, _) => unimplemented!("{} cannot be hashed", self.type_name()),
         }
     }
 }
diff --git a/src/engine.rs b/src/engine.rs
index fc6c1b44..91b20d8b 100644
--- a/src/engine.rs
+++ b/src/engine.rs
@@ -24,6 +24,7 @@ use std::{
     collections::{BTreeMap, BTreeSet},
     fmt,
     hash::{Hash, Hasher},
+    iter::{FromIterator, Rev, Zip},
     num::{NonZeroU8, NonZeroUsize},
     ops::{Deref, DerefMut},
 };
@@ -62,6 +63,12 @@ pub struct Imports {
 }
 
 impl Imports {
+    /// Create a new stack of imported [modules][Module].
+    #[inline(always)]
+    #[must_use]
+    pub fn new() -> Self {
+        Default::default()
+    }
     /// Get the length of this stack of imported [modules][Module].
     #[inline(always)]
     #[must_use]
@@ -74,20 +81,20 @@ impl Imports {
     pub fn is_empty(&self) -> bool {
         self.keys.is_empty()
     }
-    /// Get the imported [modules][Module] at a particular index.
+    /// Get the imported [module][Module] at a particular index.
     #[inline(always)]
     #[must_use]
     pub fn get(&self, index: usize) -> Option<Shared<Module>> {
         self.modules.get(index).cloned()
     }
-    /// Get the imported [modules][Module] at a particular index.
+    /// Get the imported [module][Module] at a particular index.
     #[allow(dead_code)]
     #[inline(always)]
     #[must_use]
     pub(crate) fn get_mut(&mut self, index: usize) -> Option<&mut Shared<Module>> {
         self.modules.get_mut(index)
     }
-    /// Get the index of an imported [modules][Module] by name.
+    /// Get the index of an imported [module][Module] by name.
     #[inline(always)]
     #[must_use]
     pub fn find(&self, name: &str) -> Option<usize> {
@@ -97,7 +104,7 @@ impl Imports {
             .rev()
             .find_map(|(i, key)| if key == name { Some(i) } else { None })
     }
-    /// Push an imported [modules][Module] onto the stack.
+    /// Push an imported [module][Module] onto the stack.
     #[inline(always)]
     pub fn push(&mut self, name: impl Into<Identifier>, module: impl Into<Shared<Module>>) {
         self.keys.push(name.into());
@@ -134,15 +141,6 @@ impl Imports {
     pub(crate) fn scan_raw(&self) -> impl Iterator<Item = (&Identifier, &Shared<Module>)> {
         self.keys.iter().zip(self.modules.iter())
     }
-    /// Get a consuming iterator to this stack of imported [modules][Module] in reverse order.
-    #[inline(always)]
-    #[must_use]
-    pub fn into_iter(self) -> impl Iterator<Item = (Identifier, Shared<Module>)> {
-        self.keys
-            .into_iter()
-            .rev()
-            .zip(self.modules.into_iter().rev())
-    }
     /// Does the specified function hash key exist in this stack of imported [modules][Module]?
     #[allow(dead_code)]
     #[inline(always)]
@@ -150,7 +148,7 @@ impl Imports {
     pub fn contains_fn(&self, hash: u64) -> bool {
         self.modules.iter().any(|m| m.contains_qualified_fn(hash))
     }
-    /// Get specified function via its hash key.
+    /// Get the specified function via its hash key from this stack of imported [modules][Module].
     #[inline(always)]
     #[must_use]
     pub fn get_fn(&self, hash: u64) -> Option<(&CallableFunction, Option<&Identifier>)> {
@@ -167,7 +165,8 @@ impl Imports {
     pub fn contains_iter(&self, id: TypeId) -> bool {
         self.modules.iter().any(|m| m.contains_qualified_iter(id))
     }
-    /// Get the specified [`TypeId`][std::any::TypeId] iterator.
+    /// Get the specified [`TypeId`][std::any::TypeId] iterator from this stack of imported
+    /// [modules][Module].
     #[inline(always)]
     #[must_use]
     pub fn get_iter(&self, id: TypeId) -> Option<IteratorFn> {
@@ -178,6 +177,37 @@ impl Imports {
     }
 }
 
+impl IntoIterator for Imports {
+    type Item = (Identifier, Shared<Module>);
+    type IntoIter =
+        Zip<Rev<smallvec::IntoIter<[Identifier; 4]>>, Rev<smallvec::IntoIter<[Shared<Module>; 4]>>>;
+
+    #[inline(always)]
+    fn into_iter(self) -> Self::IntoIter {
+        self.keys
+            .into_iter()
+            .rev()
+            .zip(self.modules.into_iter().rev())
+    }
+}
+
+impl<K: Into<Identifier>, M: Into<Shared<Module>>> FromIterator<(K, M)> for Imports {
+    fn from_iter<T: IntoIterator<Item = (K, M)>>(iter: T) -> Self {
+        let mut lib = Self::new();
+        lib.extend(iter);
+        lib
+    }
+}
+
+impl<K: Into<Identifier>, M: Into<Shared<Module>>> Extend<(K, M)> for Imports {
+    fn extend<T: IntoIterator<Item = (K, M)>>(&mut self, iter: T) {
+        iter.into_iter().for_each(|(k, m)| {
+            self.keys.push(k.into());
+            self.modules.push(m.into());
+        })
+    }
+}
+
 impl fmt::Debug for Imports {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.write_str("Imports")?;
@@ -348,7 +378,7 @@ impl<'a> Target<'a> {
     #[allow(dead_code)]
     #[inline(always)]
     #[must_use]
-    pub fn is_ref(&self) -> bool {
+    pub const fn is_ref(&self) -> bool {
         match self {
             Self::RefMut(_) => true,
             #[cfg(not(feature = "no_closure"))]
@@ -363,7 +393,7 @@ impl<'a> Target<'a> {
     /// Is the `Target` a temp value?
     #[inline(always)]
     #[must_use]
-    pub fn is_temp_value(&self) -> bool {
+    pub const fn is_temp_value(&self) -> bool {
         match self {
             Self::RefMut(_) => false,
             #[cfg(not(feature = "no_closure"))]
@@ -578,7 +608,12 @@ impl<T: Into<Dynamic>> From<T> for Target<'_> {
     }
 }
 
-/// An entry in a function resolution cache.
+/// _(INTERNALS)_ An entry in a function resolution cache.
+/// Exported under the `internals` feature only.
+///
+/// # Volatile Data Structure
+///
+/// This type is volatile and may change.
 #[derive(Debug, Clone)]
 pub struct FnResolutionCacheEntry {
     /// Function.
@@ -587,7 +622,12 @@ pub struct FnResolutionCacheEntry {
     pub source: Option<Identifier>,
 }
 
-/// A function resolution cache.
+/// _(INTERNALS)_ A function resolution cache.
+/// Exported under the `internals` feature only.
+///
+/// # Volatile Data Structure
+///
+/// This type is volatile and may change.
 pub type FnResolutionCache = BTreeMap<u64, Option<Box<FnResolutionCacheEntry>>>;
 
 /// _(INTERNALS)_ A type that holds all the current states of the [`Engine`].
@@ -597,15 +637,15 @@ pub type FnResolutionCache = BTreeMap<u64, Option<Box<FnResolutionCacheEntry>>>;
 ///
 /// This type is volatile and may change.
 #[derive(Debug, Clone, Default)]
-pub struct State {
+pub struct EvalState {
     /// Source of the current context.
     pub source: Option<Identifier>,
-    /// Normally, access to variables are parsed with a relative offset into the scope to avoid a lookup.
-    /// In some situation, e.g. after running an `eval` statement, subsequent offsets become mis-aligned.
-    /// When that happens, this flag is turned on to force a scope lookup by name.
-    pub always_search: bool,
-    /// Level of the current scope.  The global (root) level is zero, a new block
-    /// (or function call) is one level higher, and so on.
+    /// Normally, access to variables are parsed with a relative offset into the [`Scope`] to avoid a lookup.
+    /// In some situation, e.g. after running an `eval` statement, subsequent offsets may become mis-aligned.
+    /// When that happens, this flag is turned on to force a [`Scope`] search by name.
+    pub always_search_scope: bool,
+    /// Level of the current scope.  The global (root) level is zero, a new block (or function call)
+    /// is one level higher, and so on.
     pub scope_level: usize,
     /// Number of operations performed.
     pub operations: u64,
@@ -614,15 +654,30 @@ pub struct State {
     /// Embedded module resolver.
     #[cfg(not(feature = "no_module"))]
     pub resolver: Option<Shared<crate::module::resolvers::StaticModuleResolver>>,
-    /// Function resolution cache and free list.
-    fn_resolution_caches: StaticVec<FnResolutionCache>,
+    /// Stack of function resolution caches.
+    fn_resolution_caches: Vec<FnResolutionCache>,
 }
 
-impl State {
+impl EvalState {
+    /// Create a new [`EvalState`].
+    #[inline(always)]
+    #[must_use]
+    pub const fn new() -> Self {
+        Self {
+            source: None,
+            always_search_scope: false,
+            scope_level: 0,
+            operations: 0,
+            modules: 0,
+            #[cfg(not(feature = "no_module"))]
+            resolver: None,
+            fn_resolution_caches: Vec::new(),
+        }
+    }
     /// Is the state currently at global (root) level?
     #[inline(always)]
     #[must_use]
-    pub fn is_global(&self) -> bool {
+    pub const fn is_global(&self) -> bool {
         self.scope_level == 0
     }
     /// Get a mutable reference to the current function resolution cache.
@@ -729,13 +784,13 @@ pub struct EvalContext<'a, 'x, 'px, 'm, 's, 'b, 't, 'pt> {
     pub(crate) engine: &'a Engine,
     pub(crate) scope: &'x mut Scope<'px>,
     pub(crate) mods: &'m mut Imports,
-    pub(crate) state: &'s mut State,
+    pub(crate) state: &'s mut EvalState,
     pub(crate) lib: &'b [&'b Module],
     pub(crate) this_ptr: &'t mut Option<&'pt mut Dynamic>,
     pub(crate) level: usize,
 }
 
-impl<'x, 'px> EvalContext<'_, 'x, 'px, '_, '_, '_, '_, '_> {
+impl<'x, 'px, 'pt> EvalContext<'_, 'x, 'px, '_, '_, '_, '_, 'pt> {
     /// The current [`Engine`].
     #[inline(always)]
     #[must_use]
@@ -796,6 +851,12 @@ impl<'x, 'px> EvalContext<'_, 'x, 'px, '_, '_, '_, '_, '_> {
     pub fn this_ptr(&self) -> Option<&Dynamic> {
         self.this_ptr.as_ref().map(|v| &**v)
     }
+    /// Mutable reference to the current bound `this` pointer, if any.
+    #[inline(always)]
+    #[must_use]
+    pub fn this_ptr_mut(&mut self) -> Option<&mut &'pt mut Dynamic> {
+        self.this_ptr.as_mut()
+    }
     /// The current nesting level of function calls.
     #[inline(always)]
     #[must_use]
@@ -1002,13 +1063,13 @@ impl Engine {
     pub(crate) fn search_imports(
         &self,
         mods: &Imports,
-        state: &mut State,
+        state: &mut EvalState,
         namespace: &NamespaceRef,
     ) -> Option<Shared<Module>> {
         let root = &namespace[0].name;
 
         // Qualified - check if the root module is directly indexed
-        let index = if state.always_search {
+        let index = if state.always_search_scope {
             None
         } else {
             namespace.index()
@@ -1037,7 +1098,7 @@ impl Engine {
         &self,
         scope: &'s mut Scope,
         mods: &mut Imports,
-        state: &mut State,
+        state: &mut EvalState,
         lib: &[&Module],
         this_ptr: &'s mut Option<&mut Dynamic>,
         expr: &Expr,
@@ -1087,7 +1148,7 @@ impl Engine {
         &self,
         scope: &'s mut Scope,
         mods: &mut Imports,
-        state: &mut State,
+        state: &mut EvalState,
         lib: &[&Module],
         this_ptr: &'s mut Option<&mut Dynamic>,
         expr: &Expr,
@@ -1103,7 +1164,7 @@ impl Engine {
                     EvalAltResult::ErrorUnboundThis(*pos).into()
                 }
             }
-            _ if state.always_search => (0, expr.position()),
+            _ if state.always_search_scope => (0, expr.position()),
             Expr::Variable(Some(i), pos, _) => (i.get() as usize, *pos),
             Expr::Variable(None, pos, v) => (v.0.map(NonZeroUsize::get).unwrap_or(0), *pos),
             _ => unreachable!("Expr::Variable expected, but gets {:?}", expr),
@@ -1160,7 +1221,7 @@ impl Engine {
     fn eval_dot_index_chain_helper(
         &self,
         mods: &mut Imports,
-        state: &mut State,
+        state: &mut EvalState,
         lib: &[&Module],
         this_ptr: &mut Option<&mut Dynamic>,
         target: &mut Target,
@@ -1592,7 +1653,7 @@ impl Engine {
         &self,
         scope: &mut Scope,
         mods: &mut Imports,
-        state: &mut State,
+        state: &mut EvalState,
         lib: &[&Module],
         this_ptr: &mut Option<&mut Dynamic>,
         expr: &Expr,
@@ -1656,7 +1717,7 @@ impl Engine {
         &self,
         scope: &mut Scope,
         mods: &mut Imports,
-        state: &mut State,
+        state: &mut EvalState,
         lib: &[&Module],
         this_ptr: &mut Option<&mut Dynamic>,
         expr: &Expr,
@@ -1785,7 +1846,7 @@ impl Engine {
     fn get_indexed_mut<'t>(
         &self,
         mods: &mut Imports,
-        state: &mut State,
+        state: &mut EvalState,
         lib: &[&Module],
         target: &'t mut Dynamic,
         mut idx: Dynamic,
@@ -1843,7 +1904,7 @@ impl Engine {
             #[cfg(not(feature = "no_object"))]
             Dynamic(Union::Map(map, _, _)) => {
                 // val_map[idx]
-                let index = &*idx.read_lock::<ImmutableString>().ok_or_else(|| {
+                let index = idx.read_lock::<ImmutableString>().ok_or_else(|| {
                     self.make_type_mismatch_err::<ImmutableString>(idx.type_name(), idx_pos)
                 })?;
 
@@ -1957,7 +2018,7 @@ impl Engine {
         &self,
         scope: &mut Scope,
         mods: &mut Imports,
-        state: &mut State,
+        state: &mut EvalState,
         lib: &[&Module],
         this_ptr: &mut Option<&mut Dynamic>,
         expr: &Expr,
@@ -2158,7 +2219,7 @@ impl Engine {
         &self,
         scope: &mut Scope,
         mods: &mut Imports,
-        state: &mut State,
+        state: &mut EvalState,
         lib: &[&Module],
         this_ptr: &mut Option<&mut Dynamic>,
         statements: &[Stmt],
@@ -2170,7 +2231,7 @@ impl Engine {
         }
 
         let mut _extra_fn_resolution_cache = false;
-        let prev_always_search = state.always_search;
+        let prev_always_search_scope = state.always_search_scope;
         let prev_scope_len = scope.len();
         let prev_mods_len = mods.len();
 
@@ -2223,7 +2284,7 @@ impl Engine {
 
             // The impact of new local variables goes away at the end of a block
             // because any new variables introduced will go out of scope
-            state.always_search = prev_always_search;
+            state.always_search_scope = prev_always_search_scope;
         }
 
         result
@@ -2235,7 +2296,7 @@ impl Engine {
     pub(crate) fn eval_op_assignment(
         &self,
         mods: &mut Imports,
-        state: &mut State,
+        state: &mut EvalState,
         lib: &[&Module],
         op_info: Option<OpAssignment>,
         op_pos: Position,
@@ -2310,7 +2371,7 @@ impl Engine {
         &self,
         scope: &mut Scope,
         mods: &mut Imports,
-        state: &mut State,
+        state: &mut EvalState,
         lib: &[&Module],
         this_ptr: &mut Option<&mut Dynamic>,
         stmt: &Stmt,
@@ -3077,7 +3138,7 @@ impl Engine {
     #[must_use]
     pub(crate) fn inc_operations(
         &self,
-        state: &mut State,
+        state: &mut EvalState,
         pos: Position,
     ) -> Result<(), Box<EvalAltResult>> {
         state.operations += 1;
diff --git a/src/engine_api.rs b/src/engine_api.rs
index 6a1b350b..ab896a2a 100644
--- a/src/engine_api.rs
+++ b/src/engine_api.rs
@@ -1,7 +1,7 @@
 //! Module that defines the extern API of [`Engine`].
 
 use crate::dynamic::Variant;
-use crate::engine::{EvalContext, Imports, State};
+use crate::engine::{EvalContext, EvalState, Imports};
 use crate::fn_call::FnCallArgs;
 use crate::fn_native::SendSync;
 use crate::fn_register::RegisterNativeFunction;
@@ -1749,7 +1749,7 @@ impl Engine {
         ast: &'a AST,
         level: usize,
     ) -> RhaiResult {
-        let mut state: State = Default::default();
+        let mut state = EvalState::new();
         state.source = ast.source_raw().cloned();
         #[cfg(not(feature = "no_module"))]
         {
@@ -1831,7 +1831,7 @@ impl Engine {
         ast: &AST,
     ) -> Result<(), Box<EvalAltResult>> {
         let mods = &mut Default::default();
-        let mut state: State = Default::default();
+        let mut state = EvalState::new();
         state.source = ast.source_raw().cloned();
         #[cfg(not(feature = "no_module"))]
         {
@@ -2004,7 +2004,7 @@ impl Engine {
         this_ptr: &mut Option<&mut Dynamic>,
         args: &mut FnCallArgs,
     ) -> RhaiResult {
-        let state = &mut Default::default();
+        let state = &mut EvalState::new();
         let mods = &mut Default::default();
         let lib = &[ast.lib()];
         let statements = ast.statements();
diff --git a/src/error.rs b/src/error.rs
index 6dd09b70..acdb4499 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -98,8 +98,10 @@ impl Error for EvalAltResult {}
 impl fmt::Display for EvalAltResult {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
-            Self::ErrorSystem(s, err) if s.is_empty() => write!(f, "{}", err)?,
-            Self::ErrorSystem(s, err) => write!(f, "{}: {}", s, err)?,
+            Self::ErrorSystem(s, err) => match s.as_str() {
+                "" => write!(f, "{}", err),
+                s => write!(f, "{}: {}", s, err),
+            }?,
 
             Self::ErrorParsing(p, _) => write!(f, "Syntax error: {}", p)?,
 
@@ -128,76 +130,74 @@ impl fmt::Display for EvalAltResult {
             Self::ErrorDataRace(s, _) => {
                 write!(f, "Data race detected when accessing variable: {}", s)?
             }
-            Self::ErrorDotExpr(s, _) if !s.is_empty() => f.write_str(s)?,
+            Self::ErrorDotExpr(s, _) => match s.as_str() {
+                "" => f.write_str("Malformed dot expression"),
+                s => f.write_str(s),
+            }?,
             Self::ErrorIndexingType(s, _) => write!(f, "Indexer not registered for '{}'", s)?,
             Self::ErrorUnboundThis(_) => f.write_str("'this' is not bound")?,
             Self::ErrorFor(_) => f.write_str("For loop expects a type with an iterator defined")?,
-            Self::ErrorDotExpr(_, _) => f.write_str("Malformed dot expression")?,
             Self::ErrorTooManyOperations(_) => f.write_str("Too many operations")?,
             Self::ErrorTooManyModules(_) => f.write_str("Too many modules imported")?,
             Self::ErrorStackOverflow(_) => f.write_str("Stack overflow")?,
             Self::ErrorTerminated(_, _) => f.write_str("Script terminated")?,
 
-            Self::ErrorRuntime(d, _) if d.is::<ImmutableString>() => {
-                let s = &*d
-                    .read_lock::<ImmutableString>()
-                    .expect("never fails because the type was checked");
-
-                if s.is_empty() {
-                    f.write_str("Runtime error")?
-                } else {
-                    write!(f, "Runtime error: {}", s)?
-                }
-            }
             Self::ErrorRuntime(d, _) if d.is::<()>() => f.write_str("Runtime error")?,
+            Self::ErrorRuntime(d, _)
+                if d.read_lock::<ImmutableString>()
+                    .map_or(false, |v| v.is_empty()) =>
+            {
+                write!(f, "Runtime error")?
+            }
             Self::ErrorRuntime(d, _) => write!(f, "Runtime error: {}", d)?,
 
             Self::ErrorAssignmentToConstant(s, _) => write!(f, "Cannot modify constant {}", s)?,
-            Self::ErrorMismatchOutputType(s, r, _) => {
-                write!(f, "Output type is incorrect: {} (expecting {})", r, s)?
-            }
-            Self::ErrorMismatchDataType(s, r, _) if r.is_empty() => {
-                write!(f, "Data type is incorrect, expecting {}", s)?
-            }
-            Self::ErrorMismatchDataType(s, r, _) if s.is_empty() => {
-                write!(f, "Data type is incorrect: {}", r)?
-            }
-            Self::ErrorMismatchDataType(s, r, _) => {
-                write!(f, "Data type is incorrect: {} (expecting {})", r, s)?
-            }
-            Self::ErrorArithmetic(s, _) => f.write_str(s)?,
+            Self::ErrorMismatchOutputType(s, r, _) => match (r.as_str(), s.as_str()) {
+                ("", s) => write!(f, "Output type is incorrect, expecting {}", s),
+                (r, "") => write!(f, "Output type is incorrect: {}", r),
+                (r, s) => write!(f, "Output type is incorrect: {} (expecting {})", r, s),
+            }?,
+            Self::ErrorMismatchDataType(s, r, _) => match (r.as_str(), s.as_str()) {
+                ("", s) => write!(f, "Data type is incorrect, expecting {}", s),
+                (r, "") => write!(f, "Data type is incorrect: {}", r),
+                (r, s) => write!(f, "Data type is incorrect: {} (expecting {})", r, s),
+            }?,
+            Self::ErrorArithmetic(s, _) => match s.as_str() {
+                "" => f.write_str("Arithmetic error"),
+                s => f.write_str(s),
+            }?,
 
-            Self::LoopBreak(true, _) => f.write_str("Break statement not inside a loop")?,
-            Self::LoopBreak(false, _) => f.write_str("Continue statement not inside a loop")?,
+            Self::LoopBreak(true, _) => f.write_str("'break' not inside a loop")?,
+            Self::LoopBreak(false, _) => f.write_str("'continue' not inside a loop")?,
 
-            Self::Return(_, _) => f.write_str("NOT AN ERROR - Function returns value")?,
+            Self::Return(_, _) => f.write_str("NOT AN ERROR - function returns value")?,
 
-            Self::ErrorArrayBounds(0, index, _) => {
-                write!(f, "Array index {} out of bounds: array is empty", index)?
-            }
-            Self::ErrorArrayBounds(1, index, _) => write!(
-                f,
-                "Array index {} out of bounds: only 1 element in the array",
-                index
-            )?,
-            Self::ErrorArrayBounds(max, index, _) => write!(
-                f,
-                "Array index {} out of bounds: only {} elements in the array",
-                index, max
-            )?,
-            Self::ErrorStringBounds(0, index, _) => {
-                write!(f, "String index {} out of bounds: string is empty", index)?
-            }
-            Self::ErrorStringBounds(1, index, _) => write!(
-                f,
-                "String index {} out of bounds: only 1 character in the string",
-                index
-            )?,
-            Self::ErrorStringBounds(max, index, _) => write!(
-                f,
-                "String index {} out of bounds: only {} characters in the string",
-                index, max
-            )?,
+            Self::ErrorArrayBounds(max, index, _) => match max {
+                0 => write!(f, "Array index {} out of bounds: array is empty", index),
+                1 => write!(
+                    f,
+                    "Array index {} out of bounds: only 1 element in the array",
+                    index
+                ),
+                _ => write!(
+                    f,
+                    "Array index {} out of bounds: only {} elements in the array",
+                    index, max
+                ),
+            }?,
+            Self::ErrorStringBounds(max, index, _) => match max {
+                0 => write!(f, "String index {} out of bounds: string is empty", index),
+                1 => write!(
+                    f,
+                    "String index {} out of bounds: only 1 character in the string",
+                    index
+                ),
+                _ => write!(
+                    f,
+                    "String index {} out of bounds: only {} characters in the string",
+                    index, max
+                ),
+            }?,
             Self::ErrorBitFieldBounds(max, index, _) => write!(
                 f,
                 "Bit-field index {} out of bounds: only {} bits in the bit-field",
diff --git a/src/error_parsing.rs b/src/error_parsing.rs
index 5b797baa..d89ca226 100644
--- a/src/error_parsing.rs
+++ b/src/error_parsing.rs
@@ -125,6 +125,9 @@ pub enum ParseErrorType {
     VariableExpected,
     /// An identifier is a reserved keyword.
     Reserved(String),
+    /// An expression is of the wrong type.
+    /// Wrapped values are the type requested and type of the actual result.
+    MismatchedType(String, String),
     /// Missing an expression. Wrapped value is the expression type.
     ExprExpected(String),
     /// Defining a doc-comment in an appropriate place (e.g. not at global level).
@@ -194,12 +197,22 @@ impl fmt::Display for ParseErrorType {
 
             Self::UnknownOperator(s) => write!(f, "Unknown operator: '{}'", s),
 
-            Self::MalformedCallExpr(s) if s.is_empty() => f.write_str("Invalid expression in function call arguments"),
-            Self::MalformedIndexExpr(s) if s.is_empty() => f.write_str("Invalid index in indexing expression"),
-            Self::MalformedInExpr(s) if s.is_empty() => f.write_str("Invalid 'in' expression"),
-            Self::MalformedCapture(s) if s.is_empty() => f.write_str("Invalid capturing"),
-
-            Self::MalformedCallExpr(s) | Self::MalformedIndexExpr(s) | Self::MalformedInExpr(s) | Self::MalformedCapture(s) => f.write_str(s),
+            Self::MalformedCallExpr(s) => match s.as_str() {
+                "" => f.write_str("Invalid expression in function call arguments"),
+                s => f.write_str(s)
+            },
+            Self::MalformedIndexExpr(s) => match s.as_str() {
+                "" => f.write_str("Invalid index in indexing expression"),
+                s => f.write_str(s)
+            },
+            Self::MalformedInExpr(s) => match s.as_str() {
+                "" => f.write_str("Invalid 'in' expression"),
+                s => f.write_str(s)
+            },
+            Self::MalformedCapture(s) => match s.as_str() {
+                "" => f.write_str("Invalid capturing"),
+                s => f.write_str(s)
+            },
 
             Self::FnDuplicatedDefinition(s, n) => {
                 write!(f, "Function '{}' with ", s)?;
@@ -209,25 +222,30 @@ impl fmt::Display for ParseErrorType {
                     _ => write!(f, "{} parameters already exists", n),
                 }
             }
-
-            Self::FnMissingBody(s) if s.is_empty() => f.write_str("Expecting body statement block for anonymous function"),
-            Self::FnMissingBody(s) => write!(f, "Expecting body statement block for function '{}'", s),
-
+            Self::FnMissingBody(s) => match s.as_str() {
+                "" => f.write_str("Expecting body statement block for anonymous function"),
+                s => write!(f, "Expecting body statement block for function '{}'", s)
+            },
             Self::FnMissingParams(s) => write!(f, "Expecting parameters for function '{}'", s),
             Self::FnDuplicatedParam(s, arg) => write!(f, "Duplicated parameter '{}' for function '{}'", arg, s),
+
             Self::DuplicatedProperty(s) => write!(f, "Duplicated property '{}' for object map literal", s),
             Self::DuplicatedSwitchCase => f.write_str("Duplicated switch case"),
             Self::DuplicatedVariable(s) => write!(f, "Duplicated variable name '{}'", s),
 
+            Self::MismatchedType(r, a) => write!(f, "Expecting {}, not {}", r, a),
             Self::ExprExpected(s) => write!(f, "Expecting {} expression", s),
             Self::MissingToken(token, s) => write!(f, "Expecting '{}' {}", token, s),
             Self::MissingSymbol(s) => f.write_str(s),
 
-            Self::AssignmentToConstant(s) if s.is_empty() => f.write_str("Cannot assign to a constant value"),
-            Self::AssignmentToConstant(s) => write!(f, "Cannot assign to constant '{}'", s),
-
-            Self::AssignmentToInvalidLHS(s) if s.is_empty() => f.write_str("Expression cannot be assigned to"),
-            Self::AssignmentToInvalidLHS(s) => f.write_str(s),
+            Self::AssignmentToConstant(s) => match s.as_str() {
+                "" => f.write_str("Cannot assign to a constant value"),
+                s => write!(f, "Cannot assign to constant '{}'", s)
+            },
+            Self::AssignmentToInvalidLHS(s) => match s.as_str() {
+                "" => f.write_str("Expression cannot be assigned to"),
+                s => f.write_str(s)
+            },
 
             Self::LiteralTooLarge(typ, max) => write!(f, "{} exceeds the maximum limit ({})", typ, max),
             Self::Reserved(s) => write!(f, "'{}' is a reserved keyword", s),
diff --git a/src/fn_call.rs b/src/fn_call.rs
index 36594362..45cc65e6 100644
--- a/src/fn_call.rs
+++ b/src/fn_call.rs
@@ -2,7 +2,7 @@
 
 use crate::ast::FnCallHashes;
 use crate::engine::{
-    FnResolutionCacheEntry, Imports, State, KEYWORD_DEBUG, KEYWORD_EVAL, KEYWORD_FN_PTR,
+    EvalState, FnResolutionCacheEntry, Imports, KEYWORD_DEBUG, KEYWORD_EVAL, KEYWORD_FN_PTR,
     KEYWORD_FN_PTR_CALL, KEYWORD_FN_PTR_CURRY, KEYWORD_IS_DEF_VAR, KEYWORD_PRINT, KEYWORD_TYPE_OF,
     MAX_DYNAMIC_PARAMETERS,
 };
@@ -162,7 +162,7 @@ impl Engine {
     fn resolve_fn<'s>(
         &self,
         mods: &Imports,
-        state: &'s mut State,
+        state: &'s mut EvalState,
         lib: &[&Module],
         fn_name: &str,
         hash_script: u64,
@@ -266,7 +266,7 @@ impl Engine {
     pub(crate) fn call_native_fn(
         &self,
         mods: &Imports,
-        state: &mut State,
+        state: &mut EvalState,
         lib: &[&Module],
         name: &str,
         hash: u64,
@@ -439,7 +439,7 @@ impl Engine {
         &self,
         scope: &mut Scope,
         mods: &mut Imports,
-        state: &mut State,
+        state: &mut EvalState,
         lib: &[&Module],
         this_ptr: &mut Option<&mut Dynamic>,
         fn_def: &crate::ast::ScriptFnDef,
@@ -451,7 +451,7 @@ impl Engine {
         fn make_error(
             name: String,
             fn_def: &crate::ast::ScriptFnDef,
-            state: &State,
+            state: &EvalState,
             err: Box<EvalAltResult>,
             pos: Position,
         ) -> RhaiResult {
@@ -569,7 +569,7 @@ impl Engine {
     pub(crate) fn has_script_fn(
         &self,
         mods: Option<&Imports>,
-        state: &mut State,
+        state: &mut EvalState,
         lib: &[&Module],
         hash_script: u64,
     ) -> bool {
@@ -608,7 +608,7 @@ impl Engine {
     pub(crate) fn exec_fn_call(
         &self,
         mods: &mut Imports,
-        state: &mut State,
+        state: &mut EvalState,
         lib: &[&Module],
         fn_name: &str,
         hashes: FnCallHashes,
@@ -644,7 +644,7 @@ impl Engine {
             crate::engine::KEYWORD_IS_DEF_FN
                 if args.len() == 2 && args[0].is::<FnPtr>() && args[1].is::<crate::INT>() =>
             {
-                let fn_name = &*args[0]
+                let fn_name = args[0]
                     .read_lock::<ImmutableString>()
                     .expect("never fails because `args[0]` is `FnPtr`");
                 let num_params = args[1]
@@ -655,7 +655,7 @@ impl Engine {
                     if num_params < 0 {
                         Dynamic::FALSE
                     } else {
-                        let hash_script = calc_fn_hash(fn_name, num_params as usize);
+                        let hash_script = calc_fn_hash(fn_name.as_str(), num_params as usize);
                         self.has_script_fn(Some(mods), state, lib, hash_script)
                             .into()
                     },
@@ -788,7 +788,7 @@ impl Engine {
         &self,
         scope: &mut Scope,
         mods: &mut Imports,
-        state: &mut State,
+        state: &mut EvalState,
         statements: &[Stmt],
         lib: &[&Module],
         level: usize,
@@ -809,7 +809,7 @@ impl Engine {
         &self,
         scope: &mut Scope,
         mods: &mut Imports,
-        state: &mut State,
+        state: &mut EvalState,
         lib: &[&Module],
         script: &str,
         _pos: Position,
@@ -842,7 +842,7 @@ impl Engine {
         }
 
         // Evaluate the AST
-        let mut new_state: State = Default::default();
+        let mut new_state = EvalState::new();
         new_state.source = state.source.clone();
         new_state.operations = state.operations;
 
@@ -860,7 +860,7 @@ impl Engine {
     pub(crate) fn make_method_call(
         &self,
         mods: &mut Imports,
-        state: &mut State,
+        state: &mut EvalState,
         lib: &[&Module],
         fn_name: &str,
         mut hash: FnCallHashes,
@@ -1024,7 +1024,7 @@ impl Engine {
         &self,
         scope: &mut Scope,
         mods: &mut Imports,
-        state: &mut State,
+        state: &mut EvalState,
         lib: &[&Module],
         this_ptr: &mut Option<&mut Dynamic>,
         level: usize,
@@ -1046,7 +1046,7 @@ impl Engine {
         &self,
         scope: &mut Scope,
         mods: &mut Imports,
-        state: &mut State,
+        state: &mut EvalState,
         lib: &[&Module],
         this_ptr: &mut Option<&mut Dynamic>,
         fn_name: &str,
@@ -1202,7 +1202,7 @@ impl Engine {
                 // IMPORTANT! If the eval defines new variables in the current scope,
                 //            all variable offsets from this point on will be mis-aligned.
                 if scope.len() != prev_len {
-                    state.always_search = true;
+                    state.always_search_scope = true;
                 }
 
                 return result.map_err(|err| {
@@ -1298,7 +1298,7 @@ impl Engine {
         &self,
         scope: &mut Scope,
         mods: &mut Imports,
-        state: &mut State,
+        state: &mut EvalState,
         lib: &[&Module],
         this_ptr: &mut Option<&mut Dynamic>,
         namespace: &NamespaceRef,
diff --git a/src/fn_native.rs b/src/fn_native.rs
index 4f91cec5..82670f89 100644
--- a/src/fn_native.rs
+++ b/src/fn_native.rs
@@ -399,7 +399,7 @@ impl CallableFunction {
     /// Is this an iterator function?
     #[inline(always)]
     #[must_use]
-    pub fn is_iter(&self) -> bool {
+    pub const fn is_iter(&self) -> bool {
         match self {
             Self::Iterator(_) => true,
             Self::Pure(_) | Self::Method(_) | Self::Plugin(_) => false,
@@ -411,7 +411,7 @@ impl CallableFunction {
     /// Is this a Rhai-scripted function?
     #[inline(always)]
     #[must_use]
-    pub fn is_script(&self) -> bool {
+    pub const fn is_script(&self) -> bool {
         match self {
             #[cfg(not(feature = "no_function"))]
             Self::Script(_) => true,
@@ -422,7 +422,7 @@ impl CallableFunction {
     /// Is this a plugin function?
     #[inline(always)]
     #[must_use]
-    pub fn is_plugin_fn(&self) -> bool {
+    pub const fn is_plugin_fn(&self) -> bool {
         match self {
             Self::Plugin(_) => true,
             Self::Pure(_) | Self::Method(_) | Self::Iterator(_) => false,
@@ -434,7 +434,7 @@ impl CallableFunction {
     /// Is this a native Rust function?
     #[inline(always)]
     #[must_use]
-    pub fn is_native(&self) -> bool {
+    pub const fn is_native(&self) -> bool {
         match self {
             Self::Pure(_) | Self::Method(_) => true,
             Self::Plugin(_) => true,
diff --git a/src/immutable_string.rs b/src/immutable_string.rs
index 48c7e387..d6555698 100644
--- a/src/immutable_string.rs
+++ b/src/immutable_string.rs
@@ -522,6 +522,11 @@ impl PartialOrd<ImmutableString> for String {
 }
 
 impl ImmutableString {
+    /// Create a new [`ImmutableString`].
+    #[inline(always)]
+    pub fn new() -> Self {
+        Self(SmartString::new().into())
+    }
     /// Consume the [`ImmutableString`] and convert it into a [`String`].
     /// If there are other references to the same string, a cloned copy is returned.
     #[inline(always)]
diff --git a/src/lib.rs b/src/lib.rs
index 6d1a9aae..6b13fac0 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -233,10 +233,11 @@ pub use ast::{
 
 #[cfg(feature = "internals")]
 #[deprecated = "this type is volatile and may change"]
-pub use engine::{Imports, State as EvalState};
+pub use engine::{EvalState, FnResolutionCache, FnResolutionCacheEntry, Imports};
 
 #[cfg(feature = "internals")]
 #[cfg(not(feature = "unchecked"))]
+#[deprecated = "this type is volatile and may change"]
 pub use engine::Limits;
 
 #[cfg(feature = "internals")]
diff --git a/src/module/mod.rs b/src/module/mod.rs
index 19d674e5..1ff6e4a8 100644
--- a/src/module/mod.rs
+++ b/src/module/mod.rs
@@ -1731,6 +1731,12 @@ impl From<StaticVec<Ident>> for NamespaceRef {
 }
 
 impl NamespaceRef {
+    /// Create a new [`NamespaceRef`].
+    #[inline(always)]
+    #[must_use]
+    pub fn new(&self) -> Self {
+        Default::default()
+    }
     /// Get the [`Scope`][crate::Scope] index offset.
     #[inline(always)]
     #[must_use]
diff --git a/src/optimize.rs b/src/optimize.rs
index 22a67ac8..17468495 100644
--- a/src/optimize.rs
+++ b/src/optimize.rs
@@ -43,7 +43,7 @@ impl Default for OptimizationLevel {
 
 /// Mutable state throughout an optimization pass.
 #[derive(Debug, Clone)]
-struct State<'a> {
+struct OptimizerState<'a> {
     /// Has the [`AST`] been changed during this pass?
     changed: bool,
     /// Collection of constants to use for eager function evaluations.
@@ -58,7 +58,7 @@ struct State<'a> {
     optimization_level: OptimizationLevel,
 }
 
-impl<'a> State<'a> {
+impl<'a> OptimizerState<'a> {
     /// Create a new State.
     #[inline(always)]
     pub const fn new(
@@ -121,7 +121,7 @@ impl<'a> State<'a> {
 }
 
 // Has a system function a Rust-native override?
-fn has_native_fn(state: &State, hash_script: u64, arg_types: &[TypeId]) -> bool {
+fn has_native_fn(state: &OptimizerState, hash_script: u64, arg_types: &[TypeId]) -> bool {
     let hash_params = calc_fn_params_hash(arg_types.iter().cloned());
     let hash = combine_hashes(hash_script, hash_params);
 
@@ -135,7 +135,7 @@ fn has_native_fn(state: &State, hash_script: u64, arg_types: &[TypeId]) -> bool
 
 /// Call a registered function
 fn call_fn_with_constant_arguments(
-    state: &State,
+    state: &OptimizerState,
     fn_name: &str,
     arg_values: &mut [Dynamic],
 ) -> Option<Dynamic> {
@@ -159,7 +159,7 @@ fn call_fn_with_constant_arguments(
 /// Optimize a block of [statements][Stmt].
 fn optimize_stmt_block(
     mut statements: Vec<Stmt>,
-    state: &mut State,
+    state: &mut OptimizerState,
     preserve_result: bool,
     is_internal: bool,
     reduce_return: bool,
@@ -368,7 +368,7 @@ fn optimize_stmt_block(
 }
 
 /// Optimize a [statement][Stmt].
-fn optimize_stmt(stmt: &mut Stmt, state: &mut State, preserve_result: bool) {
+fn optimize_stmt(stmt: &mut Stmt, state: &mut OptimizerState, preserve_result: bool) {
     match stmt {
         // var = var op expr => var op= expr
         Stmt::Assignment(x, _)
@@ -682,7 +682,7 @@ fn optimize_stmt(stmt: &mut Stmt, state: &mut State, preserve_result: bool) {
 }
 
 /// Optimize an [expression][Expr].
-fn optimize_expr(expr: &mut Expr, state: &mut State, _chaining: bool) {
+fn optimize_expr(expr: &mut Expr, state: &mut OptimizerState, _chaining: bool) {
     // These keywords are handled specially
     const DONT_EVAL_KEYWORDS: &[&str] = &[
         KEYWORD_PRINT, // side effects
@@ -1051,7 +1051,7 @@ fn optimize_expr(expr: &mut Expr, state: &mut State, _chaining: bool) {
 
         // Custom syntax
         Expr::Custom(x, _) => {
-            if x.scope_changed {
+            if x.scope_may_be_changed {
                 state.propagate_constants = false;
             }
             x.keywords.iter_mut().for_each(|expr| optimize_expr(expr, state, false));
@@ -1077,7 +1077,7 @@ fn optimize_top_level(
     }
 
     // Set up the state
-    let mut state = State::new(engine, lib, optimization_level);
+    let mut state = OptimizerState::new(engine, lib, optimization_level);
 
     // Add constants and variables from the scope
     scope.iter().for_each(|(name, constant, value)| {
@@ -1142,7 +1142,7 @@ pub fn optimize_into_ast(
                     let mut fn_def = crate::fn_native::shared_take_or_clone(fn_def);
 
                     // Optimize the function body
-                    let state = &mut State::new(engine, lib2, level);
+                    let state = &mut OptimizerState::new(engine, lib2, level);
 
                     let body = mem::take(fn_def.body.statements_mut()).into_vec();
 
diff --git a/src/packages/string_basic.rs b/src/packages/string_basic.rs
index 348e602c..c361863f 100644
--- a/src/packages/string_basic.rs
+++ b/src/packages/string_basic.rs
@@ -173,6 +173,10 @@ mod number_formatting {
     pub fn int_to_octal(value: INT) -> ImmutableString {
         to_octal(value)
     }
+    #[rhai_fn(name = "to_binary")]
+    pub fn int_to_binary(value: INT) -> ImmutableString {
+        to_binary(value)
+    }
 
     #[cfg(not(feature = "only_i32"))]
     #[cfg(not(feature = "only_i64"))]
diff --git a/src/parse.rs b/src/parse.rs
index 33380e50..9d00409f 100644
--- a/src/parse.rs
+++ b/src/parse.rs
@@ -137,7 +137,7 @@ impl<'e> ParseState<'e> {
     ///
     /// If the variable is not present in the scope adds it to the list of external variables
     ///
-    /// The return value is the offset to be deducted from `ParseState::stack::len`,
+    /// The return value is the offset to be deducted from `ParseState::stack::len()`,
     /// i.e. the top element of [`ParseState`]'s variables stack is offset 1.
     ///
     /// Return `None` when the variable name is not found in the `stack`.
@@ -156,7 +156,7 @@ impl<'e> ParseState<'e> {
                     barrier = true;
                     false
                 } else {
-                    *n == name
+                    n == name
                 }
             })
             .and_then(|(i, _)| NonZeroUsize::new(i + 1));
@@ -281,9 +281,73 @@ impl Expr {
             _ => self,
         }
     }
+    /// Raise an error if the expression can never yield a boolean value.
+    fn ensure_bool_expr(self) -> Result<Expr, ParseError> {
+        let type_name = match self {
+            Expr::Unit(_) => "()",
+            Expr::DynamicConstant(ref v, _) if !v.is::<bool>() => v.type_name(),
+            Expr::IntegerConstant(_, _) => "a number",
+            #[cfg(not(feature = "no_float"))]
+            Expr::FloatConstant(_, _) => "a floating-point number",
+            Expr::CharConstant(_, _) => "a character",
+            Expr::StringConstant(_, _) => "a string",
+            Expr::InterpolatedString(_, _) => "a string",
+            Expr::Array(_, _) => "an array",
+            Expr::Map(_, _) => "an object map",
+            _ => return Ok(self),
+        };
+
+        Err(
+            PERR::MismatchedType("a boolean expression".to_string(), type_name.to_string())
+                .into_err(self.position()),
+        )
+    }
+    /// Raise an error if the expression can never yield an iterable value.
+    fn ensure_iterable(self) -> Result<Expr, ParseError> {
+        let type_name = match self {
+            Expr::Unit(_) => "()",
+            Expr::BoolConstant(_, _) => "a boolean",
+            Expr::IntegerConstant(_, _) => "a number",
+            #[cfg(not(feature = "no_float"))]
+            Expr::FloatConstant(_, _) => "a floating-point number",
+            Expr::CharConstant(_, _) => "a character",
+            Expr::StringConstant(_, _) => "a string",
+            Expr::InterpolatedString(_, _) => "a string",
+            Expr::Map(_, _) => "an object map",
+            _ => return Ok(self),
+        };
+
+        Err(
+            PERR::MismatchedType("an iterable value".to_string(), type_name.to_string())
+                .into_err(self.position()),
+        )
+    }
+}
+
+/// Make sure that the next expression is not a statement expression (i.e. wrapped in `{}`).
+#[inline(always)]
+fn ensure_not_statement_expr(input: &mut TokenStream, type_name: &str) -> Result<(), ParseError> {
+    match input.peek().expect(NEVER_ENDS) {
+        (Token::LeftBrace, pos) => Err(PERR::ExprExpected(type_name.to_string()).into_err(*pos)),
+        _ => Ok(()),
+    }
+}
+
+/// Make sure that the next expression is not a mis-typed assignment (i.e. `a = b` instead of `a == b`).
+#[inline(always)]
+fn ensure_not_assignment(input: &mut TokenStream) -> Result<(), ParseError> {
+    match input.peek().expect(NEVER_ENDS) {
+        (Token::Equals, pos) => Err(LexError::ImproperSymbol(
+            "=".to_string(),
+            "Possibly a typo of '=='?".to_string(),
+        )
+        .into_err(*pos)),
+        _ => Ok(()),
+    }
 }
 
 /// Consume a particular [token][Token], checking that it is the expected one.
+#[inline]
 fn eat_token(input: &mut TokenStream, token: Token) -> Position {
     let (t, pos) = input.next().expect(NEVER_ENDS);
 
@@ -299,6 +363,7 @@ fn eat_token(input: &mut TokenStream, token: Token) -> Position {
 }
 
 /// Match a particular [token][Token], consuming it if matched.
+#[inline]
 fn match_token(input: &mut TokenStream, token: Token) -> (bool, Position) {
     let (t, pos) = input.peek().expect(NEVER_ENDS);
     if *t == token {
@@ -1816,8 +1881,8 @@ fn parse_binary_op(
                     .expect("never fails because `||` has two arguments");
                 Expr::Or(
                     BinaryExpr {
-                        lhs: current_lhs,
-                        rhs,
+                        lhs: current_lhs.ensure_bool_expr()?,
+                        rhs: rhs.ensure_bool_expr()?,
                     }
                     .into(),
                     pos,
@@ -1832,8 +1897,8 @@ fn parse_binary_op(
                     .expect("never fails because `&&` has two arguments");
                 Expr::And(
                     BinaryExpr {
-                        lhs: current_lhs,
-                        rhs,
+                        lhs: current_lhs.ensure_bool_expr()?,
+                        rhs: rhs.ensure_bool_expr()?,
                     }
                     .into(),
                     pos,
@@ -1896,10 +1961,9 @@ fn parse_custom_syntax(
     let mut tokens: StaticVec<_> = Default::default();
 
     // Adjust the variables stack
-    if syntax.scope_changed {
-        // Add an empty variable name to the stack.
-        // Empty variable names act as a barrier so earlier variables will not be matched.
-        // Variable searches stop at the first empty variable name.
+    if syntax.scope_may_be_changed {
+        // Add a barrier variable to the stack so earlier variables will not be matched.
+        // Variable searches stop at the first barrier.
         let empty = state.get_identifier(SCOPE_SEARCH_BARRIER_MARKER);
         state.stack.push((empty, AccessMode::ReadWrite));
     }
@@ -2017,12 +2081,15 @@ fn parse_custom_syntax(
     keywords.shrink_to_fit();
     tokens.shrink_to_fit();
 
-    Ok(CustomExpr {
-        keywords,
-        tokens,
-        scope_changed: syntax.scope_changed,
-    }
-    .into_custom_syntax_expr(pos))
+    Ok(Expr::Custom(
+        CustomExpr {
+            keywords,
+            tokens,
+            scope_may_be_changed: syntax.scope_may_be_changed,
+        }
+        .into(),
+        pos,
+    ))
 }
 
 /// Parse an expression.
@@ -2070,46 +2137,6 @@ fn parse_expr(
     )
 }
 
-/// Make sure that the expression is not a statement expression (i.e. wrapped in `{}`).
-fn ensure_not_statement_expr(input: &mut TokenStream, type_name: &str) -> Result<(), ParseError> {
-    match input.peek().expect(NEVER_ENDS) {
-        // Disallow statement expressions
-        (Token::LeftBrace, pos) | (Token::EOF, pos) => {
-            Err(PERR::ExprExpected(type_name.to_string()).into_err(*pos))
-        }
-        // No need to check for others at this time - leave it for the expr parser
-        _ => Ok(()),
-    }
-}
-
-/// Make sure that the expression is not a mis-typed assignment (i.e. `a = b` instead of `a == b`).
-fn ensure_not_assignment(input: &mut TokenStream) -> Result<(), ParseError> {
-    match input.peek().expect(NEVER_ENDS) {
-        (Token::Equals, pos) => Err(LexError::ImproperSymbol(
-            "=".to_string(),
-            "Possibly a typo of '=='?".to_string(),
-        )
-        .into_err(*pos)),
-        (token @ Token::PlusAssign, pos)
-        | (token @ Token::MinusAssign, pos)
-        | (token @ Token::MultiplyAssign, pos)
-        | (token @ Token::DivideAssign, pos)
-        | (token @ Token::LeftShiftAssign, pos)
-        | (token @ Token::RightShiftAssign, pos)
-        | (token @ Token::ModuloAssign, pos)
-        | (token @ Token::PowerOfAssign, pos)
-        | (token @ Token::AndAssign, pos)
-        | (token @ Token::OrAssign, pos)
-        | (token @ Token::XOrAssign, pos) => Err(LexError::ImproperSymbol(
-            token.syntax().to_string(),
-            "Expecting a boolean expression, not an assignment".to_string(),
-        )
-        .into_err(*pos)),
-
-        _ => Ok(()),
-    }
-}
-
 /// Parse an if statement.
 fn parse_if(
     input: &mut TokenStream,
@@ -2125,7 +2152,7 @@ fn parse_if(
 
     // if guard { if_body }
     ensure_not_statement_expr(input, "a boolean")?;
-    let guard = parse_expr(input, state, lib, settings.level_up())?;
+    let guard = parse_expr(input, state, lib, settings.level_up())?.ensure_bool_expr()?;
     ensure_not_assignment(input)?;
     let if_body = parse_block(input, state, lib, settings.level_up())?;
 
@@ -2163,16 +2190,16 @@ fn parse_while_loop(
     let (guard, token_pos) = match input.next().expect(NEVER_ENDS) {
         (Token::While, pos) => {
             ensure_not_statement_expr(input, "a boolean")?;
-            let expr = parse_expr(input, state, lib, settings.level_up())?;
+            let expr = parse_expr(input, state, lib, settings.level_up())?.ensure_bool_expr()?;
+            ensure_not_assignment(input)?;
             (expr, pos)
         }
         (Token::Loop, pos) => (Expr::Unit(Position::NONE), pos),
         _ => unreachable!(),
     };
     settings.pos = token_pos;
-
-    ensure_not_assignment(input)?;
     settings.is_breakable = true;
+
     let body = parse_block(input, state, lib, settings.level_up())?;
 
     Ok(Stmt::While(guard, Box::new(body.into()), settings.pos))
@@ -2206,9 +2233,10 @@ fn parse_do(
         }
     };
 
-    ensure_not_statement_expr(input, "a boolean")?;
     settings.is_breakable = false;
-    let guard = parse_expr(input, state, lib, settings.level_up())?;
+
+    ensure_not_statement_expr(input, "a boolean")?;
+    let guard = parse_expr(input, state, lib, settings.level_up())?.ensure_bool_expr()?;
     ensure_not_assignment(input)?;
 
     Ok(Stmt::Do(
@@ -2279,7 +2307,7 @@ fn parse_for(
 
     // for name in expr { body }
     ensure_not_statement_expr(input, "a boolean")?;
-    let expr = parse_expr(input, state, lib, settings.level_up())?;
+    let expr = parse_expr(input, state, lib, settings.level_up())?.ensure_iterable()?;
 
     let prev_stack_len = state.stack.len();
 
@@ -2758,6 +2786,10 @@ fn parse_stmt(
             match input.peek().expect(NEVER_ENDS) {
                 // `return`/`throw` at <EOF>
                 (Token::EOF, _) => Ok(Stmt::Return(return_type, None, token_pos)),
+                // `return`/`throw` at end of block
+                (Token::RightBrace, _) if !settings.is_global => {
+                    Ok(Stmt::Return(return_type, None, token_pos))
+                }
                 // `return;` or `throw;`
                 (Token::SemiColon, _) => Ok(Stmt::Return(return_type, None, token_pos)),
                 // `return` or `throw` with expression
diff --git a/tests/bool_op.rs b/tests/bool_op.rs
index 18dc4ea6..e6ef023a 100644
--- a/tests/bool_op.rs
+++ b/tests/bool_op.rs
@@ -25,9 +25,9 @@ fn test_bool_op3() -> Result<(), Box<EvalAltResult>> {
     let engine = Engine::new();
 
     assert!(engine.eval::<bool>("true && (false || 123)").is_err());
-    assert_eq!(engine.eval::<bool>("true && (true || 123)")?, true);
+    assert_eq!(engine.eval::<bool>("true && (true || { throw })")?, true);
     assert!(engine.eval::<bool>("123 && (false || true)").is_err());
-    assert_eq!(engine.eval::<bool>("false && (true || 123)")?, false);
+    assert_eq!(engine.eval::<bool>("false && (true || { throw })")?, false);
 
     Ok(())
 }
diff --git a/tests/native.rs b/tests/native.rs
index 2112f6f7..349cebf8 100644
--- a/tests/native.rs
+++ b/tests/native.rs
@@ -30,6 +30,7 @@ fn test_native_context_fn_name() -> Result<(), Box<EvalAltResult>> {
 
     let mut engine = Engine::new();
 
+    #[allow(deprecated)]
     engine
         .register_raw_fn(
             "add_double",
diff --git a/tests/switch.rs b/tests/switch.rs
index 614d0876..b3bbc803 100644
--- a/tests/switch.rs
+++ b/tests/switch.rs
@@ -160,6 +160,7 @@ mod test_switch_enum {
     use super::*;
     use rhai::Array;
     #[derive(Debug, Clone)]
+    #[allow(dead_code)]
     enum MyEnum {
         Foo,
         Bar(INT),