diff --git a/Cargo.toml b/Cargo.toml
index 8d3234c3..cb68a5e0 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -11,7 +11,7 @@ edition = "2018"
 authors = ["Jonathan Turner", "Lukáš Hozda", "Stephen Chung", "jhwgh1968"]
 description = "Embedded scripting for Rust"
 homepage = "https://rhaiscript.github.io/book"
-repository = "https://github.com/rhaiscript/rhai"
+repository = "https://github.com/rhaiscript"
 readme = "README.md"
 license = "MIT OR Apache-2.0"
 include = [
diff --git a/README.md b/README.md
index caf5be6c..82aa7dec 100644
--- a/README.md
+++ b/README.md
@@ -70,15 +70,13 @@ Documentation
 
 See [The Rhai Book](https://rhaiscript.github.io/book) for details on the Rhai scripting engine and language.
 
-To build _The Book_, first install [`mdbook`](https://github.com/rust-lang/mdBook)
-and [`mdbook-tera`](https://github.com/avitex/mdbook-tera) (for templating).
-Running `mdbook build` builds it.
-
 
 Playground
 ----------
 
-An [Online Playground](https://rhaiscript.github.io/playground) is available with syntax-highlighting editor.
+An [Online Playground](https://rhaiscript.github.io/playground) is available with
+syntax-highlighting editor, powered by WebAssembly.
+
 Scripts can be evaluated directly from the editor.
 
 
diff --git a/RELEASES.md b/RELEASES.md
index 582f7273..1240a3cf 100644
--- a/RELEASES.md
+++ b/RELEASES.md
@@ -7,7 +7,9 @@ Version 0.19.10
 Bug fixes
 ---------
 
+* `no_std` feature now compiles correctly (bug introduced in `0.19.9`).
 * Bug in `FileModuleResolver::clear_cache_for_path` path mapping fixed.
+* Some optimizer fringe cases are fixed - related to constants propagation when the evil `eval` is present.
 
 Breaking changes
 ----------------
diff --git a/src/ast.rs b/src/ast.rs
index e374c83f..12bf72ec 100644
--- a/src/ast.rs
+++ b/src/ast.rs
@@ -1058,29 +1058,20 @@ impl Stmt {
     }
 }
 
-/// _(INTERNALS)_ A custom syntax definition.
+/// _(INTERNALS)_ A custom syntax expression.
 /// Exported under the `internals` feature only.
 ///
 /// # WARNING
 ///
 /// This type is volatile and may change.
-#[derive(Clone, Hash)]
+#[derive(Debug, Clone, Hash)]
 pub struct CustomExpr {
     /// List of keywords.
     pub keywords: StaticVec<Expr>,
     /// List of tokens actually parsed.
     pub tokens: Vec<ImmutableString>,
-}
-
-impl fmt::Debug for CustomExpr {
-    #[inline(always)]
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.write_str("CustomExpr { keywords:")?;
-        fmt::Debug::fmt(&self.keywords, f)?;
-        f.write_str(", tokens:")?;
-        fmt::Debug::fmt(&self.tokens, f)?;
-        f.write_str("}")
-    }
+    /// Delta number of variables in the scope.
+    pub scope_delta: isize,
 }
 
 /// _(INTERNALS)_ A binary expression.
diff --git a/src/optimize.rs b/src/optimize.rs
index 729d43e8..2cc27819 100644
--- a/src/optimize.rs
+++ b/src/optimize.rs
@@ -58,6 +58,8 @@ struct State<'a> {
     changed: bool,
     /// Collection of constants to use for eager function evaluations.
     variables: Vec<(String, AccessMode, Expr)>,
+    /// Activate constants propagation?
+    propagate_constants: bool,
     /// An [`Engine`] instance for eager function evaluation.
     engine: &'a Engine,
     /// Collection of sub-modules.
@@ -75,6 +77,7 @@ impl<'a> State<'a> {
         Self {
             changed: false,
             variables: vec![],
+            propagate_constants: true,
             engine,
             mods: (&engine.global_sub_modules).into(),
             lib,
@@ -109,6 +112,10 @@ impl<'a> State<'a> {
     /// Look up a constant from the list.
     #[inline]
     pub fn find_constant(&self, name: &str) -> Option<&Expr> {
+        if !self.propagate_constants {
+            return None;
+        }
+
         for (n, access, expr) in self.variables.iter().rev() {
             if n == name {
                 return if access.is_read_only() {
@@ -160,26 +167,37 @@ fn optimize_stmt_block(
 ) -> 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;
 
     // 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(expr), _, _) if expr.is_constant() => {
-            state.push_var(&var_def.name, AccessMode::ReadOnly, mem::take(expr));
+    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);
+
+                if value_expr.is_constant() {
+                    state.push_var(&var_def.name, AccessMode::ReadOnly, value_expr.clone());
+                }
+            }
+            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_expr(value_expr, state);
+                }
+
+                state.push_var(
+                    &var_def.name,
+                    AccessMode::ReadWrite,
+                    Expr::Unit(var_def.pos),
+                );
+            }
+            // Optimize the statement
+            _ => optimize_stmt(stmt, state, preserve_result),
         }
-        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, _, _, _) => {
-            state.push_var(
-                &var_def.name,
-                AccessMode::ReadWrite,
-                Expr::Unit(var_def.pos),
-            );
-        }
-        // Optimize the statement
-        _ => optimize_stmt(stmt, state, preserve_result),
     });
 
     // Remove all raw expression statements that are pure except for the very last statement
@@ -251,6 +269,8 @@ fn optimize_stmt_block(
     // 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
         [] => {
@@ -637,6 +657,10 @@ fn optimize_expr(expr: &mut Expr, state: &mut State) {
             (lhs, rhs) => { optimize_expr(lhs, state); optimize_expr(rhs, state); }
         },
 
+        // eval!
+        Expr::FnCall(x, _) if x.name == KEYWORD_EVAL => {
+            state.propagate_constants = false;
+        }
         // Do not call some special keywords
         Expr::FnCall(x, _) if DONT_EVAL_KEYWORDS.contains(&x.name.as_ref()) => {
             x.args.iter_mut().for_each(|a| optimize_expr(a, state));
@@ -726,7 +750,12 @@ fn optimize_expr(expr: &mut Expr, state: &mut State) {
         }
 
         // Custom syntax
-        Expr::Custom(x, _) => x.keywords.iter_mut().for_each(|expr| optimize_expr(expr, state)),
+        Expr::Custom(x, _) => {
+            if x.scope_delta != 0 {
+                state.propagate_constants = false;
+            }
+            x.keywords.iter_mut().for_each(|expr| optimize_expr(expr, state));
+        }
 
         // All other expressions - skip
         _ => (),
@@ -790,7 +819,11 @@ fn optimize_top_level(
                 Stmt::Const(var_def, None, _, _) => {
                     state.push_var(&var_def.name, AccessMode::ReadOnly, Expr::Unit(var_def.pos));
                 }
-                Stmt::Let(var_def, _, _, _) => {
+                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,
diff --git a/src/parser.rs b/src/parser.rs
index 4c462f66..fbf17165 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -1995,7 +1995,14 @@ fn parse_custom_syntax(
         }
     }
 
-    Ok(Expr::Custom(Box::new(CustomExpr { keywords, tokens }), pos))
+    Ok(Expr::Custom(
+        Box::new(CustomExpr {
+            keywords,
+            tokens,
+            scope_delta: syntax.scope_delta,
+        }),
+        pos,
+    ))
 }
 
 /// Parse an expression.
diff --git a/src/syntax.rs b/src/syntax.rs
index 65b6513c..78ce64f7 100644
--- a/src/syntax.rs
+++ b/src/syntax.rs
@@ -101,6 +101,14 @@ impl Engine {
     /// * `keywords` holds a slice of strings that define the custom syntax.  
     /// * `new_vars` is the number of new variables declared by this custom syntax, or the number of variables removed (if negative).
     /// * `func` is the implementation function.
+    ///
+    /// # Notes
+    ///
+    /// If `new_vars` is positive, then a number of new variables are expected to be pushed into the
+    /// current [`Scope`][crate::Scope].
+    ///
+    /// If `new_vars` is negative, then it is expected that only the top `new_vars` variables in the
+    /// current [`Scope`][crate::Scope] will be _popped_.  Do not randomly remove variables.
     pub fn register_custom_syntax<S: AsRef<str> + Into<ImmutableString>>(
         &mut self,
         keywords: impl AsRef<[S]>,
diff --git a/tests/optimizer.rs b/tests/optimizer.rs
index 94d77d3c..d062ec5f 100644
--- a/tests/optimizer.rs
+++ b/tests/optimizer.rs
@@ -55,7 +55,7 @@ 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(Unit(0:0)), false, 1:3), Expr(IntegerConstant(123, 1:53))], 1:1)]"#));
+    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)]"#));
 
     let ast = engine.compile("if 1 == 2 { 42 }")?;