Implement annexB Block-Level Function Declarations (#2910)

* Implement annexB Block-Level Function Declarations

* Apply suggestions

boa_ast/src/operations.rs

@@ -9,7 +9,7 @@ use boa_interner::{Interner, Sym};
 use rustc_hash::{FxHashMap, FxHashSet};
 
 use crate::{
-    declaration::{ExportDeclaration, ImportDeclaration, VarDeclaration, Variable},
+    declaration::{Binding, ExportDeclaration, ImportDeclaration, VarDeclaration, Variable},
     expression::{access::SuperPropertyAccess, Await, Identifier, SuperCall, Yield},
     function::{
         ArrowFunction, AsyncArrowFunction, AsyncFunction, AsyncGenerator, Class, ClassElement,
@@ -1663,3 +1663,213 @@ impl<'ast> Visitor<'ast> for TopLevelVarScopedDeclarationsVisitor<'_> {
         }
     }
 }
+
+/// Returns a list function declaration names that are directly contained in a statement lists
+/// `Block`, `CaseClause` or `DefaultClause`.
+/// If the function declaration would cause an early error it is not included in the list.
+///
+/// This behavior is used in the following annexB sections:
+/// * [B.3.2.1 Changes to FunctionDeclarationInstantiation][spec0]
+/// * [B.3.2.2 Changes to GlobalDeclarationInstantiation][spec1]
+/// * [B.3.2.3 Changes to EvalDeclarationInstantiation][spec2]
+///
+/// [spec0]: https://tc39.es/ecma262/#sec-web-compat-functiondeclarationinstantiation
+/// [spec1]: https://tc39.es/ecma262/#sec-web-compat-globaldeclarationinstantiation
+/// [spec2]: https://tc39.es/ecma262/#sec-web-compat-evaldeclarationinstantiation
+#[must_use]
+pub fn annex_b_function_declarations_names<'a, N>(node: &'a N) -> Vec<Identifier>
+where
+    &'a N: Into<NodeRef<'a>>,
+{
+    let mut declarations = Vec::new();
+    AnnexBFunctionDeclarationNamesVisitor(&mut declarations).visit(node.into());
+    declarations
+}
+
+/// The [`Visitor`] used for [`annex_b_function_declarations_names`].
+#[derive(Debug)]
+struct AnnexBFunctionDeclarationNamesVisitor<'a>(&'a mut Vec<Identifier>);
+
+impl<'ast> Visitor<'ast> for AnnexBFunctionDeclarationNamesVisitor<'_> {
+    type BreakTy = Infallible;
+
+    fn visit_statement_list_item(
+        &mut self,
+        node: &'ast StatementListItem,
+    ) -> ControlFlow<Self::BreakTy> {
+        match node {
+            StatementListItem::Statement(node) => self.visit(node),
+            StatementListItem::Declaration(_) => ControlFlow::Continue(()),
+        }
+    }
+
+    fn visit_statement(&mut self, node: &'ast Statement) -> ControlFlow<Self::BreakTy> {
+        match node {
+            Statement::Block(node) => self.visit(node),
+            Statement::If(node) => self.visit(node),
+            Statement::DoWhileLoop(node) => self.visit(node),
+            Statement::WhileLoop(node) => self.visit(node),
+            Statement::ForLoop(node) => self.visit(node),
+            Statement::ForInLoop(node) => self.visit(node),
+            Statement::ForOfLoop(node) => self.visit(node),
+            Statement::Switch(node) => self.visit(node),
+            Statement::Labelled(node) => self.visit(node),
+            Statement::Try(node) => self.visit(node),
+            Statement::With(node) => self.visit(node),
+            _ => ControlFlow::Continue(()),
+        }
+    }
+
+    fn visit_block(&mut self, node: &'ast crate::statement::Block) -> ControlFlow<Self::BreakTy> {
+        self.visit(node.statement_list());
+        for statement in node.statement_list().statements() {
+            if let StatementListItem::Declaration(Declaration::Function(function)) = statement {
+                let name = function
+                    .name()
+                    .expect("function declaration must have name");
+                self.0.push(name);
+            }
+        }
+
+        let lexically_declared_names = lexically_declared_names_legacy(node.statement_list());
+
+        self.0
+            .retain(|name| !lexically_declared_names.contains(&(*name, false)));
+
+        ControlFlow::Continue(())
+    }
+
+    fn visit_switch(&mut self, node: &'ast crate::statement::Switch) -> ControlFlow<Self::BreakTy> {
+        for case in node.cases() {
+            self.visit(case);
+            for statement in case.body().statements() {
+                if let StatementListItem::Declaration(Declaration::Function(function)) = statement {
+                    let name = function
+                        .name()
+                        .expect("function declaration must have name");
+                    self.0.push(name);
+                }
+            }
+        }
+        if let Some(default) = node.default() {
+            self.visit(default);
+            for statement in default.statements() {
+                if let StatementListItem::Declaration(Declaration::Function(function)) = statement {
+                    let name = function
+                        .name()
+                        .expect("function declaration must have name");
+                    self.0.push(name);
+                }
+            }
+        }
+
+        let lexically_declared_names = lexically_declared_names_legacy(node);
+
+        self.0
+            .retain(|name| !lexically_declared_names.contains(&(*name, false)));
+
+        ControlFlow::Continue(())
+    }
+
+    fn visit_try(&mut self, node: &'ast crate::statement::Try) -> ControlFlow<Self::BreakTy> {
+        self.visit(node.block());
+        if let Some(catch) = node.catch() {
+            self.visit(catch.block());
+
+            if let Some(Binding::Pattern(pattern)) = catch.parameter() {
+                let bound_names = bound_names(pattern);
+
+                self.0.retain(|name| !bound_names.contains(name));
+            }
+        }
+        if let Some(finally) = node.finally() {
+            self.visit(finally.block());
+        }
+        ControlFlow::Continue(())
+    }
+
+    fn visit_if(&mut self, node: &'ast crate::statement::If) -> ControlFlow<Self::BreakTy> {
+        if let Some(node) = node.else_node() {
+            self.visit(node);
+        }
+        self.visit(node.body())
+    }
+
+    fn visit_do_while_loop(
+        &mut self,
+        node: &'ast crate::statement::DoWhileLoop,
+    ) -> ControlFlow<Self::BreakTy> {
+        self.visit(node.body())
+    }
+
+    fn visit_while_loop(
+        &mut self,
+        node: &'ast crate::statement::WhileLoop,
+    ) -> ControlFlow<Self::BreakTy> {
+        self.visit(node.body())
+    }
+
+    fn visit_for_loop(
+        &mut self,
+        node: &'ast crate::statement::ForLoop,
+    ) -> ControlFlow<Self::BreakTy> {
+        self.visit(node.body());
+
+        if let Some(ForLoopInitializer::Lexical(node)) = node.init() {
+            let bound_names = bound_names(node);
+            self.0.retain(|name| !bound_names.contains(name));
+        }
+
+        ControlFlow::Continue(())
+    }
+
+    fn visit_for_in_loop(
+        &mut self,
+        node: &'ast crate::statement::ForInLoop,
+    ) -> ControlFlow<Self::BreakTy> {
+        self.visit(node.body());
+
+        if let IterableLoopInitializer::Let(node) = node.initializer() {
+            let bound_names = bound_names(node);
+            self.0.retain(|name| !bound_names.contains(name));
+        }
+        if let IterableLoopInitializer::Const(node) = node.initializer() {
+            let bound_names = bound_names(node);
+            self.0.retain(|name| !bound_names.contains(name));
+        }
+
+        ControlFlow::Continue(())
+    }
+
+    fn visit_for_of_loop(
+        &mut self,
+        node: &'ast crate::statement::ForOfLoop,
+    ) -> ControlFlow<Self::BreakTy> {
+        self.visit(node.body());
+
+        if let IterableLoopInitializer::Let(node) = node.initializer() {
+            let bound_names = bound_names(node);
+            self.0.retain(|name| !bound_names.contains(name));
+        }
+        if let IterableLoopInitializer::Const(node) = node.initializer() {
+            let bound_names = bound_names(node);
+            self.0.retain(|name| !bound_names.contains(name));
+        }
+
+        ControlFlow::Continue(())
+    }
+
+    fn visit_labelled(
+        &mut self,
+        node: &'ast crate::statement::Labelled,
+    ) -> ControlFlow<Self::BreakTy> {
+        if let LabelledItem::Statement(node) = node.item() {
+            self.visit(node);
+        }
+        ControlFlow::Continue(())
+    }
+
+    fn visit_with(&mut self, node: &'ast crate::statement::With) -> ControlFlow<Self::BreakTy> {
+        self.visit(node.statement())
+    }
+}

boa_engine/src/builtins/eval/mod.rs

@@ -234,7 +234,7 @@ impl Eval {
         let push_env = compiler.emit_opcode_with_two_operands(Opcode::PushDeclarativeEnvironment);
 
         compiler.eval_declaration_instantiation(&body, strict)?;
-        compiler.compile_statement_list(&body, true);
+        compiler.compile_statement_list(&body, true, false);
 
         let env_info = compiler.pop_compile_environment();
         compiler.patch_jump_with_target(push_env.0, env_info.num_bindings as u32);

boa_engine/src/builtins/json/mod.rs

@@ -123,7 +123,7 @@ impl Json {
                 context.realm().environment().compile_env(),
                 context,
             );
-            compiler.compile_statement_list(&statement_list, true);
+            compiler.compile_statement_list(&statement_list, true, false);
             Gc::new(compiler.finish())
         };
         let unfiltered = context.execute(code_block)?;

boa_engine/src/bytecompiler/class.rs

@@ -47,7 +47,7 @@ impl ByteCompiler<'_, '_> {
                 false,
             );
 
-            compiler.compile_statement_list(expr.body(), false);
+            compiler.compile_statement_list(expr.body(), false, false);
 
             let env_info = compiler.pop_compile_environment();
 
@@ -374,7 +374,7 @@ impl ByteCompiler<'_, '_> {
                         false,
                     );
 
-                    compiler.compile_statement_list(statement_list, false);
+                    compiler.compile_statement_list(statement_list, false, false);
                     let env_info = compiler.pop_compile_environment();
                     compiler.pop_compile_environment();
                     compiler.num_bindings = env_info.num_bindings;

boa_engine/src/bytecompiler/declarations.rs

@@ -15,6 +15,9 @@ use boa_ast::{
 };
 use boa_interner::Sym;
 
+#[cfg(feature = "annex-b")]
+use boa_ast::operations::annex_b_function_declarations_names;
+
 impl ByteCompiler<'_, '_> {
     /// `GlobalDeclarationInstantiation ( script, env )`
     ///
@@ -151,8 +154,55 @@ impl ByteCompiler<'_, '_> {
         // 11. NOTE: No abnormal terminations occur after this algorithm step if the global object is an ordinary object.
         //     However, if the global object is a Proxy exotic object it may exhibit behaviours
         //     that cause abnormal terminations in some of the following steps.
+
         // 12. NOTE: Annex B.3.2.2 adds additional steps at this point.
-        // TODO: Support B.3.2.2.
+        // 12. Perform the following steps:
+        // a. Let strict be IsStrict of script.
+        // b. If strict is false, then
+        #[cfg(feature = "annex-b")]
+        if !script.strict() {
+            let lex_names = top_level_lexically_declared_names(script);
+
+            // i. Let declaredFunctionOrVarNames be the list-concatenation of declaredFunctionNames and declaredVarNames.
+            // ii. For each FunctionDeclaration f that is directly contained in the StatementList of a Block, CaseClause,
+            //     or DefaultClause Contained within script, do
+            for f in annex_b_function_declarations_names(script) {
+                // 1. Let F be StringValue of the BindingIdentifier of f.
+                // 2. If replacing the FunctionDeclaration f with a VariableStatement that has F as a BindingIdentifier
+                //    would not produce any Early Errors for script, then
+                if !lex_names.contains(&f) {
+                    // a. If env.HasLexicalDeclaration(F) is false, then
+                    if !self.current_environment.borrow().has_lex_binding(f) {
+                        // i. Let fnDefinable be ? env.CanDeclareGlobalVar(F).
+                        let fn_definable = self.context.can_declare_global_function(f)?;
+
+                        // ii. If fnDefinable is true, then
+                        if fn_definable {
+                            // i. NOTE: A var binding for F is only instantiated here if it is neither
+                            //          a VarDeclaredName nor the name of another FunctionDeclaration.
+                            // ii. If declaredFunctionOrVarNames does not contain F, then
+                            if !declared_function_names.contains(&f)
+                                && !declared_var_names.contains(&f)
+                            {
+                                // i. Perform ? env.CreateGlobalVarBinding(F, false).
+                                self.context.create_global_var_binding(f, false)?;
+
+                                // ii. Append F to declaredFunctionOrVarNames.
+                                declared_function_names.push(f);
+                            }
+                            // iii. When the FunctionDeclaration f is evaluated, perform the following
+                            //      steps in place of the FunctionDeclaration Evaluation algorithm provided in 15.2.6:
+                            //     i. Let genv be the running execution context's VariableEnvironment.
+                            //     ii. Let benv be the running execution context's LexicalEnvironment.
+                            //     iii. Let fobj be ! benv.GetBindingValue(F, false).
+                            //     iv. Perform ? genv.SetMutableBinding(F, fobj, false).
+                            //     v. Return unused.
+                            self.annex_b_function_names.push(f);
+                        }
+                    }
+                }
+            }
+        }
 
         // 13. Let lexDeclarations be the LexicallyScopedDeclarations of script.
         // 14. Let privateEnv be null.
@@ -253,6 +303,8 @@ impl ByteCompiler<'_, '_> {
     where
         &'a N: Into<NodeRef<'a>>,
     {
+        let mut env = self.current_environment.borrow_mut();
+
         // 1. Let declarations be the LexicallyScopedDeclarations of code.
         let declarations = lexically_scoped_declarations(block);
 
@@ -266,19 +318,28 @@ impl ByteCompiler<'_, '_> {
                 // a. For each element dn of the BoundNames of d, do
                 for dn in bound_names::<'_, VariableList>(d) {
                     // 1. Perform ! env.CreateImmutableBinding(dn, true).
-                    self.create_immutable_binding(dn, true);
+                    env.create_immutable_binding(dn, true);
                 }
             }
             // ii. Else,
             else {
                 // a. For each element dn of the BoundNames of d, do
                 for dn in bound_names::<'_, Declaration>(d) {
+                    #[cfg(not(feature = "annex-b"))]
                     // 1. Perform ! env.CreateMutableBinding(dn, false). NOTE: This step is replaced in section B.3.2.6.
-                    // TODO: Support B.3.2.6.
+                    env.create_mutable_binding(dn, false);
-                    self.create_mutable_binding(dn, false);
+
+                    #[cfg(feature = "annex-b")]
+                    // 1. If ! env.HasBinding(dn) is false, then
+                    if !env.has_binding(dn) {
+                        // a. Perform  ! env.CreateMutableBinding(dn, false).
+                        env.create_mutable_binding(dn, false);
                     }
                 }
             }
+        }
+
+        drop(env);
 
         // Note: Not sure if the spec is wrong here or if our implementation just differs too much,
         //       but we need 3.a to be finished for all declarations before 3.b can be done.
@@ -343,7 +404,8 @@ impl ByteCompiler<'_, '_> {
             // c. Assert: The following loop will terminate.
             // d. Repeat, while thisEnv is not varEnv,
             //     i. If thisEnv is not an Object Environment Record, then
-            //         1. NOTE: The environment of with statements cannot contain any lexical declaration so it doesn't need to be checked for var/let hoisting conflicts.
+            //         1. NOTE: The environment of with statements cannot contain any lexical
+            //            declaration so it doesn't need to be checked for var/let hoisting conflicts.
             //         2. For each element name of varNames, do
             //             a. If ! thisEnv.HasBinding(name) is true, then
             //                 i. Throw a SyntaxError exception.
@@ -420,7 +482,93 @@ impl ByteCompiler<'_, '_> {
         functions_to_initialize.reverse();
 
         // 11. NOTE: Annex B.3.2.3 adds additional steps at this point.
-        // TODO: Support B.3.2.3
+        // 11. If strict is false, then
+        #[cfg(feature = "annex-b")]
+        if !strict {
+            let lexically_declared_names = top_level_lexically_declared_names(body);
+
+            // a. Let declaredFunctionOrVarNames be the list-concatenation of declaredFunctionNames and declaredVarNames.
+            // b. For each FunctionDeclaration f that is directly contained in the StatementList
+            //    of a Block, CaseClause, or DefaultClause Contained within body, do
+            for f in annex_b_function_declarations_names(body) {
+                // i. Let F be StringValue of the BindingIdentifier of f.
+                // ii. If replacing the FunctionDeclaration f with a VariableStatement that has F
+                //     as a BindingIdentifier would not produce any Early Errors for body, then
+                if !lexically_declared_names.contains(&f) {
+                    // 1. Let bindingExists be false.
+                    // 2. Let thisEnv be lexEnv.
+                    // 3. Assert: The following loop will terminate.
+                    // 4. Repeat, while thisEnv is not varEnv,
+                    // a. If thisEnv is not an Object Environment Record, then
+                    // i. If ! thisEnv.HasBinding(F) is true, then
+                    // i. Let bindingExists be true.
+                    // b. Set thisEnv to thisEnv.[[OuterEnv]].
+                    let binding_exists = self.has_binding_until_var(f);
+
+                    // 5. If bindingExists is false and varEnv is a Global Environment Record, then
+                    let fn_definable = if !binding_exists && var_environment_is_global {
+                        // a. If varEnv.HasLexicalDeclaration(F) is false, then
+                        // b. Else,
+                        if self.current_environment.borrow().has_lex_binding(f) {
+                            // i. Let fnDefinable be false.
+                            false
+                        } else {
+                            // i. Let fnDefinable be ? varEnv.CanDeclareGlobalVar(F).
+                            self.context.can_declare_global_var(f)?
+                        }
+                    }
+                    // 6. Else,
+                    else {
+                        // a. Let fnDefinable be true.
+                        true
+                    };
+
+                    // 7. If bindingExists is false and fnDefinable is true, then
+                    if !binding_exists && fn_definable {
+                        let mut function_names = Vec::new();
+
+                        // a. If declaredFunctionOrVarNames does not contain F, then
+                        if !declared_function_names.contains(&f)
+                            //&& !var_names.contains(&f)
+                            && !function_names.contains(&f)
+                        {
+                            // i. If varEnv is a Global Environment Record, then
+                            if var_environment_is_global {
+                                // i. Perform ? varEnv.CreateGlobalVarBinding(F, true).
+                                self.context.create_global_var_binding(f, true)?;
+                            }
+                            // ii. Else,
+                            else {
+                                // i. Let bindingExists be ! varEnv.HasBinding(F).
+                                // ii. If bindingExists is false, then
+                                if !self.has_binding(f) {
+                                    // i. Perform ! varEnv.CreateMutableBinding(F, true).
+                                    self.create_mutable_binding(f, true);
+
+                                    // ii. Perform ! varEnv.InitializeBinding(F, undefined).
+                                    let binding = self.initialize_mutable_binding(f, true);
+                                    let index = self.get_or_insert_binding(binding);
+                                    self.emit_opcode(Opcode::PushUndefined);
+                                    self.emit(Opcode::DefInitVar, &[index]);
+                                }
+                            }
+
+                            // iii. Append F to declaredFunctionOrVarNames.
+                            function_names.push(f);
+                        }
+
+                        // b. When the FunctionDeclaration f is evaluated, perform the following steps
+                        //    in place of the FunctionDeclaration Evaluation algorithm provided in 15.2.6:
+                        //     i. Let genv be the running execution context's VariableEnvironment.
+                        //     ii. Let benv be the running execution context's LexicalEnvironment.
+                        //     iii. Let fobj be ! benv.GetBindingValue(F, false).
+                        //     iv. Perform ? genv.SetMutableBinding(F, fobj, false).
+                        //     v. Return unused.
+                        self.annex_b_function_names.push(f);
+                    }
+                }
+            }
+        }
 
         // 12. Let declaredVarNames be a new empty List.
         let mut declared_var_names = Vec::new();
@@ -752,13 +900,16 @@ impl ByteCompiler<'_, '_> {
             // a. If strict is true or simpleParameterList is false, then
             //     i. Let ao be CreateUnmappedArgumentsObject(argumentsList).
             // b. Else,
-            //     i. NOTE: A mapped argument object is only provided for non-strict functions that don't have a rest parameter, any parameter default value initializers, or any destructured parameters.
+            //     i. NOTE: A mapped argument object is only provided for non-strict functions
+            //              that don't have a rest parameter, any parameter
+            //              default value initializers, or any destructured parameters.
             //     ii. Let ao be CreateMappedArgumentsObject(func, formals, argumentsList, env).
 
             // c. If strict is true, then
             if strict {
                 // i. Perform ! env.CreateImmutableBinding("arguments", false).
-                // ii. NOTE: In strict mode code early errors prevent attempting to assign to this binding, so its mutability is not observable.
+                // ii. NOTE: In strict mode code early errors prevent attempting to assign
+                //           to this binding, so its mutability is not observable.
                 self.create_immutable_binding(arguments, false);
                 self.arguments_binding = Some(self.initialize_immutable_binding(arguments));
             }
@@ -777,7 +928,7 @@ impl ByteCompiler<'_, '_> {
         }
         // 23. Else,
         //     a. Let parameterBindings be parameterNames.
-        let parameter_bindings = parameter_names;
+        let parameter_bindings = parameter_names.clone();
 
         // 24. Let iteratorRecord be CreateListIteratorRecord(argumentsList).
         // 25. If hasDuplicates is true, then
@@ -821,8 +972,11 @@ impl ByteCompiler<'_, '_> {
 
         // 27. If hasParameterExpressions is false, then
         // 28. Else,
-        if has_parameter_expressions {
+        #[allow(unused_variables, unused_mut)]
-            // a. NOTE: A separate Environment Record is needed to ensure that closures created by expressions in the formal parameter list do not have visibility of declarations in the function body.
+        let mut instantiated_var_names = if has_parameter_expressions {
+            // a. NOTE: A separate Environment Record is needed to ensure that closures created by
+            //          expressions in the formal parameter list do not have
+            //          visibility of declarations in the function body.
             // b. Let varEnv be NewDeclarativeEnvironment(env).
             // c. Set the VariableEnvironment of calleeContext to varEnv.
             self.push_compile_environment(true);
@@ -865,6 +1019,8 @@ impl ByteCompiler<'_, '_> {
                     //          the same value as the corresponding initialized parameter.
                 }
             }
+
+            instantiated_var_names
         } else {
             // a. NOTE: Only a single Environment Record is needed for the parameters and top-level vars.
             // b. Let instantiatedVarNames be a copy of the List parameterBindings.
@@ -889,21 +1045,67 @@ impl ByteCompiler<'_, '_> {
             }
 
             // d. Let varEnv be env.
+
+            instantiated_var_names
         };
 
         // 29. NOTE: Annex B.3.2.1 adds additional steps at this point.
-        // TODO: Support B.3.2.1
-
         // 29. If strict is false, then
-        //      a. Let lexEnv be NewDeclarativeEnvironment(varEnv).
+        #[cfg(feature = "annex-b")]
-        //      b. NOTE: Non-strict functions use a separate Environment Record for top-level lexical declarations so that a direct eval can determine whether any var scoped declarations introduced by the eval code conflict with pre-existing top-level lexically scoped declarations. This is not needed for strict functions because a strict direct eval always places all declarations into a new Environment Record.
+        if !strict {
+            // a. For each FunctionDeclaration f that is directly contained in the StatementList
+            //    of a Block, CaseClause, or DefaultClause, do
+            for f in annex_b_function_declarations_names(code) {
+                // i. Let F be StringValue of the BindingIdentifier of f.
+                // ii. If replacing the FunctionDeclaration f with a VariableStatement that has F
+                //     as a BindingIdentifier would not produce any Early Errors
+                //     for func and parameterNames does not contain F, then
+                if !lexical_names.contains(&f) && !parameter_names.contains(&f) {
+                    // 1. NOTE: A var binding for F is only instantiated here if it is neither a
+                    //    VarDeclaredName, the name of a formal parameter, or another FunctionDeclaration.
+
+                    // 2. If initializedBindings does not contain F and F is not "arguments", then
+                    if !instantiated_var_names.contains(&f) && f != arguments {
+                        // a. Perform ! varEnv.CreateMutableBinding(F, false).
+                        self.create_mutable_binding(f, true);
+
+                        // b. Perform ! varEnv.InitializeBinding(F, undefined).
+                        let binding = self.initialize_mutable_binding(f, true);
+                        let index = self.get_or_insert_binding(binding);
+                        self.emit_opcode(Opcode::PushUndefined);
+                        self.emit(Opcode::DefInitVar, &[index]);
+
+                        // c. Append F to instantiatedVarNames.
+                        instantiated_var_names.push(f);
+                    }
+
+                    // 3. When the FunctionDeclaration f is evaluated, perform the following steps
+                    //    in place of the FunctionDeclaration Evaluation algorithm provided in 15.2.6:
+                    //     a. Let fenv be the running execution context's VariableEnvironment.
+                    //     b. Let benv be the running execution context's LexicalEnvironment.
+                    //     c. Let fobj be ! benv.GetBindingValue(F, false).
+                    //     d. Perform ! fenv.SetMutableBinding(F, fobj, false).
+                    //     e. Return unused.
+                    self.annex_b_function_names.push(f);
+                }
+            }
+        }
+
+        // 30. If strict is false, then
+        // 30.a. Let lexEnv be NewDeclarativeEnvironment(varEnv).
+        // 30.b. NOTE: Non-strict functions use a separate Environment Record for top-level lexical
+        //      declarations so that a direct eval can determine whether any var scoped declarations
+        //      introduced by the eval code conflict with pre-existing top-level lexically scoped declarations.
+        //      This is not needed for strict functions because a strict direct eval always
+        //      places all declarations into a new Environment Record.
         // 31. Else,
         //     a. Let lexEnv be varEnv.
         // 32. Set the LexicalEnvironment of calleeContext to lexEnv.
 
         // 33. Let lexDeclarations be the LexicallyScopedDeclarations of code.
         // 34. For each element d of lexDeclarations, do
-        //     a. NOTE: A lexically declared name cannot be the same as a function/generator declaration, formal parameter, or a var name. Lexically declared names are only instantiated here but not initialized.
+        //     a. NOTE: A lexically declared name cannot be the same as a function/generator declaration,
+        //        formal parameter, or a var name. Lexically declared names are only instantiated here but not initialized.
         //     b. For each element dn of the BoundNames of d, do
         //         i. If IsConstantDeclaration of d is true, then
         //             1. Perform ! lexEnv.CreateImmutableBinding(dn, true).

boa_engine/src/bytecompiler/env.rs

@@ -66,6 +66,15 @@ impl ByteCompiler<'_, '_> {
             .has_binding_eval(name, strict)
     }
 
+    #[cfg(feature = "annex-b")]
+    /// Check if a binding name exists in a environment.
+    /// Stop when a function scope is reached.
+    pub(crate) fn has_binding_until_var(&self, name: Identifier) -> bool {
+        self.current_environment
+            .borrow()
+            .has_binding_until_var(name)
+    }
+
     /// Create a mutable binding at bytecode compile time.
     /// This function returns a syntax error, if the binding is a redeclaration.
     ///
@@ -119,4 +128,12 @@ impl ByteCompiler<'_, '_> {
             .borrow()
             .set_mutable_binding_recursive(name)
     }
+
+    #[cfg(feature = "annex-b")]
+    /// Return the binding locator for a set operation on an existing var binding.
+    pub(crate) fn set_mutable_binding_var(&self, name: Identifier) -> BindingLocator {
+        self.current_environment
+            .borrow()
+            .set_mutable_binding_var_recursive(name)
+    }
 }

boa_engine/src/bytecompiler/function.rs

@@ -125,7 +125,7 @@ impl FunctionCompiler {
             self.generator,
         );
 
-        compiler.compile_statement_list(body, false);
+        compiler.compile_statement_list(body, false, false);
 
         if let Some(env_labels) = env_labels {
             let env_info = compiler.pop_compile_environment();

boa_engine/src/bytecompiler/mod.rs

@@ -290,6 +290,9 @@ pub struct ByteCompiler<'ctx, 'host> {
 
     // TODO: remove when we separate scripts from the context
     context: &'ctx mut Context<'host>,
+
+    #[cfg(feature = "annex-b")]
+    annex_b_function_names: Vec<Identifier>,
 }
 
 impl<'ctx, 'host> ByteCompiler<'ctx, 'host> {
@@ -337,6 +340,9 @@ impl<'ctx, 'host> ByteCompiler<'ctx, 'host> {
             json_parse,
             current_environment,
             context,
+
+            #[cfg(feature = "annex-b")]
+            annex_b_function_names: Vec::new(),
         }
     }
 
@@ -403,11 +409,6 @@ impl<'ctx, 'host> ByteCompiler<'ctx, 'host> {
                 let index = self.get_or_insert_binding(binding);
                 self.emit(Opcode::DefVar, &[index]);
             }
-            BindingOpcode::Let => {
-                let binding = self.initialize_mutable_binding(name, false);
-                let index = self.get_or_insert_binding(binding);
-                self.emit(Opcode::DefLet, &[index]);
-            }
             BindingOpcode::InitVar => {
                 let binding = if self.has_binding(name) {
                     self.set_mutable_binding(name)
@@ -420,12 +421,12 @@ impl<'ctx, 'host> ByteCompiler<'ctx, 'host> {
             BindingOpcode::InitLet => {
                 let binding = self.initialize_mutable_binding(name, false);
                 let index = self.get_or_insert_binding(binding);
-                self.emit(Opcode::DefInitLet, &[index]);
+                self.emit(Opcode::PutLexicalValue, &[index]);
             }
             BindingOpcode::InitConst => {
                 let binding = self.initialize_immutable_binding(name);
                 let index = self.get_or_insert_binding(binding);
-                self.emit(Opcode::DefInitConst, &[index]);
+                self.emit(Opcode::PutLexicalValue, &[index]);
             }
             BindingOpcode::SetName => {
                 let binding = self.set_mutable_binding(name);
@@ -742,7 +743,7 @@ impl<'ctx, 'host> ByteCompiler<'ctx, 'host> {
     }
 
     /// Compile a [`StatementList`].
-    pub fn compile_statement_list(&mut self, list: &StatementList, use_expr: bool) {
+    pub fn compile_statement_list(&mut self, list: &StatementList, use_expr: bool, block: bool) {
         if use_expr {
             let expr_index = list
                 .statements()
@@ -758,11 +759,11 @@ impl<'ctx, 'host> ByteCompiler<'ctx, 'host> {
                 .count();
 
             for (i, item) in list.statements().iter().enumerate() {
-                self.compile_stmt_list_item(item, i + 1 == expr_index);
+                self.compile_stmt_list_item(item, i + 1 == expr_index, block);
             }
         } else {
             for item in list.statements() {
-                self.compile_stmt_list_item(item, false);
+                self.compile_stmt_list_item(item, false, block);
             }
         }
     }
@@ -980,10 +981,10 @@ impl<'ctx, 'host> ByteCompiler<'ctx, 'host> {
                             let ident = ident;
                             if let Some(expr) = variable.init() {
                                 self.compile_expr(expr, true);
-                                self.emit_binding(BindingOpcode::InitLet, *ident);
                             } else {
-                                self.emit_binding(BindingOpcode::Let, *ident);
+                                self.emit_opcode(Opcode::PushUndefined);
                             }
+                            self.emit_binding(BindingOpcode::InitLet, *ident);
                         }
                         Binding::Pattern(pattern) => {
                             if let Some(init) = variable.init() {
@@ -1023,18 +1024,33 @@ impl<'ctx, 'host> ByteCompiler<'ctx, 'host> {
     }
 
     /// Compile a [`StatementListItem`].
-    fn compile_stmt_list_item(&mut self, item: &StatementListItem, use_expr: bool) {
+    fn compile_stmt_list_item(&mut self, item: &StatementListItem, use_expr: bool, block: bool) {
         match item {
             StatementListItem::Statement(stmt) => {
                 self.compile_stmt(stmt, use_expr);
             }
-            StatementListItem::Declaration(decl) => self.compile_decl(decl),
+            StatementListItem::Declaration(decl) => self.compile_decl(decl, block),
         }
     }
 
     /// Compile a [`Declaration`].
-    pub fn compile_decl(&mut self, decl: &Declaration) {
+    #[allow(unused_variables)]
+    pub fn compile_decl(&mut self, decl: &Declaration, block: bool) {
         match decl {
+            #[cfg(feature = "annex-b")]
+            Declaration::Function(function) if block => {
+                let name = function
+                    .name()
+                    .expect("function declaration must have name");
+                if self.annex_b_function_names.contains(&name) {
+                    let binding = self.get_binding_value(name);
+                    let index = self.get_or_insert_binding(binding);
+                    self.emit(Opcode::GetName, &[index]);
+                    let binding = self.set_mutable_binding_var(name);
+                    let index = self.get_or_insert_binding(binding);
+                    self.emit(Opcode::SetName, &[index]);
+                }
+            }
             Declaration::Class(class) => self.class(class, false),
             Declaration::Lexical(lexical) => self.compile_lexical_decl(lexical),
             _ => {}

boa_engine/src/bytecompiler/module.rs

@@ -18,7 +18,7 @@ impl ByteCompiler<'_, '_> {
     pub fn compile_module_item(&mut self, item: &ModuleItem) {
         match item {
             ModuleItem::StatementListItem(stmt) => {
-                self.compile_stmt_list_item(stmt, false);
+                self.compile_stmt_list_item(stmt, false, false);
             }
             _ => {
                 // TODO: Remove after implementing modules.

boa_engine/src/bytecompiler/statement/block.rs

@@ -8,7 +8,7 @@ impl ByteCompiler<'_, '_> {
         let push_env = self.emit_opcode_with_two_operands(Opcode::PushDeclarativeEnvironment);
 
         self.block_declaration_instantiation(block);
-        self.compile_statement_list(block.statement_list(), use_expr);
+        self.compile_statement_list(block.statement_list(), use_expr, true);
 
         let env_info = self.pop_compile_environment();
         self.patch_jump_with_target(push_env.0, env_info.num_bindings as u32);

boa_engine/src/bytecompiler/statement/switch.rs

@@ -43,7 +43,7 @@ impl ByteCompiler<'_, '_> {
                 label
             };
             self.patch_jump(label);
-            self.compile_statement_list(case.body(), false);
+            self.compile_statement_list(case.body(), false, true);
         }
 
         if !default_label_set {

boa_engine/src/context/mod.rs

@@ -262,7 +262,7 @@ impl<'host> Context<'host> {
             self,
         );
         compiler.global_declaration_instantiation(statement_list)?;
-        compiler.compile_statement_list(statement_list, true);
+        compiler.compile_statement_list(statement_list, true, false);
         Ok(Gc::new(compiler.finish()))
     }
 

boa_engine/src/environments/compile.rs

@@ -56,6 +56,12 @@ impl CompileTimeEnvironment {
             .map_or(false, |binding| binding.lex)
     }
 
+    #[cfg(feature = "annex-b")]
+    /// Check if the environment has a binding with the given name.
+    pub(crate) fn has_binding(&self, name: Identifier) -> bool {
+        self.bindings.contains_key(&name)
+    }
+
     /// Checks if `name` is a lexical binding.
     pub(crate) fn is_lex_binding(&self, name: Identifier) -> bool {
         if let Some(binding) = self.bindings.get(&name) {
@@ -123,6 +129,23 @@ impl CompileTimeEnvironment {
         }
     }
 
+    #[cfg(feature = "annex-b")]
+    /// Check if a binding name exists in a environment.
+    /// Stop when a function scope is reached.
+    pub(crate) fn has_binding_until_var(&self, name: Identifier) -> bool {
+        if self.function_scope {
+            return false;
+        }
+        if self.bindings.contains_key(&name) {
+            return true;
+        }
+        if let Some(outer) = &self.outer {
+            outer.borrow().has_binding_until_var(name)
+        } else {
+            false
+        }
+    }
+
     /// Create a mutable binding.
     ///
     /// If the binding is a function scope binding and this is a declarative environment, try the outer environment.
@@ -236,6 +259,29 @@ impl CompileTimeEnvironment {
         }
     }
 
+    #[cfg(feature = "annex-b")]
+    /// Return the binding locator for a set operation on an existing var binding.
+    pub(crate) fn set_mutable_binding_var_recursive(&self, name: Identifier) -> BindingLocator {
+        if !self.is_function() {
+            return self.outer.as_ref().map_or_else(
+                || BindingLocator::global(name),
+                |outer| outer.borrow().set_mutable_binding_var_recursive(name),
+            );
+        }
+
+        match self.bindings.get(&name) {
+            Some(binding) if binding.mutable => {
+                BindingLocator::declarative(name, self.environment_index, binding.index)
+            }
+            Some(binding) if binding.strict => BindingLocator::mutate_immutable(name),
+            Some(_) => BindingLocator::silent(name),
+            None => self.outer.as_ref().map_or_else(
+                || BindingLocator::global(name),
+                |outer| outer.borrow().set_mutable_binding_var_recursive(name),
+            ),
+        }
+    }
+
     /// Gets the outer environment of this environment.
     pub(crate) fn outer(&self) -> Option<Gc<GcRefCell<Self>>> {
         self.outer.clone()

boa_engine/src/environments/runtime.rs

@@ -613,13 +613,13 @@ impl DeclarativeEnvironmentStack {
         }
     }
 
-    /// Set the value of a declarative binding.
+    /// Set the value of a lexical binding.
     ///
     /// # Panics
     ///
     /// Panics if the environment or binding index are out of range.
     #[track_caller]
-    pub(crate) fn put_declarative_value(
+    pub(crate) fn put_lexical_value(
         &mut self,
         environment_index: usize,
         binding_index: usize,

boa_engine/src/vm/code_block.rs

@@ -330,12 +330,9 @@ impl CodeBlock {
                     self.functions[operand as usize].length
                 )
             }
-            Opcode::DefInitArg
+            Opcode::DefVar
-            | Opcode::DefVar
             | Opcode::DefInitVar
-            | Opcode::DefLet
+            | Opcode::PutLexicalValue
-            | Opcode::DefInitLet
-            | Opcode::DefInitConst
             | Opcode::GetName
             | Opcode::GetLocator
             | Opcode::GetNameAndLocator
@@ -977,7 +974,7 @@ impl JsObject {
             context
                 .vm
                 .environments
-                .put_declarative_value(index, 0, class_object.into());
+                .put_lexical_value(index, 0, class_object.into());
             last_env -= 1;
         }
 
@@ -989,7 +986,7 @@ impl JsObject {
             context
                 .vm
                 .environments
-                .put_declarative_value(index, 0, self.clone().into());
+                .put_lexical_value(index, 0, self.clone().into());
             last_env -= 1;
         }
 
@@ -1023,7 +1020,7 @@ impl JsObject {
                     context,
                 )
             };
-            context.vm.environments.put_declarative_value(
+            context.vm.environments.put_lexical_value(
                 binding.environment_index(),
                 binding.binding_index(),
                 arguments_obj.into(),
@@ -1245,7 +1242,7 @@ impl JsObject {
                     context
                         .vm
                         .environments
-                        .put_declarative_value(index, 0, self.clone().into());
+                        .put_lexical_value(index, 0, self.clone().into());
                     last_env -= 1;
                 }
 
@@ -1278,7 +1275,7 @@ impl JsObject {
                             context,
                         )
                     };
-                    context.vm.environments.put_declarative_value(
+                    context.vm.environments.put_lexical_value(
                         binding.environment_index(),
                         binding.binding_index(),
                         arguments_obj.into(),

boa_engine/src/vm/flowgraph/mod.rs

@@ -405,12 +405,9 @@ impl CodeBlock {
                     graph.add_node(previous_pc, NodeShape::None, label.into(), Color::None);
                     graph.add_edge(previous_pc, pc, None, Color::None, EdgeStyle::Line);
                 }
-                Opcode::DefInitArg
+                Opcode::DefVar
-                | Opcode::DefVar
                 | Opcode::DefInitVar
-                | Opcode::DefLet
+                | Opcode::PutLexicalValue
-                | Opcode::DefInitLet
-                | Opcode::DefInitConst
                 | Opcode::GetName
                 | Opcode::GetLocator
                 | Opcode::GetNameAndLocator

boa_engine/src/vm/opcode/define/mod.rs

@@ -66,47 +66,22 @@ impl Operation for DefInitVar {
     }
 }
 
-/// `DefLet` implements the Opcode Operation for `Opcode::DefLet`
+/// `PutLexicalValue` implements the Opcode Operation for `Opcode::PutLexicalValue`
 ///
 /// Operation:
-///  - Declare `let` type variable.
+///  - Initialize a lexical binding.
 #[derive(Debug, Clone, Copy)]
-pub(crate) struct DefLet;
+pub(crate) struct PutLexicalValue;
 
-impl Operation for DefLet {
+impl Operation for PutLexicalValue {
-    const NAME: &'static str = "DefLet";
+    const NAME: &'static str = "PutLexicalValue";
-    const INSTRUCTION: &'static str = "INST - DefLet";
+    const INSTRUCTION: &'static str = "INST - PutLexicalValue";
-
-    fn execute(context: &mut Context<'_>) -> JsResult<CompletionType> {
-        let index = context.vm.read::<u32>();
-        let binding_locator = context.vm.frame().code_block.bindings[index as usize];
-        context.vm.environments.put_declarative_value(
-            binding_locator.environment_index(),
-            binding_locator.binding_index(),
-            JsValue::Undefined,
-        );
-        Ok(CompletionType::Normal)
-    }
-}
-
-macro_rules! implement_declaratives {
-    ($name:ident, $doc_string:literal) => {
-        #[doc= concat!("`", stringify!($name), "` implements the OpCode Operation for `Opcode::", stringify!($name), "`\n")]
-        #[doc= "\n"]
-        #[doc="Operation:\n"]
-        #[doc= concat!(" - ", $doc_string)]
-        #[derive(Debug, Clone, Copy)]
-        pub(crate) struct $name;
-
-        impl Operation for $name {
-            const NAME: &'static str = stringify!($name);
-            const INSTRUCTION: &'static str = stringify!("INST - " + $name);
 
     fn execute(context: &mut Context<'_>) -> JsResult<CompletionType> {
         let index = context.vm.read::<u32>();
         let value = context.vm.pop();
         let binding_locator = context.vm.frame().code_block.bindings[index as usize];
-                context.vm.environments.put_declarative_value(
+        context.vm.environments.put_lexical_value(
             binding_locator.environment_index(),
             binding_locator.binding_index(),
             value,
@@ -114,9 +89,3 @@ macro_rules! implement_declaratives {
         Ok(CompletionType::Normal)
     }
 }
-    };
-}
-
-implement_declaratives!(DefInitLet, "Declare and initialize `let` type variable");
-implement_declaratives!(DefInitConst, "Declare and initialize `const` type variable");
-implement_declaratives!(DefInitArg, "Declare and initialize function arguments");

boa_engine/src/vm/opcode/mod.rs

@@ -638,13 +638,6 @@ generate_impl! {
         /// Stack: value **=>** (ToNumeric(value)), (value - 1)
         DecPost,
 
-        /// Declare and initialize a function argument.
-        ///
-        /// Operands: name_index: `u32`
-        ///
-        /// Stack: value **=>**
-        DefInitArg,
-
         /// Declare `var` type variable.
         ///
         /// Operands: name_index: `u32`
@@ -659,26 +652,12 @@ generate_impl! {
         /// Stack: value **=>**
         DefInitVar,
 
-        /// Declare `let` type variable.
+        /// Initialize a lexical binding.
-        ///
-        /// Operands: name_index: `u32`
-        ///
-        /// Stack: **=>**
-        DefLet,
-
-        /// Declare and initialize `let` type variable.
-        ///
-        /// Operands: name_index: `u32`
-        ///
-        /// Stack: value **=>**
-        DefInitLet,
-
-        /// Declare and initialize `const` type variable.
         ///
         /// Operands: name_index: `u32`
         ///
         /// Stack: value **=>**
-        DefInitConst,
+        PutLexicalValue,
 
         /// Find a binding on the environment chain and push its value.
         ///
@@ -1633,7 +1612,6 @@ generate_impl! {
 #[derive(Clone, Copy, Debug)]
 pub(crate) enum BindingOpcode {
     Var,
-    Let,
     InitVar,
     InitLet,
     InitConst,