Implement var initializers in for-in loops (#2842)

This Pull Request implements [Initializers in ForIn Statement Heads](https://tc39.es/ecma262/#sec-initializers-in-forin-statement-heads) from the Annex B. This also cleans up the "annex-b" feature to be able to disable it with `--no-default-features`, since I couldn't test the error messages when the feature is disabled.

boa_ast/src/statement/iteration/mod.rs

@@ -9,7 +9,7 @@ mod for_of_loop;
 mod while_loop;
 
 use crate::{
-    declaration::Binding,
+    declaration::{Binding, Variable},
     expression::{access::PropertyAccess, Identifier},
     pattern::Pattern,
 };
@@ -43,7 +43,7 @@ pub enum IterableLoopInitializer {
     /// A property access.
     Access(PropertyAccess),
     /// A new var declaration.
-    Var(Binding),
+    Var(Variable),
     /// A new let declaration.
     Let(Binding),
     /// A new const declaration.
@@ -58,12 +58,12 @@ impl ToInternedString for IterableLoopInitializer {
             Self::Identifier(ident) => return ident.to_interned_string(interner),
             Self::Pattern(pattern) => return pattern.to_interned_string(interner),
             Self::Access(access) => return access.to_interned_string(interner),
-            Self::Var(binding) => (binding, "var"),
+            Self::Var(binding) => (binding.to_interned_string(interner), "var"),
-            Self::Let(binding) => (binding, "let"),
+            Self::Let(binding) => (binding.to_interned_string(interner), "let"),
-            Self::Const(binding) => (binding, "const"),
+            Self::Const(binding) => (binding.to_interned_string(interner), "const"),
         };
 
-        format!("{pre} {}", binding.to_interned_string(interner))
+        format!("{pre} {binding}")
     }
 }
 
@@ -75,7 +75,8 @@ impl VisitWith for IterableLoopInitializer {
         match self {
             Self::Identifier(id) => visitor.visit_identifier(id),
             Self::Access(pa) => visitor.visit_property_access(pa),
-            Self::Var(b) | Self::Let(b) | Self::Const(b) => visitor.visit_binding(b),
+            Self::Var(b) => visitor.visit_variable(b),
+            Self::Let(b) | Self::Const(b) => visitor.visit_binding(b),
             Self::Pattern(p) => visitor.visit_pattern(p),
         }
     }
@@ -87,7 +88,8 @@ impl VisitWith for IterableLoopInitializer {
         match self {
             Self::Identifier(id) => visitor.visit_identifier_mut(id),
             Self::Access(pa) => visitor.visit_property_access_mut(pa),
-            Self::Var(b) | Self::Let(b) | Self::Const(b) => visitor.visit_binding_mut(b),
+            Self::Var(b) => visitor.visit_variable_mut(b),
+            Self::Let(b) | Self::Const(b) => visitor.visit_binding_mut(b),
             Self::Pattern(p) => visitor.visit_pattern_mut(p),
         }
     }

boa_cli/Cargo.toml

@@ -12,7 +12,7 @@ repository.workspace = true
 rust-version.workspace = true
 
 [dependencies]
-boa_engine = { workspace = true, features = ["deser", "console", "flowgraph", "trace", "annex-b"] }
+boa_engine = { workspace = true, features = ["deser", "flowgraph", "trace", "console"] }
 boa_ast = { workspace = true, features = ["serde"] }
 boa_parser.workspace = true
 boa_gc.workspace = true
@@ -26,8 +26,7 @@ phf = { version = "0.11.1", features = ["macros"] }
 pollster = "0.3.0"
 
 [features]
-default = ["intl"]
+default = ["boa_engine/annex-b", "boa_engine/intl"]
-intl = ["boa_engine/intl"]
 
 [target.x86_64-unknown-linux-gnu.dependencies]
 jemallocator = "0.5.0"

boa_engine/src/bytecompiler/statement/loop.rs

@@ -89,6 +89,16 @@ impl ByteCompiler<'_, '_> {
         label: Option<Sym>,
         configurable_globals: bool,
     ) {
+        // Handle https://tc39.es/ecma262/#prod-annexB-ForInOfStatement
+        if let IterableLoopInitializer::Var(var) = for_in_loop.initializer() {
+            if let Binding::Identifier(ident) = var.binding() {
+                if let Some(init) = var.init() {
+                    self.compile_expr(init, true);
+                    self.create_mutable_binding(*ident, true, true);
+                    self.emit_binding(BindingOpcode::InitVar, *ident);
+                }
+            }
+        }
         let initializer_bound_names = match for_in_loop.initializer() {
             IterableLoopInitializer::Let(declaration)
             | IterableLoopInitializer::Const(declaration) => bound_names(declaration),
@@ -137,9 +147,7 @@ impl ByteCompiler<'_, '_> {
         match for_in_loop.initializer() {
             IterableLoopInitializer::Identifier(ident) => {
                 self.create_mutable_binding(*ident, true, true);
-                let binding = self.set_mutable_binding(*ident);
+                self.emit_binding(BindingOpcode::InitVar, *ident);
-                let index = self.get_or_insert_binding(binding);
-                self.emit(Opcode::DefInitVar, &[index]);
             }
             IterableLoopInitializer::Access(access) => {
                 self.access_set(
@@ -148,7 +156,7 @@ impl ByteCompiler<'_, '_> {
                     ByteCompiler::access_set_top_of_stack_expr_fn,
                 );
             }
-            IterableLoopInitializer::Var(declaration) => match declaration {
+            IterableLoopInitializer::Var(declaration) => match declaration.binding() {
                 Binding::Identifier(ident) => {
                     self.create_mutable_binding(*ident, true, configurable_globals);
                     self.emit_binding(BindingOpcode::InitVar, *ident);
@@ -284,18 +292,22 @@ impl ByteCompiler<'_, '_> {
                     ByteCompiler::access_set_top_of_stack_expr_fn,
                 );
             }
-            IterableLoopInitializer::Var(declaration) => match declaration {
+            IterableLoopInitializer::Var(declaration) => {
-                Binding::Identifier(ident) => {
+                // ignore initializers since those aren't allowed on for-of loops.
-                    self.create_mutable_binding(*ident, true, false);
+                assert!(declaration.init().is_none());
-                    self.emit_binding(BindingOpcode::InitVar, *ident);
+                match declaration.binding() {
-                }
+                    Binding::Identifier(ident) => {
-                Binding::Pattern(pattern) => {
+                        self.create_mutable_binding(*ident, true, false);
-                    for ident in bound_names(pattern) {
+                        self.emit_binding(BindingOpcode::InitVar, *ident);
-                        self.create_mutable_binding(ident, true, false);
+                    }
+                    Binding::Pattern(pattern) => {
+                        for ident in bound_names(pattern) {
+                            self.create_mutable_binding(ident, true, false);
+                        }
+                        self.compile_declaration_pattern(pattern, BindingOpcode::InitVar);
                     }
-                    self.compile_declaration_pattern(pattern, BindingOpcode::InitVar);
                 }
-            },
+            }
             IterableLoopInitializer::Let(declaration) => match declaration {
                 Binding::Identifier(ident) => {
                     self.create_mutable_binding(*ident, false, false);

boa_parser/src/error.rs

@@ -125,18 +125,22 @@ impl Error {
         }
     }
 
-    /// Creates a "general" parsing error with the specific error message for a wrong function declaration in non-strict mode.
+    /// Creates a "general" parsing error with the specific error message for a misplaced function declaration.
-    pub(crate) fn wrong_function_declaration_non_strict(position: Position) -> Self {
+    pub(crate) fn misplaced_function_declaration(position: Position, strict: bool) -> Self {
         Self::General {
-            message: "In non-strict mode code, functions can only be declared at top level, inside a block, or as the body of an if statement.".into(),
+            message: format!(
-            position
+                "{}functions can only be declared at the top level or inside a block.",
+                if strict { "in strict mode code, " } else { "" }
+            )
+            .into(),
+            position,
         }
     }
 
     /// Creates a "general" parsing error with the specific error message for a wrong function declaration with label.
     pub(crate) fn wrong_labelled_function_declaration(position: Position) -> Self {
         Self::General {
-            message: "Labelled functions can only be declared at top level or inside a block"
+            message: "labelled functions can only be declared at the top level or inside a block"
                 .into(),
             position,
         }

boa_parser/src/lexer/mod.rs

@@ -178,7 +178,7 @@ impl<R> Lexer<R> {
     where
         R: Read,
     {
-        if !cfg!(feature = "annex-b") || self.module() {
+        if cfg!(not(feature = "annex-b")) || self.module() {
             return Ok(());
         }
 

boa_parser/src/parser/statement/if_stm/mod.rs

@@ -77,9 +77,8 @@ where
             TokenKind::Keyword((Keyword::Function, _)) => {
                 // FunctionDeclarations in IfStatement Statement Clauses
                 // https://tc39.es/ecma262/#sec-functiondeclarations-in-ifstatement-statement-clauses
-                if strict {
+                if cfg!(not(feature = "annex-b")) || strict {
-                    // This production only applies when parsing non-strict code.
+                    return Err(Error::misplaced_function_declaration(position, strict));
-                    return Err(Error::wrong_function_declaration_non_strict(position));
                 }
                 // Source text matched by this production is processed as if each matching
                 // occurrence of FunctionDeclaration[?Yield, ?Await, ~Default] was the sole
@@ -117,8 +116,10 @@ where
                         TokenKind::Keyword((Keyword::Function, _)) => {
                             // FunctionDeclarations in IfStatement Statement Clauses
                             // https://tc39.es/ecma262/#sec-functiondeclarations-in-ifstatement-statement-clauses
-                            if strict {
+                            if cfg!(not(feature = "annex-b")) || strict {
-                                return Err(Error::wrong_function_declaration_non_strict(position));
+                                return Err(Error::misplaced_function_declaration(
+                                    position, strict,
+                                ));
                             }
 
                             // Source text matched by this production is processed as if each matching

boa_parser/src/parser/statement/iteration/for_statement.rs

@@ -17,7 +17,10 @@ use crate::{
     },
     Error,
 };
-use ast::operations::{bound_names, var_declared_names};
+use ast::{
+    declaration::Binding,
+    operations::{bound_names, var_declared_names},
+};
 use boa_ast::{
     self as ast,
     statement::{
@@ -157,9 +160,13 @@ where
                 ));
             }
             (Some(init), TokenKind::Keyword((kw @ (Keyword::In | Keyword::Of), false))) => {
-                let kw = *kw;
+                let in_loop = kw == &Keyword::In;
-                let init =
+                let init = initializer_to_iterable_loop_initializer(
-                    initializer_to_iterable_loop_initializer(init, position, cursor.strict())?;
+                    init,
+                    position,
+                    cursor.strict(),
+                    in_loop,
+                )?;
 
                 cursor.advance(interner);
                 let expr = Expression::new(None, true, self.allow_yield, self.allow_await)
@@ -208,7 +215,7 @@ where
                         }
                     }
                 }
-                return Ok(if kw == Keyword::In {
+                return Ok(if in_loop {
                     ForInLoop::new(init, expr, body).into()
                 } else {
                     ForOfLoop::new(init, expr, body, r#await).into()
@@ -288,7 +295,9 @@ fn initializer_to_iterable_loop_initializer(
     initializer: ForLoopInitializer,
     position: Position,
     strict: bool,
+    in_loop: bool,
 ) -> ParseResult<IterableLoopInitializer> {
+    let loop_type = if in_loop { "for-in" } else { "for-of" };
     match initializer {
         ForLoopInitializer::Expression(mut expr) => {
             while let ast::Expression::Parenthesized(p) = expr {
@@ -338,7 +347,7 @@ fn initializer_to_iterable_loop_initializer(
             [declaration] => {
                 if declaration.init().is_some() {
                     return Err(Error::lex(LexError::Syntax(
-                        "a declaration in the head of a for-of loop can't have an initializer"
+                        format!("a lexical declaration in the head of a {loop_type} loop can't have an initializer")
                             .into(),
                         position,
                     )));
@@ -353,25 +362,38 @@ fn initializer_to_iterable_loop_initializer(
                 })
             }
             _ => Err(Error::lex(LexError::Syntax(
-                "only one variable can be declared in the head of a for-of loop".into(),
+                format!("only one variable can be declared in the head of a {loop_type} loop")
+                    .into(),
                 position,
             ))),
         },
         ForLoopInitializer::Var(decl) => match decl.0.as_ref() {
             [declaration] => {
-                // TODO: implement initializers in ForIn heads
                 // https://tc39.es/ecma262/#sec-initializers-in-forin-statement-heads
-                if declaration.init().is_some() {
+                let is_pattern = matches!(declaration.binding(), Binding::Pattern(_));
+                if declaration.init().is_some()
+                    && (cfg!(not(feature = "annex-b")) || strict || !in_loop || is_pattern)
+                {
                     return Err(Error::lex(LexError::Syntax(
-                        "a declaration in the head of a for-of loop can't have an initializer"
+                        format!(
-                            .into(),
+                            "{}a {} declaration in the head of a {loop_type} loop \
+                            cannot have an initializer",
+                            if strict { "in strict mode, " } else { "" },
+                            if is_pattern {
+                                "binding pattern"
+                            } else {
+                                "binding declaration"
+                            }
+                        )
+                        .into(),
                         position,
                     )));
                 }
-                Ok(IterableLoopInitializer::Var(declaration.binding().clone()))
+                Ok(IterableLoopInitializer::Var(declaration.clone()))
             }
             _ => Err(Error::lex(LexError::Syntax(
-                "only one variable can be declared in the head of a for-of loop".into(),
+                format!("only one variable can be declared in the head of a {loop_type} loop")
+                    .into(),
                 position,
             ))),
         },

boa_parser/src/parser/statement/labelled_stm/mod.rs

@@ -65,18 +65,22 @@ where
             // Early Error: It is a Syntax Error if any strict mode source code matches this rule.
             // https://tc39.es/ecma262/#sec-labelled-statements-static-semantics-early-errors
             // https://tc39.es/ecma262/#sec-labelled-function-declarations
-            TokenKind::Keyword((Keyword::Function, _)) if strict => {
+            TokenKind::Keyword((Keyword::Function, _))
-                return Err(Error::general(
+                if cfg!(not(feature = "annex-b")) || strict =>
-                    "In strict mode code, functions can only be declared at top level or inside a block.",
+            {
-                    next_token.span().start()
+                return Err(Error::misplaced_function_declaration(
+                    next_token.span().start(),
+                    strict,
                 ))
             }
             TokenKind::Keyword((Keyword::Function, _)) => {
                 FunctionDeclaration::new(self.allow_yield, self.allow_await, false)
-                .parse(cursor, interner)?
+                    .parse(cursor, interner)?
-                .into()
+                    .into()
             }
-            _ => Statement::new(self.allow_yield, self.allow_await, self.allow_return).parse(cursor, interner)?.into()
+            _ => Statement::new(self.allow_yield, self.allow_await, self.allow_return)
+                .parse(cursor, interner)?
+                .into(),
         };
 
         Ok(ast::statement::Labelled::new(labelled_item, label))

boa_parser/src/parser/statement/variable/mod.rs

@@ -214,8 +214,13 @@ where
                     .is_some()
                 {
                     Some(
-                        Initializer::new(Some(ident), true, self.allow_yield, self.allow_await)
+                        Initializer::new(
-                            .parse(cursor, interner)?,
+                            Some(ident),
+                            self.allow_in,
+                            self.allow_yield,
+                            self.allow_await,
+                        )
+                        .parse(cursor, interner)?,
                     )
                 } else {
                     None

boa_tester/Cargo.toml

@@ -12,7 +12,7 @@ repository.workspace = true
 rust-version.workspace = true
 
 [dependencies]
-boa_engine = { workspace = true, features = ["annex-b"] }
+boa_engine.workspace = true
 boa_gc.workspace = true
 clap = { version = "4.2.4", features = ["derive"] }
 serde = { version = "1.0.160", features = ["derive"] }
@@ -31,5 +31,4 @@ comfy-table = "6.1.4"
 serde_repr = "0.1.12"
 
 [features]
-default = ["intl"]
+default = ["boa_engine/intl", "boa_engine/annex-b"]
-intl = ["boa_engine/intl"]

boa_wasm/Cargo.toml

@@ -12,11 +12,14 @@ repository.workspace = true
 rust-version.workspace = true
 
 [dependencies]
-boa_engine = { workspace = true, features = ["console", "annex-b"] }
+boa_engine = { workspace = true, features = ["console"] }
 wasm-bindgen = "0.2.84"
 getrandom = { version = "0.2.9", features = ["js"] }
 chrono = { version = "0.4.24", features = ["clock", "std", "wasmbind"] }
 
+[features]
+default = ["boa_engine/annex-b"]
+
 [lib]
 crate-type = ["cdylib", "lib"]
 name = "boa_wasm"