Use `Rc` instead of `Gc` for `CompileTimeEnvironment`s (#3025)

* Use `Rc` instead of `Gc` for `CompileTimeEnvironment`s

* Add comment

boa_engine/src/bytecompiler/env.rs

@@ -1,12 +1,13 @@
+use std::{cell::RefCell, rc::Rc};
+
 use super::ByteCompiler;
 use crate::environments::{BindingLocator, BindingLocatorError, CompileTimeEnvironment};
 use boa_ast::expression::Identifier;
-use boa_gc::{Gc, GcRefCell};
 
 impl ByteCompiler<'_, '_> {
     /// Push either a new declarative or function environment on the compile time environment stack.
     pub(crate) fn push_compile_environment(&mut self, function_scope: bool) {
-        self.current_environment = Gc::new(GcRefCell::new(CompileTimeEnvironment::new(
+        self.current_environment = Rc::new(RefCell::new(CompileTimeEnvironment::new(
             self.current_environment.clone(),
             function_scope,
         )));

boa_engine/src/bytecompiler/function.rs

@@ -1,3 +1,5 @@
+use std::{cell::RefCell, rc::Rc};
+
 use crate::{
     builtins::function::ThisMode,
     bytecompiler::ByteCompiler,
@@ -6,7 +8,7 @@ use crate::{
     Context,
 };
 use boa_ast::function::{FormalParameterList, FunctionBody};
-use boa_gc::{Gc, GcRefCell};
+use boa_gc::Gc;
 use boa_interner::Sym;
 
 /// `FunctionCompiler` is used to compile AST functions to bytecode.
@@ -88,7 +90,7 @@ impl FunctionCompiler {
         mut self,
         parameters: &FormalParameterList,
         body: &FunctionBody,
-        outer_env: Gc<GcRefCell<CompileTimeEnvironment>>,
+        outer_env: Rc<RefCell<CompileTimeEnvironment>>,
         context: &mut Context<'_>,
     ) -> Gc<CodeBlock> {
         self.strict = self.strict || body.strict();

boa_engine/src/bytecompiler/mod.rs

@@ -11,7 +11,10 @@ mod module;
 mod statement;
 mod utils;
 
-use std::cell::Cell;
+use std::{
+    cell::{Cell, RefCell},
+    rc::Rc,
+};
 
 use crate::{
     builtins::function::ThisMode,
@@ -35,7 +38,7 @@ use boa_ast::{
     pattern::Pattern,
     Declaration, Expression, Statement, StatementList, StatementListItem,
 };
-use boa_gc::{Gc, GcRefCell};
+use boa_gc::Gc;
 use boa_interner::{Interner, Sym};
 use rustc_hash::FxHashMap;
 
@@ -241,10 +244,10 @@ pub struct ByteCompiler<'ctx, 'host> {
     pub(crate) functions: Vec<Gc<CodeBlock>>,
 
     /// Compile time environments in this function.
-    pub(crate) compile_environments: Vec<Gc<GcRefCell<CompileTimeEnvironment>>>,
+    pub(crate) compile_environments: Vec<Rc<RefCell<CompileTimeEnvironment>>>,
 
     /// The environment that is currently active.
-    pub(crate) current_environment: Gc<GcRefCell<CompileTimeEnvironment>>,
+    pub(crate) current_environment: Rc<RefCell<CompileTimeEnvironment>>,
 
     pub(crate) code_block_flags: CodeBlockFlags,
 
@@ -273,7 +276,7 @@ impl<'ctx, 'host> ByteCompiler<'ctx, 'host> {
         name: Sym,
         strict: bool,
         json_parse: bool,
-        current_environment: Gc<GcRefCell<CompileTimeEnvironment>>,
+        current_environment: Rc<RefCell<CompileTimeEnvironment>>,
         // TODO: remove when we separate scripts from the context
         context: &'ctx mut Context<'host>,
     ) -> ByteCompiler<'ctx, 'host> {

boa_engine/src/environments/compile.rs

@@ -1,6 +1,8 @@
+use std::{cell::RefCell, rc::Rc};
+
 use crate::environments::runtime::BindingLocator;
 use boa_ast::expression::Identifier;
-use boa_gc::{Finalize, Gc, GcRefCell, Trace};
+use boa_gc::{empty_trace, Finalize, Trace};
 
 use rustc_hash::FxHashMap;
 
@@ -20,15 +22,19 @@ struct CompileTimeBinding {
 /// A compile time environment maps bound identifiers to their binding positions.
 ///
 /// A compile time environment also indicates, if it is a function environment.
-#[derive(Debug, Finalize, Trace)]
+#[derive(Debug, Finalize)]
 pub(crate) struct CompileTimeEnvironment {
-    outer: Option<Gc<GcRefCell<Self>>>,
+    outer: Option<Rc<RefCell<Self>>>,
     environment_index: u32,
-    #[unsafe_ignore_trace]
     bindings: FxHashMap<Identifier, CompileTimeBinding>,
     function_scope: bool,
 }
 
+// Safety: Nothing in this struct needs tracing, so this is safe.
+unsafe impl Trace for CompileTimeEnvironment {
+    empty_trace!();
+}
+
 impl CompileTimeEnvironment {
     /// Creates a new global compile time environment.
     pub(crate) fn new_global() -> Self {
@@ -41,7 +47,7 @@ impl CompileTimeEnvironment {
     }
 
     /// Creates a new compile time environment.
-    pub(crate) fn new(parent: Gc<GcRefCell<Self>>, function_scope: bool) -> Self {
+    pub(crate) fn new(parent: Rc<RefCell<Self>>, function_scope: bool) -> Self {
         let index = parent.borrow().environment_index + 1;
         Self {
             outer: Some(parent),
@@ -291,7 +297,7 @@ impl CompileTimeEnvironment {
     }
 
     /// Gets the outer environment of this environment.
-    pub(crate) fn outer(&self) -> Option<Gc<GcRefCell<Self>>> {
+    pub(crate) fn outer(&self) -> Option<Rc<RefCell<Self>>> {
         self.outer.clone()
     }
 

boa_engine/src/environments/runtime/declarative/mod.rs

@@ -3,9 +3,12 @@ mod global;
 mod lexical;
 mod module;
 
-use std::cell::Cell;
+use std::{
+    cell::{Cell, RefCell},
+    rc::Rc,
+};
 
-use boa_gc::{Finalize, Gc, GcRefCell, Trace};
+use boa_gc::{Finalize, GcRefCell, Trace};
 pub(crate) use function::{FunctionEnvironment, FunctionSlots, ThisBindingStatus};
 pub(crate) use global::GlobalEnvironment;
 pub(crate) use lexical::LexicalEnvironment;
@@ -36,7 +39,10 @@ use crate::{environments::CompileTimeEnvironment, JsObject, JsResult, JsValue};
 #[derive(Debug, Trace, Finalize)]
 pub(crate) struct DeclarativeEnvironment {
     kind: DeclarativeEnvironmentKind,
-    compile: Gc<GcRefCell<CompileTimeEnvironment>>,
+
+    // Safety: Nothing in CompileTimeEnvironment needs tracing.
+    #[unsafe_ignore_trace]
+    compile: Rc<RefCell<CompileTimeEnvironment>>,
 }
 
 impl DeclarativeEnvironment {
@@ -44,20 +50,20 @@ impl DeclarativeEnvironment {
     pub(crate) fn global(global_this: JsObject) -> Self {
         Self {
             kind: DeclarativeEnvironmentKind::Global(GlobalEnvironment::new(global_this)),
-            compile: Gc::new(GcRefCell::new(CompileTimeEnvironment::new_global())),
+            compile: Rc::new(RefCell::new(CompileTimeEnvironment::new_global())),
         }
     }
 
     /// Creates a new `DeclarativeEnvironment` from its kind and compile environment.
     pub(crate) fn new(
         kind: DeclarativeEnvironmentKind,
-        compile: Gc<GcRefCell<CompileTimeEnvironment>>,
+        compile: Rc<RefCell<CompileTimeEnvironment>>,
     ) -> Self {
         Self { kind, compile }
     }
 
     /// Gets the compile time environment of this environment.
-    pub(crate) fn compile_env(&self) -> Gc<GcRefCell<CompileTimeEnvironment>> {
+    pub(crate) fn compile_env(&self) -> Rc<RefCell<CompileTimeEnvironment>> {
         self.compile.clone()
     }
 

boa_engine/src/environments/runtime/mod.rs

@@ -1,10 +1,12 @@
+use std::{cell::RefCell, rc::Rc};
+
 use crate::{
     environments::CompileTimeEnvironment,
     object::{JsObject, PrivateName},
     Context, JsResult, JsString, JsSymbol, JsValue,
 };
 use boa_ast::expression::Identifier;
-use boa_gc::{empty_trace, Finalize, Gc, GcRefCell, Trace};
+use boa_gc::{empty_trace, Finalize, Gc, Trace};
 use rustc_hash::FxHashSet;
 
 mod declarative;
@@ -220,7 +222,7 @@ impl EnvironmentStack {
     #[track_caller]
     pub(crate) fn push_lexical(
         &mut self,
-        compile_environment: Gc<GcRefCell<CompileTimeEnvironment>>,
+        compile_environment: Rc<RefCell<CompileTimeEnvironment>>,
     ) -> u32 {
         let num_bindings = compile_environment.borrow().num_bindings();
 
@@ -265,7 +267,7 @@ impl EnvironmentStack {
     #[track_caller]
     pub(crate) fn push_function(
         &mut self,
-        compile_environment: Gc<GcRefCell<CompileTimeEnvironment>>,
+        compile_environment: Rc<RefCell<CompileTimeEnvironment>>,
         function_slots: FunctionSlots,
     ) {
         let num_bindings = compile_environment.borrow().num_bindings();
@@ -309,7 +311,7 @@ impl EnvironmentStack {
     #[track_caller]
     pub(crate) fn push_function_inherit(
         &mut self,
-        compile_environment: Gc<GcRefCell<CompileTimeEnvironment>>,
+        compile_environment: Rc<RefCell<CompileTimeEnvironment>>,
     ) {
         let num_bindings = compile_environment.borrow().num_bindings();
 
@@ -361,10 +363,7 @@ impl EnvironmentStack {
     ///
     /// Panics if no environment exists on the stack.
     #[track_caller]
-    pub(crate) fn push_module(
-        &mut self,
-        compile_environment: Gc<GcRefCell<CompileTimeEnvironment>>,
-    ) {
+    pub(crate) fn push_module(&mut self, compile_environment: Rc<RefCell<CompileTimeEnvironment>>) {
         let num_bindings = compile_environment.borrow().num_bindings();
         self.stack.push(Environment::Declarative(Gc::new(
             DeclarativeEnvironment::new(
@@ -401,7 +400,7 @@ impl EnvironmentStack {
     /// # Panics
     ///
     /// Panics if no environment exists on the stack.
-    pub(crate) fn current_compile_environment(&self) -> Gc<GcRefCell<CompileTimeEnvironment>> {
+    pub(crate) fn current_compile_environment(&self) -> Rc<RefCell<CompileTimeEnvironment>> {
         self.stack
             .iter()
             .filter_map(Environment::as_declarative)

boa_engine/src/module/source.rs

@@ -1,5 +1,5 @@
 use std::{
-    cell::Cell,
+    cell::{Cell, RefCell},
     collections::HashSet,
     hash::{BuildHasherDefault, Hash},
     rc::Rc,
@@ -1403,7 +1403,7 @@ impl SourceTextModule {
         // 6. Set module.[[Environment]] to env.
         let global_env = realm.environment().clone();
         let global_compile_env = global_env.compile_env();
-        let module_compile_env = Gc::new(GcRefCell::new(CompileTimeEnvironment::new(
+        let module_compile_env = Rc::new(RefCell::new(CompileTimeEnvironment::new(
             global_compile_env,
             true,
         )));

boa_engine/src/vm/code_block.rs

@@ -20,10 +20,15 @@ use crate::{
 };
 use bitflags::bitflags;
 use boa_ast::function::FormalParameterList;
-use boa_gc::{empty_trace, Finalize, Gc, GcRefCell, Trace};
+use boa_gc::{empty_trace, Finalize, Gc, Trace};
 use boa_interner::Sym;
 use boa_profiler::Profiler;
-use std::{cell::Cell, collections::VecDeque, mem::size_of};
+use std::{
+    cell::{Cell, RefCell},
+    collections::VecDeque,
+    mem::size_of,
+    rc::Rc,
+};
 use thin_vec::ThinVec;
 
 #[cfg(any(feature = "trace", feature = "flowgraph"))]
@@ -129,7 +134,12 @@ pub struct CodeBlock {
     pub(crate) functions: Box<[Gc<Self>]>,
 
     /// Compile time environments in this function.
-    pub(crate) compile_environments: Box<[Gc<GcRefCell<CompileTimeEnvironment>>]>,
+    ///
+    // Safety: Nothing in CompileTimeEnvironment needs tracing, so this is safe.
+    //
+    // TODO(#3034): Maybe changing this to Gc after garbage collection would be better than Rc.
+    #[unsafe_ignore_trace]
+    pub(crate) compile_environments: Box<[Rc<RefCell<CompileTimeEnvironment>>]>,
 }
 
 /// ---- `CodeBlock` public API ----