[oscars-integration] Integrate mark_sweep_branded backend

[shruti2522]

Stacked on #5460

Integrate oscars's mark_sweep_branded backend across boa_engine, replacing the custom GC with a compile time safe, branded lifetime alternative.

boa_gc now wraps mark_sweep_branded::Gc and MutationContext<'id, 'gc>. Lifetimes are strictly validated at compile time via branding, eliminating runtime tracking overhead.

Bumped oscars to include 5–8 element tuple Trace implementations required by Boa's AST.

Not covered, deliberately

Performance tuning. This PR focuses entirely on soundness and compilation under the new GC model. Benchmarks and optimizations will be done in follow up PRs

[Copilot]

Pull request overview

This PR continues the oscars GC migration by wiring the mark_sweep_branded backend through boa_gc, updating Boa to allocate via a GC context, and adjusting many call sites to compile under the new branded/guard-based APIs.

Changes:

• Introduces boa_gc::GcContext and threads allocation through Context::alloc()/Context::gc_collector(), replacing many direct MutationContext::dummy() uses.
• Adds oscars_backend feature plumbing across boa_gc, boa_engine, and boa_string, including a dummy WeakMap shim for oscars.
• Updates a broad set of engine/runtime code to handle new borrow/guard return types (GcRef, GcRefMut) by dereferencing before cloning.

Reviewed changes

Copilot reviewed 79 out of 80 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/macros/tests/gcd_callback.rs	Updates test to use MutationContext::global() and adjusts lints.
tests/fuzz/Cargo.toml	Adds a local patch override for boa_string in fuzz crate.
examples/src/bin/jstypedarray.rs	Updates example allocation to use MutationContext::global().
examples/src/bin/derive.rs	Adds dead_code allow to keep example compiling.
core/string/src/tests.rs	Replaces assert!(.. == ..) with assert_eq! for clearer failures.
core/string/src/lib.rs	Adds oscars Trace/Finalize impls for JsString behind oscars_backend.
core/string/src/builder.rs	Small refactors/attributes around latin1 conversion code paths.
core/string/Cargo.toml	Adds optional oscars dep + oscars_backend feature.
core/runtime/src/test262.rs	Adjusts downcast/clone patterns for new guard-based borrows.
core/runtime/src/microtask/tests.rs	Derefs borrow guards before cloning logs.
core/runtime/src/console/tests.rs	Derefs borrow guards; loosens some log substring assertions.
core/runtime/src/abort/mod.rs	Derefs borrow guard before cloning abort reason.
core/macros/src/lib.rs	Updates Trace derive implementation to better handle ref bindings and cloning.
core/interner/src/sym.rs	Switches Sym to #[derive(Trace)] with #[unsafe_ignore_trace] field.
core/gc/src/trace.rs	Wraps custom trace bodies in an unsafe block for consistency.
core/gc/src/test/weak.rs	Updates ephemeron size test for new Gc<'_, T> form.
core/gc/src/pointers/weak_map.rs	Adds WeakMap::get_value() helper.
core/gc/src/pointers/mutation_context.rs	Makes dummy() const; adds global() alias.
core/gc/src/oscars_weak_map.rs	Adds oscars-backend dummy/leaky WeakMap implementation.
core/gc/src/lib.rs	Switches oscars backend exports to mark_sweep_branded, adds type_id_of, macros, and weak map shim.
core/gc/src/context.rs	Adds GcContext abstraction used by Context::alloc()/gc_collector().
core/gc/src/cell.rs	Assertion style cleanup (assert_eq!, debug_assert_eq!).
core/gc/Cargo.toml	Updates feature wiring and oscars dependency; adds typeid optional dep.
core/engine/src/vm/tests.rs	Disables a GC-stress test under oscars_backend.
core/engine/src/vm/opcode/push/environment.rs	Threads global mutation context into environment pushes; uses Context::alloc() in one path.
core/engine/src/vm/opcode/function.rs	Explicitly types closure arg to satisfy inference under new API.
core/engine/src/vm/opcode/await/mod.rs	Uses Context::alloc() for captures; adjusts clone usage.
core/engine/src/vm/mod.rs	Switches initial CodeBlock allocation to MutationContext::global().
core/engine/src/vm/inline_cache/mod.rs	Adds an explicit Shape binding to satisfy type expectations.
core/engine/src/vm/code_block.rs	Adds clippy allow for clone_on_copy on constants accessor.
core/engine/src/value/integer.rs	Assertion style cleanup (assert_eq!/assert_ne!).
core/engine/src/value/inner/nan_boxed.rs	Adjusts raw pointer handling to match new JsObject::{into_raw,from_raw} signatures.
core/engine/src/value/inner/legacy.rs	Updates trace match style to avoid borrowing patterns.
core/engine/src/value/equality.rs	Assertion style cleanup (debug_assert_eq!).
core/engine/src/script.rs	Allocates script internals and compiled code via Context::alloc().
core/engine/src/realm.rs	Uses MutationContext::global() for realm/env allocations.
core/engine/src/object/shape/unique_shape.rs	Updates allocations/upgrades to global(); derefs guard before cloning prototype.
core/engine/src/object/shape/shared_shape/mod.rs	Updates allocations/upgrades; disables tests under oscars_backend.
core/engine/src/object/shape/shared_shape/forward_transition.rs	Updates WeakGc::new calls; disables tests under oscars_backend; adds clippy allows.
core/engine/src/object/mod.rs	Documents TypeId differences between Any and oscars typeid.
core/engine/src/object/jsobject.rs	Changes raw object pointer API to *const (); uses global() for allocations.
core/engine/src/object/builtins/jsweakset.rs	Constructs weak set map with context.gc_collector().
core/engine/src/object/builtins/jsweakmap.rs	Constructs weak map with context.gc_collector().
core/engine/src/object/builtins/jstypedarray.rs	Updates docs to use context.alloc(...) in example.
core/engine/src/object/builtins/jspromise.rs	Uses Context::alloc() for promise state; adds Clone for JsFuture.
core/engine/src/native_function/mod.rs	Updates closure allocation and downcast cloning patterns for guard-based borrows.
core/engine/src/native_function/continuation.rs	Updates coroutine closure allocation for global() and new from_raw typing.
core/engine/src/module/synthetic.rs	Uses Context::alloc() for compiled code; threads mutation context into env stack.
core/engine/src/module/source.rs	Uses Context::alloc() for compiled module code; threads mutation context into env stack.
core/engine/src/module/mod.rs	Uses Context::alloc() for module repr; updates a test to deref borrow guard before cloning.
core/engine/src/module/loader/mod.rs	Adds GC derives on module cache key used in GcRefCell map.
core/engine/src/lib.rs	Adds clippy allows to accommodate Gc being Copy under oscars backend.
core/engine/src/host_defined.rs	Relaxes Finalize bound for HostDefined<T>.
core/engine/src/error/mod.rs	Updates downcast clone patterns; minor trace match style change.
core/engine/src/environments/runtime/mod.rs	Threads mutation context into env stack push APIs; uses global() for env nodes.
core/engine/src/context/mod.rs	Adds GcContext to Context, plus alloc()/gc_collector() helpers.
core/engine/src/bytecompiler/function.rs	Switches code block allocation to MutationContext::global().
core/engine/src/bytecompiler/class.rs	Switches multiple code block allocations to MutationContext::global().
core/engine/src/builtins/weak/weak_ref.rs	Uses context.gc_collector() for weak refs; disables tests under oscars_backend.
core/engine/src/builtins/weak_set/mod.rs	Uses context.gc_collector(); disables tests under oscars_backend.
core/engine/src/builtins/weak_map/mod.rs	Migrates weak map storage access patterns; uses get_value(); adjusts borrows for oscars.
core/engine/src/builtins/set/ordered_set.rs	Minor trace loop variable rename cleanup.
core/engine/src/builtins/promise/mod.rs	Uses Context::alloc() for multiple GC allocations.
core/engine/src/builtins/json/mod.rs	Uses Context::alloc() for compiled code; derefs guard before cloning bigint.
core/engine/src/builtins/intl/locale/utils.rs	Derefs guard before cloning locale.
core/engine/src/builtins/intl/locale/mod.rs	Derefs guard before cloning locale in maximize/minimize.
core/engine/src/builtins/intl/list_format/mod.rs	Assertion style cleanup (assert_eq!).
core/engine/src/builtins/generator/mod.rs	Updates trace match style to avoid borrowing patterns.
core/engine/src/builtins/function/mod.rs	Threads mutation context into environment stack pushes.
core/engine/src/builtins/function/arguments.rs	Derefs environment guard before calling .get(...); adds clippy allows.
core/engine/src/builtins/finalization_registry/tests.rs	Disables miri tests under oscars_backend.
core/engine/src/builtins/finalization_registry/mod.rs	Updates weak GC usage to context.gc_collector()/global(), adjusts register/unregister signatures.
core/engine/Cargo.toml	Adds oscars_backend feature wiring to gc/string crates.
Cargo.toml	Adds typeid dependency and patches boa_string for git URL builds.
Cargo.lock	Updates oscars git revision and adds typeid + feature deps.
.github/workflows/webassembly.yml	Enables workflow on dev/oscars-gc branch.
.github/workflows/test262.yml	Enables workflow on dev/oscars-gc branch; skips job when targeting that base branch.
.github/workflows/rust.yml	Enables workflow on dev/oscars-gc branch.
.github/workflows/pull_request.yml	Enables workflow on dev/oscars-gc branch.

Suppressed comments (1)

core/engine/src/module/loader/mod.rs:294

• Deriving boa_gc::Trace for ModuleCacheKey will likely fail because PathBuf and Box<[ImportAttribute]> don't implement Trace (and they don't contain GC pointers anyway). Since this key is stored inside a GC-traced GcRefCell<FxHashMap<...>>, it should use an empty trace implementation and explicitly ignore the non-GC fields.

#[derive(Debug, Clone, PartialEq, Eq, Hash, boa_gc::Trace, boa_gc::Finalize)]
struct ModuleCacheKey {
    path: PathBuf,
    attributes: Box<[ImportAttribute]>,
}

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