[oscars-integration] Thread MutationContext token, resolve Gc borrow conflicts in environments

[shruti2522]

follow up to #5458

Implemented next part of the oscars GC migration.

• added Context::gc() to wrap token access. in the final phases of integration, we would only have to update this one spot.
• threaded MutationContext by value into the environments api (push_lexical, push_function, etc)
• fixed some mutable aliasing issues in the eval and json modules
• also enabled ci workflow for dev/oscars-gc branch

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	53,125	53,125	0
Passed	51,073	51,072	-1
Ignored	1,482	1,482	0
Failed	570	571	+1
Panics	0	0	0
Conformance	96.14%	96.14%	-0.00%

Broken tests (1):

test/intl402/DateTimeFormat/constructor-options-fractionalSecondDigits-valid.js (previously Passed)

Tested main commit: f54077467b4b01eb0fe221cfff470a8546ebe36c
Tested PR commit: 64b57065aa7e5cea6682ce8a2b6a944dc79293b8
Compare commits: f540774...64b5706

[Copilot]

Pull request overview

Copilot reviewed 80 out of 81 changed files in this pull request and generated 3 comments.

Suppressed comments (7)

core/engine/src/builtins/set/ordered_set.rs:20

• Trace implementation is effectively a no-op, so OrderedSet won't mark any JsValue keys. This can lead to use-after-free under the mark-sweep GC backend and is inconsistent with the existing MapKey/JsValue storage.

unsafe impl Trace for OrderedSet {
    custom_trace!(this, mark, {
        // Values in an IndexSet cannot be mutably borrowed. Since this is a null collector, we skip
    });
}

core/engine/src/builtins/map/ordered_map.rs:50

• Trace only marks values, not keys. OrderedMap stores GC-managed keys (MapKey::Key(JsValue)), so omitting key tracing can allow keys (and the objects they reference) to be collected prematurely under the mark-sweep backend.

unsafe impl<V: Trace> Trace for OrderedMap<V> {
    custom_trace!(this, mark, {
        for v in this.map.values() {
            mark(v);
        }
    });
}

core/engine/src/object/shape/shared_shape/forward_transition.rs:102

• Using .copied() here forces WeakGc<SharedShapeInner> to be Copy, which breaks compilation with the mark-sweep GC backend. Prefer .cloned() for backend-agnostic code.
  .github/workflows/test262.yml:20
• This disables the entire test262 job for every PR/event (if: false). If this is only meant to temporarily reduce CI load on dev/oscars-gc, consider scoping the condition to that branch; otherwise it removes an important correctness gate for all PRs.

  run_test262:
    if: false
    name: Run the test262 test suite

core/engine/Cargo.toml:16

• oscars_backend becomes enabled by default for boa_engine. This is a major behavioral change (e.g. null collector semantics + dummy WeakMap) that affects all downstream users who rely on default features, and it also makes it harder to compare regressions vs the existing mark-sweep backend.

[features]
default = ["float16", "xsum", "temporal", "oscars_backend"]

core/gc/Cargo.toml:26

• boa_gc now enables oscars_backend by default. Since the oscars null collector has very different semantics than the existing GC (and currently disables/changes weak structures), flipping the default is likely to be a breaking change for crates that depend on boa_gc with default features.
  Cargo.toml:275
• Adding a workspace-level [patch."https://github.com/boa-dev/boa.git"] causes all consumers to override boa_string for that git source, which is surprising and can break downstream dependency resolution. This kind of patch is usually kept in a local dev config or confined to a non-workspace crate (e.g. fuzz).

[patch."https://github.com/boa-dev/boa.git"]
boa_string = { path = "core/string" }

[Copilot]

Pull request overview

Copilot reviewed 78 out of 79 changed files in this pull request and generated 3 comments.

Suppressed comments (1)

core/gc/src/pointers/weak_map.rs:70

• WeakMap::get_value calls Ephemeron::value using MutationContext::dummy(), which bypasses the token requirement and makes this API usable without providing a real MutationContext. That defeats the purpose of threading MutationContext through APIs and makes it harder to audit which code paths rely on the dummy token.

[shruti2522]

@nekevss @jedel1043 this one ready for review, we can merge this