Replace type_name with TypeId using typeid crate

[shruti2522]

Related to #98 (comment)

Replaced the string based type_name method with the standard core::any::TypeId. This makes downcasting safer and fully supports #![no_std] environments without cross thread memory bugs.

Because TypeId requires a 'static lifetime (and our GC pointers use a temporary 'gc lifetime), added a StaticId proxy type to safely bridge the gap

• added type StaticId to the Trace trait. Every GC type now links to a safe 'static version of itself (for example, Gc<'gc, T> links to Gc<'static, T::StaticId>)
• GcBox uses TypeId instead of strings. When Gc::is<U>() is called, it safely checks the type using TypeId::of::<U::StaticId>()
• To satisfy the compiler's size requirements without causing infinite type resolution loops, used Vec as the static proxy for slices ([T]) and String as the proxy for str
• Removed the thread_local! requirement in the null collector to prevent cross thread uaf bugs and brings back full #![no_std] support.

[nekevss]

Huh, why can't we use TypeId::of::<U>() here?

[shruti2522]

Replced StaticId with dtonlay's typeid crate, everything compiles, certainly an improvement on the earlier approach.
It gives us a TypeId equivalent that works without requiring 'static, so U no longer needs the StaticId proxy at all

[Copilot]

Pull request overview

This PR replaces string-based runtime type checks (type_name) with TypeId-based checks (via the typeid crate) in the branded collectors, aiming to make Gc::is/downcasting safer and more compatible with #![no_std].

Changes:

• Store a runtime type identifier in GcBox and switch Gc::is to compare TypeIds instead of type_name strings.
• Update branded Trace blanket impls and add/adjust safety documentation in the null collector’s global mutation context.
• Introduce the typeid dependency and update lockfile accordingly.

Reviewed changes

Copilot reviewed 11 out of 12 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
oscars/src/collectors/null_collector_branded/weak.rs	Adds comment about static proxy mapping for weak pointers.
oscars/src/collectors/null_collector_branded/trace.rs	Updates several Trace blanket impl bounds and comments; adds Trace for str.
oscars/src/collectors/null_collector_branded/mutation_ctx.rs	Expands safety documentation around MutationContext::global() TLS usage.
oscars/src/collectors/null_collector_branded/gc.rs	Switches Gc::is from string comparison to typeid::of.
oscars/src/collectors/null_collector_branded/gc_box.rs	Stores TypeId in GcBox and updates constructor bounds.
oscars/src/collectors/null_collector_branded/ephemeron.rs	Adds comment describing intended static proxy mapping.
oscars/src/collectors/null_collector_branded/cell.rs	Uses Clone trait for GcRef; adds comment about static mapping.
oscars/src/collectors/mark_sweep_branded/weak.rs	Minor simplification in is_upgradable; adds comment about static proxy mapping.
oscars/src/collectors/mark_sweep_branded/trace.rs	Updates several Trace blanket impl bounds/comments; adds Trace for str.
oscars/src/collectors/mark_sweep_branded/tests/uaf.rs	Adds comment describing intended static proxy mapping for test type.
oscars/src/collectors/mark_sweep_branded/tests/mod.rs	Adds comment on Trace impl for JsObject.
oscars/src/collectors/mark_sweep_branded/mod.rs	Adds Default impl for Collector.
oscars/src/collectors/mark_sweep_branded/gc.rs	Switches Gc::is to typeid::of; updates Trace impl signature.
oscars/src/collectors/mark_sweep_branded/gc_box.rs	Stores TypeId in GcBox and updates constructor bounds.
oscars/src/collectors/mark_sweep_branded/ephemeron.rs	Adds comment describing intended static proxy mapping.
oscars/src/collectors/mark_sweep_branded/cell.rs	Adds comment about branding/static mapping for GcRefCell.
oscars/Cargo.toml	Adds the typeid dependency.
Cargo.lock	Records the new typeid package in the lockfile.

Suppressed comments (3)

oscars/src/collectors/mark_sweep_branded/trace.rs:272

• PhantomData<T> is a marker and has no runtime data to trace, so requiring T: 'static here is an unnecessary restriction. It also conflicts with the unbranded collector’s Trace for PhantomData<T> blanket impl (collectors/mark_sweep/trace.rs:398-401). Unless there is a concrete reason to restrict this, consider restoring the unrestricted impl.

unsafe impl Trace for core::any::TypeId {
    #[inline]
    unsafe fn trace(&self, _tracer: &mut Tracer) {}
}

oscars/src/collectors/null_collector_branded/trace.rs:194

• PhantomData<T> is a marker and has no runtime data to trace, so requiring T: 'static here is an unnecessary restriction (and inconsistent with the unbranded collector’s blanket impl in collectors/mark_sweep/trace.rs:398-401). Unless there is a concrete reason to restrict this, consider restoring the unrestricted impl.

unsafe impl<T: Trace + Default> Trace for Cell<T> {
    unsafe fn trace(&self, tracer: &mut Tracer) {
        let v = self.take();
        v.trace(tracer);

oscars/src/collectors/mark_sweep_branded/trace.rs:426

• This comment says “Gc requires &mut self to trace”, but Trace::trace takes &self and Gc itself is traced via a shared reference. If the intent is to forbid branded GC pointers in BTreeSet by using the T: 'static bound, the comment should state that instead to avoid spreading incorrect guidance.

    use crate::collectors::mark_sweep_branded::{Trace, Tracer};
    use icu_locale_core::{LanguageIdentifier, Locale};

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