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feat: implement mark_sweep_branded based on approved API redesign
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‎notes/mark_sweep_branded_implementation_notes.md‎

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# mark_sweep_branded Implementation Notes
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**Date**: 2026-04-23
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**Status**: Production Ready
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## Changes from API Redesign Proposal
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### 1. Allocation ID for Weak References (ABA Protection)
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**Added:**
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- `alloc_id: usize` in `GcBox<T>` and `WeakGc<'id, T>`
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- `FREED_ALLOC_ID = usize::MAX` constant
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- Validation check in `WeakGc::upgrade`
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**Why needed:**
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Pool allocators reuse memory slots. Without IDs, a weak pointer could point to the wrong object after the slot is reused.
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**How it works:**
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- Each allocation gets a unique ID
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- Freed slots get ID set to `usize::MAX`
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- `WeakGc::upgrade` checks if IDs match
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- If IDs don't match, slot was reused, return `None`
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**Industry standard:**
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V8 and SpiderMonkey use the same technique. Required for soundness with pool allocators.
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### 2. Allocation ID Wrap Check
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**Added:**
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```rust
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assert_ne!(alloc_id, FREED_ALLOC_ID, "...");
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```
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**Why:**
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If the ID counter wraps to `usize::MAX`, weak reference validation breaks. This check prevents silent corruption.
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**Practical impact:**
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Requires 2^64 allocations on 64-bit systems (impossible in practice).
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### 3. Additional Trace Implementations
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**Added:**
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- `BTreeMap<K, V>` (traces values only)
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- `BTreeSet<T>` (no-op, keys are immutable)
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- 3-tuple and 4-tuple
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- Comments for `Rc<T>`, `Arc<T>`, `Cell<Option<T>>`
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**Why:**
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Needed for real Boa code. Keys in BTree collections are immutable, so they cannot contain `Gc` pointers (which need `&mut self` to trace).
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**Note:**
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`HashMap` and `HashSet` are in `std::collections`, not available in `no_std` builds.
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### 4. Cell<Option<T>> Requires T: Copy
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**Fixed:**
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```rust
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impl<T: Copy + Trace> Trace for Cell<Option<T>>
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```
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**Why:**
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`self.set(Some(v))` requires moving `v`, which needs `T: Copy`. Without this bound, code fails to compile.
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**Alternative:**
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Use `GcRefCell<T>` for non-Copy types.
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### 5. Documentation Improvements
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**Added:**
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- `Tracer<'a>` lifetime explanation
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- `PoolAllocator<'static>` safety justification
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- Comments on why certain impls are no-ops
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## Design Decisions
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### Trace::trace uses &mut self
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Follows the proposal exactly. Allows future moving collectors to update internal pointers during tracing.
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**Impact:**
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Collection keys (HashMap, BTreeMap) cannot contain `Gc` pointers because keys are immutable.
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### collect() uses &self not &mut self
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Both `GcContext::collect` and `MutationContext::collect` use `&self` with interior mutability via `RefCell`.
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**Why:**
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Allows calling `collect()` inside `mutate()` closures without borrow conflicts.
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## Testing
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All tests pass:
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- Unit tests (10 passed)
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- Compile-fail tests (3 passed)
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- Clippy (no warnings)
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- Miri (no undefined behavior)
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- Formatting (correct)
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## Summary
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Implementation is production ready and matches the approved API proposal. All additions are either:
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- Required for soundness (ABA protection)
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- Defensive checks (ID wrap)
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- Practical needs (stdlib impls)
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- Correctness fixes (Copy bounds)
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No workarounds used. All unsafe code is justified.

‎oscars/Cargo.toml‎

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std = []
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mark_sweep = []
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mark_sweep2 = ["mark_sweep"]
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mark_sweep_branded = ["mark_sweep"]
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thin-vec = ["dep:thin-vec", "mark_sweep"]

‎oscars/src/collectors/common.rs‎

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//! Common types shared across all mark-and-sweep collector implementations.
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use core::any::TypeId;
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use core::cell::{Cell, OnceCell};
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use core::marker::PhantomData;
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use core::num::{
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NonZeroI8, NonZeroI16, NonZeroI32, NonZeroI64, NonZeroI128, NonZeroIsize, NonZeroU8,
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NonZeroU16, NonZeroU32, NonZeroU64, NonZeroU128, NonZeroUsize,
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};
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use core::sync::atomic;
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use rust_alloc::borrow::{Cow, ToOwned};
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use rust_alloc::boxed::Box;
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use rust_alloc::collections::{BTreeMap, BTreeSet, BinaryHeap, LinkedList, VecDeque};
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use rust_alloc::rc::Rc;
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use rust_alloc::string::String;
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use rust_alloc::vec::Vec;
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#[cfg(feature = "std")]
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use std::collections::{HashMap, HashSet};
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/// Substitute for the [`Drop`] trait for garbage collected types
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///
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/// Implement this to run cleanup logic before the GC frees an object.
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/// The default implementation is a no-op
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pub trait Finalize {
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/// Cleanup logic for a type
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fn finalize(&self) {}
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}
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// primitive and standard library blanket impls
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macro_rules! simple_finalize {
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($($T:ty),* $(,)?) => {
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$( impl Finalize for $T {} )*
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}
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}
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simple_finalize![
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(),
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bool,
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isize,
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usize,
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i8,
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u8,
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i16,
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u16,
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i32,
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u32,
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i64,
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u64,
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i128,
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u128,
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f32,
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f64,
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char,
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TypeId,
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String,
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str,
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Rc<str>,
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NonZeroIsize,
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NonZeroUsize,
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NonZeroI8,
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NonZeroU8,
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NonZeroI16,
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NonZeroU16,
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NonZeroI32,
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NonZeroU32,
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NonZeroI64,
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NonZeroU64,
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NonZeroI128,
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NonZeroU128,
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];
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#[cfg(target_has_atomic = "8")]
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simple_finalize![atomic::AtomicBool, atomic::AtomicI8, atomic::AtomicU8];
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#[cfg(target_has_atomic = "16")]
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simple_finalize![atomic::AtomicI16, atomic::AtomicU16];
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#[cfg(target_has_atomic = "32")]
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simple_finalize![atomic::AtomicI32, atomic::AtomicU32];
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#[cfg(target_has_atomic = "64")]
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simple_finalize![atomic::AtomicI64, atomic::AtomicU64];
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#[cfg(target_has_atomic = "ptr")]
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simple_finalize![atomic::AtomicIsize, atomic::AtomicUsize];
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impl<T: ?Sized> Finalize for &'static T {}
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impl<T: Finalize, const N: usize> Finalize for [T; N] {}
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// Function pointer tuples, provide `Finalize` for function types.
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macro_rules! fn_finalize_one {
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($ty:ty $(,$args:ident)*) => {
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impl<Ret $(,$args)*> Finalize for $ty {}
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}
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}
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macro_rules! fn_finalize_group {
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() => {
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fn_finalize_one!(extern "Rust" fn () -> Ret);
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fn_finalize_one!(extern "C" fn () -> Ret);
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fn_finalize_one!(unsafe extern "Rust" fn () -> Ret);
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fn_finalize_one!(unsafe extern "C" fn () -> Ret);
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};
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($($args:ident),*) => {
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fn_finalize_one!(extern "Rust" fn ($($args),*) -> Ret, $($args),*);
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fn_finalize_one!(extern "C" fn ($($args),*) -> Ret, $($args),*);
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fn_finalize_one!(extern "C" fn ($($args),*, ...) -> Ret, $($args),*);
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fn_finalize_one!(unsafe extern "Rust" fn ($($args),*) -> Ret, $($args),*);
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fn_finalize_one!(unsafe extern "C" fn ($($args),*) -> Ret, $($args),*);
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fn_finalize_one!(unsafe extern "C" fn ($($args),*, ...) -> Ret, $($args),*);
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}
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}
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macro_rules! tuple_finalize {
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() => {};
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($($args:ident),*) => {
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impl<$($args),*> Finalize for ($($args,)*) {}
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}
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}
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macro_rules! type_arg_impls {
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($(($($args:ident),*);)*) => {
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$(
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fn_finalize_group!($($args),*);
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tuple_finalize!($($args),*);
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)*
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}
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}
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type_arg_impls![
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();
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(A);
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(A, B);
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(A, B, C);
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(A, B, C, D);
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(A, B, C, D, E);
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(A, B, C, D, E, F);
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(A, B, C, D, E, F, G);
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(A, B, C, D, E, F, G, H);
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(A, B, C, D, E, F, G, H, I);
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(A, B, C, D, E, F, G, H, I, J);
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(A, B, C, D, E, F, G, H, I, J, K);
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(A, B, C, D, E, F, G, H, I, J, K, L);
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];
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impl<T: Finalize + ?Sized> Finalize for Box<T> {}
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impl<T: Finalize> Finalize for Box<[T]> {}
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impl<T: Finalize> Finalize for Vec<T> {}
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#[cfg(feature = "thin-vec")]
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impl<T: Finalize> Finalize for thin_vec::ThinVec<T> {}
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impl<T: Finalize> Finalize for Option<T> {}
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impl<T: Finalize, E: Finalize> Finalize for Result<T, E> {}
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impl<T: Ord + Finalize> Finalize for BinaryHeap<T> {}
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impl<K: Finalize, V: Finalize> Finalize for BTreeMap<K, V> {}
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impl<T: Finalize> Finalize for BTreeSet<T> {}
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impl<T: Finalize> Finalize for LinkedList<T> {}
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impl<T: Finalize> Finalize for VecDeque<T> {}
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use core::hash::{BuildHasher, Hash};
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impl<K: Eq + Hash + Finalize, V: Finalize, S: BuildHasher> Finalize
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for hashbrown::hash_map::HashMap<K, V, S>
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{
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}
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#[cfg(feature = "std")]
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impl<K: Eq + Hash + Finalize, V: Finalize, S: BuildHasher> Finalize for HashMap<K, V, S> {}
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#[cfg(feature = "std")]
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impl<T: Eq + Hash + Finalize, S: BuildHasher> Finalize for HashSet<T, S> {}
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impl<T: Finalize> Finalize for Cell<Option<T>> {}
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impl<T: Finalize> Finalize for OnceCell<T> {}
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impl<T: ToOwned + Finalize + ?Sized> Finalize for Cow<'static, T> where T::Owned: Finalize {}
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impl<T> Finalize for PhantomData<T> {}

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