Implement null collector for lifetime branded GC

oscars/Cargo.toml

@@ -37,4 +37,5 @@ mark_sweep = []
 mark_sweep2 = ["mark_sweep"]
 mark_sweep_branded = ["mark_sweep"]
 null_collector = ["mark_sweep"]
+null_collector_branded = ["mark_sweep"]
 thin-vec = ["dep:thin-vec", "mark_sweep"]

oscars/src/collectors/mark_sweep_branded/tests/ui/gc_cannot_escape_mutate.stderr

@@ -4,6 +4,6 @@ error: lifetime may not live long enough
 14 |         let _escaped = ctx.mutate(|cx| {
    |                                    --- return type of closure is oscars::collectors::mark_sweep_branded::Gc<'2, i32>
    |                                    |
-   |                                    has type `&MutationContext<'_, '1>`
+   |                                    has type `&oscars::collectors::mark_sweep_branded::MutationContext<'_, '1>`
 15 |             cx.try_alloc(42i32).unwrap() // ERROR: Gc<'gc, i32> cannot escape mutate()
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ returning this value requires that `'1` must outlive `'2`

oscars/src/collectors/mark_sweep_branded/tests/ui/root_cross_context.stderr

@@ -4,7 +4,7 @@ error: lifetime may not live long enough
 13 |     with_gc(|ctx1| {
    |              ---- lifetime `'2` appears in the type of `ctx1`
 14 |         with_gc(|ctx2| {
-   |                  ---- has type `GcContext<'1>`
+   |                  ---- has type `oscars::collectors::mark_sweep_branded::GcContext<'1>`
 15 |             // root carries 'id of ctx1
 16 |             let root = ctx1.mutate(|cx| cx.root(cx.try_alloc(123i32).unwrap()).unwrap());
    |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ returning this value requires that `'1` must outlive `'2`
@@ -13,7 +13,7 @@ error: lifetime may not live long enough
   --> src/collectors/mark_sweep_branded/tests/ui/root_cross_context.rs:16:41
    |
 13 |     with_gc(|ctx1| {
-   |              ---- has type `GcContext<'1>`
+   |              ---- has type `oscars::collectors::mark_sweep_branded::GcContext<'1>`
 ...
 16 |             let root = ctx1.mutate(|cx| cx.root(cx.try_alloc(123i32).unwrap()).unwrap());
    |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ returning this value requires that `'1` must outlive `'static`

oscars/src/collectors/mod.rs

@@ -8,6 +8,8 @@ pub mod mark_sweep_arena2;
 #[cfg(feature = "null_collector")]
 pub mod null_collector;
 
-// TODO: Implement a null collector for the branded API as well
+#[cfg(feature = "null_collector_branded")]
+pub mod null_collector_branded;
+
 #[cfg(feature = "mark_sweep_branded")]
 pub mod mark_sweep_branded;

oscars/src/collectors/null_collector_branded/cell.rs

@@ -0,0 +1,68 @@
+use crate::collectors::null_collector_branded::trace::{Finalize, Trace, Tracer};
+use core::cell::{Ref, RefCell, RefMut};
+use core::ops::{Deref, DerefMut};
+
+/// GC aware wrapper around [`RefCell<T>`]
+pub struct GcRefCell<T: Trace> {
+    inner: RefCell<T>,
+}
+
+impl<T: Trace> GcRefCell<T> {
+    pub fn new(value: T) -> Self {
+        Self {
+            inner: RefCell::new(value),
+        }
+    }
+
+    /// Acquires a shared borrow of the inner value.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the value is currently mutably borrowed.
+    pub fn borrow(&self) -> GcRef<'_, T> {
+        GcRef(self.inner.borrow())
+    }
+
+    /// Acquires a mutable borrow of the inner value.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the value is currently borrowed.
+    pub fn borrow_mut(&self) -> GcRefMut<'_, T> {
+        GcRefMut(self.inner.borrow_mut())
+    }
+}
+
+/// Shared borrow guard returned by [`GcRefCell::borrow`]
+pub struct GcRef<'a, T: Trace>(Ref<'a, T>);
+
+impl<T: Trace> Deref for GcRef<'_, T> {
+    type Target = T;
+    fn deref(&self) -> &T {
+        &self.0
+    }
+}
+
+/// A mutable borrow guard returned by [`GcRefCell::borrow_mut`]
+pub struct GcRefMut<'a, T: Trace>(RefMut<'a, T>);
+
+impl<T: Trace> Deref for GcRefMut<'_, T> {
+    type Target = T;
+    fn deref(&self) -> &T {
+        &self.0
+    }
+}
+
+impl<T: Trace> DerefMut for GcRefMut<'_, T> {
+    fn deref_mut(&mut self) -> &mut T {
+        &mut self.0
+    }
+}
+
+impl<T: Trace> Finalize for GcRefCell<T> {}
+
+impl<T: Trace> Trace for GcRefCell<T> {
+    fn trace(&mut self, tracer: &mut Tracer) {
+        self.inner.get_mut().trace(tracer);
+    }
+}

oscars/src/collectors/null_collector_branded/ephemeron.rs

@@ -0,0 +1,45 @@
+use crate::{
+    alloc::mempool3::PoolPointer,
+    collectors::null_collector_branded::{
+        gc::Gc,
+        gc_box::GcBox,
+        mutation_ctx::MutationContext,
+        trace::{Finalize, Trace, Tracer},
+    },
+};
+use core::marker::PhantomData;
+
+pub struct Ephemeron<'id, K: Trace, V: Trace> {
+    pub(crate) key_ptr: Option<PoolPointer<'static, GcBox<K>>>,
+    pub(crate) value_ptr: PoolPointer<'static, GcBox<V>>,
+    pub(crate) _marker: PhantomData<*mut &'id ()>,
+}
+
+impl<'id, K: Trace, V: Trace> Ephemeron<'id, K, V> {
+    /// Returns the value if the key is alive.
+    pub fn get_value<'gc>(&self, _cx: &MutationContext<'id, 'gc>) -> Option<Gc<'gc, V>> {
+        // In the null collector, everything stays alive until context drops.
+        if self.key_ptr.is_some() {
+            Some(Gc {
+                ptr: self.value_ptr,
+                _marker: PhantomData,
+            })
+        } else {
+            None
+        }
+    }
+}
+
+impl<'id, K: Trace, V: Trace> Clone for Ephemeron<'id, K, V> {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl<'id, K: Trace, V: Trace> Copy for Ephemeron<'id, K, V> {}
+
+impl<'id, K: Trace, V: Trace> Finalize for Ephemeron<'id, K, V> {}
+
+impl<'id, K: Trace, V: Trace> Trace for Ephemeron<'id, K, V> {
+    fn trace(&mut self, _tracer: &mut Tracer) {}
+}

oscars/src/collectors/null_collector_branded/gc.rs

@@ -0,0 +1,55 @@
+//! Core pointer types.
+
+use crate::{
+    alloc::mempool3::PoolPointer,
+    collectors::null_collector_branded::{
+        gc_box::GcBox,
+        trace::{Finalize, Trace},
+    },
+};
+use core::fmt;
+use core::marker::PhantomData;
+use core::ops::Deref;
+
+/// Transient pointer to a GC managed value.
+#[derive(Debug)]
+pub struct Gc<'gc, T: Trace + ?Sized + 'gc> {
+    pub(crate) ptr: PoolPointer<'static, GcBox<T>>,
+    pub(crate) _marker: PhantomData<(&'gc T, *const ())>,
+}
+
+impl<'gc, T: Trace + ?Sized + 'gc> Copy for Gc<'gc, T> {}
+impl<'gc, T: Trace + ?Sized + 'gc> Clone for Gc<'gc, T> {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl<'gc, T: Trace + 'gc> Gc<'gc, T> {
+    /// Returns a shared reference to the value.
+    #[inline]
+    pub fn get(&self) -> &T {
+        // SAFETY: `ptr` is non-null and valid for `'gc` by construction.
+        unsafe { &(*self.ptr.as_ptr().as_ptr()).0.value }
+    }
+}
+
+impl<'gc, T: Trace + fmt::Display + 'gc> fmt::Display for Gc<'gc, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt(self.get(), f)
+    }
+}
+
+impl<'gc, T: Trace + 'gc> Deref for Gc<'gc, T> {
+    type Target = T;
+    fn deref(&self) -> &T {
+        self.get()
+    }
+}
+
+impl<T: Trace> Finalize for Gc<'_, T> {}
+impl<T: Trace> Trace for Gc<'_, T> {
+    fn trace(&mut self, tracer: &mut crate::collectors::null_collector_branded::trace::Tracer) {
+        tracer.mark(self);
+    }
+}

oscars/src/collectors/null_collector_branded/gc_box.rs

@@ -0,0 +1,22 @@
+use core::ptr::NonNull;
+
+use crate::alloc::mempool3::PoolAllocator;
+
+pub(crate) type DropFn = unsafe fn(&mut PoolAllocator<'static>, NonNull<u8>);
+
+/// Heap wrapper for a garbage collected value.
+///
+/// Allocated via [`PoolAllocator`]
+pub(crate) struct GcBox<T: ?Sized> {
+    /// Type erased finalize and free fn
+    pub(crate) drop_fn: DropFn,
+    /// User value
+    pub(crate) value: T,
+}
+
+impl<T> GcBox<T> {
+    /// Create a [`GcBox`] for `value`
+    pub(crate) fn new(value: T, drop_fn: DropFn) -> Self {
+        Self { drop_fn, value }
+    }
+}

oscars/src/collectors/null_collector_branded/mod.rs

@@ -0,0 +1,127 @@
+//! Lifetime branded null GC
+#![cfg_attr(not(any(test, feature = "std")), allow(unused_imports))]
+
+pub mod cell;
+pub mod ephemeron;
+pub mod gc;
+pub mod gc_box;
+pub mod mutation_ctx;
+pub mod root;
+pub mod trace;
+pub mod weak;
+
+pub use cell::GcRefCell;
+pub use ephemeron::Ephemeron;
+pub use gc::Gc;
+pub use mutation_ctx::MutationContext;
+pub use root::Root;
+pub use trace::{Finalize, Trace, Tracer};
+pub use weak::WeakGc;
+
+use crate::alloc::mempool3::{PoolAllocError, PoolAllocator};
+use core::cell::RefCell;
+use core::marker::PhantomData;
+use core::ptr::NonNull;
+use gc_box::{DropFn, GcBox};
+use rust_alloc::vec::Vec;
+
+pub(crate) struct Collector {
+    // SAFETY: We use 'static here because the PoolAllocator owns its memory,
+    // and we ensure that `Gc` objects and pool allocations do not outlive
+    // the `Collector` instance
+    pub(crate) pool: RefCell<PoolAllocator<'static>>,
+}
+
+impl Collector {
+    fn new() -> Self {
+        Self {
+            pool: RefCell::new(PoolAllocator::default()),
+        }
+    }
+
+    /// Allocates a value from the pool.
+    pub(crate) fn try_alloc<'gc, T: trace::Trace + trace::Finalize + 'gc>(
+        &'gc self,
+        value: T,
+    ) -> Result<Gc<'gc, T>, PoolAllocError> {
+        unsafe fn drop_and_free<T: trace::Trace + trace::Finalize>(
+            pool: &mut PoolAllocator<'static>,
+            ptr: NonNull<u8>,
+        ) {
+            use crate::alloc::mempool3::PoolItem;
+            unsafe {
+                let typed_ptr = ptr.cast::<PoolItem<GcBox<T>>>();
+                (*typed_ptr.as_ptr()).0.value.finalize();
+                core::ptr::drop_in_place(typed_ptr.as_ptr());
+                pool.free_slot(ptr);
+            }
+        }
+
+        let mut pool = self.pool.borrow_mut();
+        let ptr = pool.try_alloc(GcBox::new(value, drop_and_free::<T>))?;
+
+        drop(pool);
+
+        Ok(Gc {
+            ptr: unsafe { ptr.extend_lifetime() },
+            _marker: PhantomData,
+        })
+    }
+
+    /// Runs a collection cycle (no-op for null collector)
+    pub(crate) fn collect(&self) {}
+}
+
+impl Drop for Collector {
+    /// Frees all remaining allocations
+    fn drop(&mut self) {
+        use crate::alloc::mempool3::PoolItem;
+
+        // Free all GC allocations
+        let all: Vec<(NonNull<u8>, DropFn)> = self
+            .pool
+            .borrow()
+            .iter_live_slots()
+            .map(|ptr| unsafe {
+                let drop_fn = (*ptr.cast::<PoolItem<GcBox<()>>>().as_ptr()).0.drop_fn;
+                (ptr, drop_fn)
+            })
+            .collect();
+        let mut pool = self.pool.borrow_mut();
+        for (ptr, drop_fn) in all {
+            unsafe {
+                (drop_fn)(&mut pool, ptr);
+            }
+        }
+    }
+}
+
+/// Owns the GC and carries the `'id` context brand
+pub struct GcContext<'id> {
+    collector: Collector,
+    _marker: PhantomData<*mut &'id ()>,
+}
+
+impl<'id> GcContext<'id> {
+    /// Opens a mutation window and passes a [`MutationContext`] to `f`
+    /// Triggers a gc cycle
+    pub fn collect(&self) {
+        self.collector.collect();
+    }
+
+    pub fn mutate<R>(&self, f: impl for<'gc> FnOnce(&MutationContext<'id, 'gc>) -> R) -> R {
+        let cx = MutationContext {
+            collector: &self.collector,
+            _marker: PhantomData,
+        };
+        f(&cx)
+    }
+}
+
+/// Create new GC context
+pub fn with_gc<R, F: for<'id> FnOnce(GcContext<'id>) -> R>(f: F) -> R {
+    f(GcContext {
+        collector: Collector::new(),
+        _marker: PhantomData,
+    })
+}