Implement GcErase + GcRebrand for GcCell

Add #[zerogc(collector_id(SimpleCollectorId))] annotation
to the binary_trees example.

Author: Techcable

libs/simple/examples/binary_trees.rs

@@ -2,12 +2,13 @@
     arbitrary_self_types, // Unfortunately this is required for methods on Gc refs
 )]
 use zerogc::prelude::*;
-use zerogc_simple::{SimpleCollector, SimpleCollectorContext, Gc};
+use zerogc_simple::{SimpleCollector, SimpleCollectorContext, Gc, CollectorId as SimpleCollectorId};
 use zerogc_derive::Trace;
 
 use slog::{Logger, Drain, o};
 
 #[derive(Trace)]
+#[zerogc(collector_id(SimpleCollectorId))]
 struct Tree<'gc> {
     #[zerogc(mutable(public))]
     children: GcCell<Option<(Gc<'gc, Tree<'gc>>, Gc<'gc, Tree<'gc>>)>>,

src/cell.rs

@@ -15,7 +15,7 @@
 //! and it'll generate a safe wrapper.
 use core::cell::Cell;
 
-use crate::{GcSafe, Trace, GcVisitor, NullTrace, TraceImmutable, GcDirectBarrier,};
+use crate::{GcSafe, Trace, GcVisitor, NullTrace, TraceImmutable, GcDirectBarrier, CollectorId, GcErase, GcRebrand};
 
 /// A `Cell` pointing to a garbage collected object.
 ///
@@ -104,3 +104,13 @@ unsafe impl<T: GcSafe + Copy> GcSafe for GcCell<T> {
     /// Since T is Copy, we shouldn't need to be dropped
     const NEEDS_DROP: bool = false;
 }
+unsafe impl<'min, T, Id> GcErase<'min, Id> for GcCell<T>
+    where T: Trace + Copy + GcErase<'min, Id>, Id: CollectorId,
+          T::Erased: Copy + Trace {
+    type Erased = GcCell<T::Erased>;
+}
+unsafe impl<'new_gc, T, Id> GcRebrand<'new_gc, Id> for GcCell<T>
+    where T: Trace + Copy + GcRebrand<'new_gc, Id>, Id: CollectorId,
+          T::Branded: Copy + Trace {
+    type Branded = GcCell<T::Branded>;
+}