Allow Rootable impls to have ?Sized Root types.

src/gc-arena/src/arena.rs

@@ -66,7 +66,7 @@ impl ArenaParameters {
 /// `Rootable<'a>` for *any* possible `'a`. This is necessary so that the `Root` types can be
 /// branded by the unique, invariant lifetimes that makes an `Arena` sound.
 pub trait Rootable<'a> {
-    type Root: Collect + 'a;
+    type Root: ?Sized + Collect + 'a;
 }
 
 /// A convenience macro for quickly creating type that implements of `Rootable`.
@@ -139,13 +139,19 @@ pub type Root<'a, R> = <R as Rootable<'a>>::Root;
 /// this way, incremental garbage collection can be achieved (assuming "sufficiently small" calls
 /// to `mutate`) that is both extremely safe and zero overhead vs what you would write in C with raw
 /// pointers and manually ensuring that invariants are held.
-pub struct Arena<R: for<'a> Rootable<'a> + ?Sized> {
+pub struct Arena<R: for<'a> Rootable<'a> + ?Sized>
+where
+    Root<'static, R>: Sized,
+{
     // We rely on the implicit drop order here, `root` *must* be dropped before `context`!
     root: Root<'static, R>,
     context: Context,
 }
 
-impl<R: for<'a> Rootable<'a> + ?Sized> Arena<R> {
+impl<R: for<'a> Rootable<'a> + ?Sized> Arena<R>
+where
+    Root<'static, R>: Sized,
+{
     /// Create a new arena with the given garbage collector tuning parameters. You must provide a
     /// closure that accepts a `MutationContext` and returns the appropriate root.
     pub fn new<F>(arena_parameters: ArenaParameters, f: F) -> Arena<R>
@@ -216,7 +222,10 @@ impl<R: for<'a> Rootable<'a> + ?Sized> Arena<R> {
     pub fn map_root<R2: for<'a> Rootable<'a> + ?Sized>(
         self,
         f: impl for<'gc> FnOnce(MutationContext<'gc, '_>, Root<'gc, R>) -> Root<'gc, R2>,
-    ) -> Arena<R2> {
+    ) -> Arena<R2>
+    where
+        Root<'static, R2>: Sized,
+    {
         self.context.root_barrier();
         let new_root: Root<'static, R2> = unsafe { f(self.context.mutation_context(), self.root) };
         Arena {
@@ -229,7 +238,10 @@ impl<R: for<'a> Rootable<'a> + ?Sized> Arena<R> {
     pub fn try_map_root<R2: for<'a> Rootable<'a> + ?Sized, E>(
         self,
         f: impl for<'gc> FnOnce(MutationContext<'gc, '_>, Root<'gc, R>) -> Result<Root<'gc, R2>, E>,
-    ) -> Result<Arena<R2>, E> {
+    ) -> Result<Arena<R2>, E>
+    where
+        Root<'static, R2>: Sized,
+    {
         self.context.root_barrier();
         let new_root: Root<'static, R2> = unsafe { f(self.context.mutation_context(), self.root)? };
         Ok(Arena {

src/gc-arena/tests/tests.rs

@@ -415,6 +415,10 @@ fn test_dynamic_roots() {
         )
     });
 
+    let root3 = arena.mutate(|mc, root_set| {
+        root_set.stash::<Rootable![[u8]]>(mc, unsize!(Gc::allocate(mc, [1, 2, 3]) => [u8]))
+    });
+
     arena.collect_all();
     arena.collect_all();
 
@@ -427,10 +431,14 @@ fn test_dynamic_roots() {
         let root2 = root_set.fetch(&root2);
         assert_eq!(*root2.0, 27);
         assert_eq!(*root2.1, true);
+
+        let root3 = root_set.fetch::<Rootable![[u8]]>(&root3);
+        assert_eq!(*root3, [1, 2, 3]);
     });
 
     drop(root1);
     drop(root2);
+    drop(root3);
 
     arena.collect_all();
     arena.collect_all();