mark_sweep: add Gc::cast_ref_unchecked parity API and tests

oscars/src/collectors/mark_sweep/pointers/gc.rs

@@ -93,6 +93,17 @@ impl<T: Trace> Gc<T> {
             marker: PhantomData,
         }
     }
+
+    /// Cast a `&Gc<T>` to `&Gc<U>` without consuming the handle.
+    ///
+    /// # Safety
+    ///
+    /// Caller must ensure that `U` is the correct runtime type for `this`.
+    #[inline]
+    #[must_use]
+    pub unsafe fn cast_ref_unchecked<U: Trace + 'static>(this: &Self) -> &Gc<U> {
+        unsafe { &*(this as *const Self).cast::<Gc<U>>() }
+    }
 }
 
 impl<T: Trace> Gc<T> {

oscars/src/collectors/mark_sweep/tests.rs

@@ -149,6 +149,28 @@ fn ptr_eq_distinguishes_equal_values() {
     );
 }
 
+#[test]
+fn cast_ref_unchecked_preserves_identity_and_value() {
+    let collector = &mut MarkSweepGarbageCollector::default()
+        .with_page_size(256)
+        .with_heap_threshold(512);
+
+    let typed = Gc::new_in(13u32, collector);
+    let erased_as_u64: Gc<u64> = unsafe { Gc::cast_unchecked(typed.clone()) };
+
+    // SAFETY: `erased_as_u64` points to an allocation that actually stores `u32`.
+    let recovered_ref: &Gc<u32> = unsafe { Gc::cast_ref_unchecked(&erased_as_u64) };
+
+    assert_eq!(
+        **recovered_ref, 13u32,
+        "cast_ref_unchecked recovered wrong value"
+    );
+    assert!(
+        Gc::ptr_eq(&typed, recovered_ref),
+        "cast_ref_unchecked should preserve pointer identity"
+    );
+}
+
 #[test]
 fn multi_gc() {
     let collector = &mut MarkSweepGarbageCollector::default()

oscars/src/collectors/mark_sweep_arena2/pointers/gc.rs

@@ -97,6 +97,17 @@ impl<T: Trace> Gc<T> {
             marker: PhantomData,
         }
     }
+
+    /// Cast a `&Gc<T>` to `&Gc<U>` without consuming the handle.
+    ///
+    /// # Safety
+    ///
+    /// Caller must ensure that `U` is the correct runtime type for `this`.
+    #[inline]
+    #[must_use]
+    pub unsafe fn cast_ref_unchecked<U: Trace + 'static>(this: &Self) -> &Gc<U> {
+        unsafe { &*(this as *const Self).cast::<Gc<U>>() }
+    }
 }
 
 impl<T: Trace> Gc<T> {

oscars/src/collectors/mark_sweep_arena2/tests.rs

@@ -160,6 +160,28 @@ fn ptr_eq_distinguishes_equal_values() {
     );
 }
 
+#[test]
+fn cast_ref_unchecked_preserves_identity_and_value() {
+    let collector = &mut MarkSweepGarbageCollector::default()
+        .with_arena_size(256)
+        .with_heap_threshold(512);
+
+    let typed = Gc::new_in(13u32, collector);
+    let erased_as_u64: Gc<u64> = unsafe { Gc::cast_unchecked(typed.clone()) };
+
+    // SAFETY: `erased_as_u64` points to an allocation that actually stores `u32`.
+    let recovered_ref: &Gc<u32> = unsafe { Gc::cast_ref_unchecked(&erased_as_u64) };
+
+    assert_eq!(
+        **recovered_ref, 13u32,
+        "cast_ref_unchecked recovered wrong value"
+    );
+    assert!(
+        Gc::ptr_eq(&typed, recovered_ref),
+        "cast_ref_unchecked should preserve pointer identity"
+    );
+}
+
 #[test]
 fn multi_gc() {
     let collector = &mut MarkSweepGarbageCollector::default()