Ignore fat pointer vtables for Gc::ptr_eq

Rust doesn’t guarantee that fat pointer comparison works as expected
due to duplicated vtables:

rust-lang/rust#46139

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

gc/src/gc.rs

@@ -1,7 +1,7 @@
 use crate::trace::{Finalize, Trace};
 use std::cell::{Cell, RefCell};
 use std::mem;
-use std::ptr::NonNull;
+use std::ptr::{self, NonNull};
 
 const INITIAL_THRESHOLD: usize = 100;
 
@@ -119,6 +119,13 @@ impl<T: Trace> GcBox<T> {
 }
 
 impl<T: Trace + ?Sized> GcBox<T> {
+ /// Returns `true` if the two references refer to the same `GcBox`.
+ pub(crate) fn ptr_eq(this: &GcBox<T>, other: &GcBox<T>) -> bool {
+ // Use .header to ignore fat pointer vtables, to work around
+ // https://github.com/rust-lang/rust/issues/46139
+ ptr::eq(&this.header, &other.header)
+ }
+
  /// Marks this `GcBox` and marks through its data.
  pub(crate) unsafe fn trace_inner(&self) {
  let marked = self.header.marked.get();

gc/src/lib.rs

@@ -90,7 +90,7 @@ impl<T: Trace> Gc<T> {
 impl<T: Trace + ?Sized> Gc<T> {
  /// Returns `true` if the two `Gc`s point to the same allocation.
  pub fn ptr_eq(this: &Gc<T>, other: &Gc<T>) -> bool {
- ptr::eq(this.inner(), other.inner())
+ GcBox::ptr_eq(this.inner(), other.inner())
  }
 }