From a7d573562c7876f9d97416d20ad3b69a2cd44acb Mon Sep 17 00:00:00 2001
From: Anders Kaseorg <andersk@mit.edu>
Date: Wed, 30 Dec 2020 02:13:46 -0800
Subject: [PATCH] Ignore fat pointer vtables for Gc::ptr_eq
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Rust doesn’t guarantee that fat pointer comparison works as expected
due to duplicated vtables:

https://github.com/rust-lang/rust/issues/46139

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
---
 gc/src/gc.rs  | 9 ++++++++-
 gc/src/lib.rs | 2 +-
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/gc/src/gc.rs b/gc/src/gc.rs
index d2a728d..a841cb2 100644
--- a/gc/src/gc.rs
+++ b/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();
diff --git a/gc/src/lib.rs b/gc/src/lib.rs
index bf99b72..efc6e8b 100644
--- a/gc/src/lib.rs
+++ b/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())
     }
 }