Ignore vtables in {Rc, Arc, Weak}::ptr_eq

We sometimes generate duplicate vtables depending on the number of
codegen units (rust-lang#46139), so we need to exclude the vtable part when
comparing fat pointers.

This is safe for the case of Rc, Arc, and Weak because these always
point to an entire allocation.  These methods can’t be used to
compare, e.g., a struct with one of its fields, a slice with one of
its subslices, or consecutive ZST fields.

Fixes rust-lang#63021.

Signed-off-by: Anders Kaseorg <[email protected]>

library/alloc/src/rc.rs

@@ -1002,7 +1002,9 @@ impl<T: ?Sized> Rc<T> {
     ///
     /// [`ptr::eq`]: core::ptr::eq
     pub fn ptr_eq(this: &Self, other: &Self) -> bool {
-        this.ptr.as_ptr() == other.ptr.as_ptr()
+        // The *const u8 cast discards the vtable to work around
+        // https://github.com/rust-lang/rust/issues/46139
+        this.ptr.as_ptr() as *const u8 == other.ptr.as_ptr() as *const u8
     }
 }
 
@@ -2136,7 +2138,9 @@ impl<T: ?Sized> Weak<T> {
     #[inline]
     #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
     pub fn ptr_eq(&self, other: &Self) -> bool {
-        self.ptr.as_ptr() == other.ptr.as_ptr()
+        // The *const u8 cast discards the vtable to work around
+        // https://github.com/rust-lang/rust/issues/46139
+        self.ptr.as_ptr() as *const u8 == other.ptr.as_ptr() as *const u8
     }
 }
 

library/alloc/src/sync.rs

@@ -1066,7 +1066,9 @@ impl<T: ?Sized> Arc<T> {
     ///
     /// [`ptr::eq`]: core::ptr::eq
     pub fn ptr_eq(this: &Self, other: &Self) -> bool {
-        this.ptr.as_ptr() == other.ptr.as_ptr()
+        // The *const u8 cast discards the vtable to work around
+        // https://github.com/rust-lang/rust/issues/46139
+        this.ptr.as_ptr() as *const u8 == other.ptr.as_ptr() as *const u8
     }
 }
 
@@ -1936,7 +1938,9 @@ impl<T: ?Sized> Weak<T> {
     #[inline]
     #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
     pub fn ptr_eq(&self, other: &Self) -> bool {
-        self.ptr.as_ptr() == other.ptr.as_ptr()
+        // The *const u8 cast discards the vtable to work around
+        // https://github.com/rust-lang/rust/issues/46139
+        self.ptr.as_ptr() as *const u8 == other.ptr.as_ptr() as *const u8
     }
 }
 

library/alloc/tests/arc.rs

@@ -195,3 +195,20 @@ fn shared_from_iter_trustedlen_no_fuse() {
     assert_trusted_len(&iter);
     assert_eq!(&[Box::new(42), Box::new(24)], &*iter.collect::<Rc<[_]>>());
 }
+
+mod other_mod {
+    use std::sync::Arc;
+
+    pub fn cast(r: Arc<()>) -> Arc<dyn Send> {
+        r
+    }
+}
+
+#[test]
+fn ptr_eq_ignores_duplicated_vtable() {
+    let a = Arc::new(());
+    let b = other_mod::cast(Arc::clone(&a));
+    let c: Arc<dyn Send> = a;
+    assert!(Arc::ptr_eq(&b, &c));
+    assert!(Weak::ptr_eq(&Arc::downgrade(&b), &Arc::downgrade(&c)));
+}

library/alloc/tests/rc.rs

@@ -191,3 +191,20 @@ fn shared_from_iter_trustedlen_no_fuse() {
     assert_trusted_len(&iter);
     assert_eq!(&[Box::new(42), Box::new(24)], &*iter.collect::<Rc<[_]>>());
 }
+
+mod other_mod {
+    use std::rc::Rc;
+
+    pub fn cast(r: Rc<()>) -> Rc<dyn Send> {
+        r
+    }
+}
+
+#[test]
+fn ptr_eq_ignores_duplicated_vtable() {
+    let a = Rc::new(());
+    let b = other_mod::cast(Rc::clone(&a));
+    let c: Rc<dyn Send> = a;
+    assert!(Rc::ptr_eq(&b, &c));
+    assert!(Weak::ptr_eq(&Rc::downgrade(&b), &Rc::downgrade(&c)));
+}