Rollup merge of rust-lang#129673 - matthewpipie:arc-weak-debug-trait,…

… r=dtolnay

Add fmt::Debug to sync::Weak<T, A>

Currently, `sync::Weak<T>` implements `Debug`, but `sync::Weak<T, A>` does not. This appears to be an oversight, as `rc::Weak<T, A>` implements `Debug`. (Note: `sync::Weak` is the weak for `Arc`, and `rc::Weak` is the weak for `Rc`.)

This PR adds the Debug trait for `sync::Weak<T, A>`. The issue was initially brought up here: rust-lang/wg-allocators#131

library/alloc/src/sync.rs

@@ -335,7 +335,7 @@ impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Weak<U, A>> f
 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
 
 #[stable(feature = "arc_weak", since = "1.4.0")]
-impl<T: ?Sized> fmt::Debug for Weak<T> {
+impl<T: ?Sized, A: Allocator> fmt::Debug for Weak<T, A> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "(Weak)")
     }