Auto merge of rust-lang#117138 - zachs18:rwlock_guard_debug_unsized, …

…r=dtolnay

Add T: ?Sized to `RwLockReadGuard` and `RwLockWriteGuard`'s Debug impls.

For context, `MutexGuard` has `+ ?Sized` on its `Debug` impl, and all three have `+ ?Sized` on their `Display` impls.

It looks like the `?Sized` was just missed when the impls were added (the impl for `MutexGuard` was added in the same PR (rust-lang#38006) with support for `T: Debug + ?Sized`, and `RwLock*Guard`s did allow `T: ?Sized` types already); the `Display` impls were added later (rust-lang#42822) with support for `T: Debug + ?Sized` types.

I think this needs a T-libs-api FCP? I'm not sure if this also needs an ACP. If so I can make one.

These are changes to (stable) trait impls on stable types so will be insta-stable.

`@rustbot` label +T-libs-api

library/std/src/sync/rwlock.rs

@@ -532,7 +532,7 @@ impl<'rwlock, T: ?Sized> RwLockWriteGuard<'rwlock, T> {
 }
 
 #[stable(feature = "std_debug", since = "1.16.0")]
-impl<T: fmt::Debug> fmt::Debug for RwLockReadGuard<'_, T> {
+impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLockReadGuard<'_, T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         (**self).fmt(f)
     }
@@ -546,7 +546,7 @@ impl<T: ?Sized + fmt::Display> fmt::Display for RwLockReadGuard<'_, T> {
 }
 
 #[stable(feature = "std_debug", since = "1.16.0")]
-impl<T: fmt::Debug> fmt::Debug for RwLockWriteGuard<'_, T> {
+impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLockWriteGuard<'_, T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         (**self).fmt(f)
     }