Add manual Sync impl for ReentrantLockGuard

Fixes: #125526

library/std/src/sync/reentrant_lock.rs

@@ -116,6 +116,9 @@ pub struct ReentrantLockGuard<'a, T: ?Sized + 'a> {
 #[unstable(feature = "reentrant_lock", issue = "121440")]
 impl<T: ?Sized> !Send for ReentrantLockGuard<'_, T> {}
 
+#[unstable(feature = "reentrant_lock", issue = "121440")]
+unsafe impl<T: ?Sized + Sync> Sync for ReentrantLockGuard<'_, T> {}
+
 #[unstable(feature = "reentrant_lock", issue = "121440")]
 impl<T> ReentrantLock<T> {
     /// Creates a new re-entrant lock in an unlocked state ready for use.

tests/ui/sync/reentrantlockguard-sync.rs

@@ -0,0 +1,15 @@
+#![feature(reentrant_lock)]
+use std::sync::ReentrantLock;
+use std::cell::Cell;
+
+// ReentrantLockGuard<Cell<i32>> must not be Sync, that would be unsound.
+
+fn test_sync<T: Sync>(_t: T) {}
+
+fn main()
+{
+    let m = ReentrantLock::new(Cell::new(0i32));
+    let guard = m.lock();
+    test_sync(guard);
+    //~^ ERROR `Cell<i32>` cannot be shared between threads safely [E0277]
+}

tests/ui/sync/reentrantlockguard-sync.stderr

@@ -0,0 +1,20 @@
+error[E0277]: `Cell<i32>` cannot be shared between threads safely
+  --> $DIR/reentrantlockguard-sync.rs:13:15
+   |
+LL |     test_sync(guard);
+   |     --------- ^^^^^ `Cell<i32>` cannot be shared between threads safely
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Sync` is not implemented for `Cell<i32>`, which is required by `ReentrantLockGuard<'_, Cell<i32>>: Sync`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI32` instead
+   = note: required for `ReentrantLockGuard<'_, Cell<i32>>` to implement `Sync`
+note: required by a bound in `test_sync`
+  --> $DIR/reentrantlockguard-sync.rs:7:17
+   |
+LL | fn test_sync<T: Sync>(_t: T) {}
+   |                 ^^^^ required by this bound in `test_sync`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.