Rollup merge of rust-lang#126539 - lukaslueg:patch-1, r=jhpratt

Update `Arc::try_unwrap()` docs

Clarify the language wrt "race condition" not meaning "memory unsafety".

The docs make an important point about a 'logical' race condition that can occur if the `Err`-case in `Arc::try_unwrap()` is not handled properly. The language as is uses the term "race condition", which the reader may associate with "memory unsafety". This PR tries to clarify the scenario and qualify "race condition without memory unsafety".

Author: jhpratt

alloc/src/sync.rs

@@ -940,15 +940,18 @@ impl<T, A: Allocator> Arc<T, A> {
     /// This will succeed even if there are outstanding weak references.
     ///
     /// It is strongly recommended to use [`Arc::into_inner`] instead if you don't
-    /// want to keep the `Arc` in the [`Err`] case.
-    /// Immediately dropping the [`Err`] payload, like in the expression
-    /// `Arc::try_unwrap(this).ok()`, can still cause the strong count to
-    /// drop to zero and the inner value of the `Arc` to be dropped:
-    /// For instance if two threads each execute this expression in parallel, then
-    /// there is a race condition. The threads could first both check whether they
-    /// have the last clone of their `Arc` via `Arc::try_unwrap`, and then
-    /// both drop their `Arc` in the call to [`ok`][`Result::ok`],
-    /// taking the strong count from two down to zero.
+    /// keep the `Arc` in the [`Err`] case.
+    /// Immediately dropping the [`Err`]-value, as the expression
+    /// `Arc::try_unwrap(this).ok()` does, can cause the strong count to
+    /// drop to zero and the inner value of the `Arc` to be dropped.
+    /// For instance, if two threads execute such an expression in parallel,
+    /// there is a race condition without the possibility of unsafety:
+    /// The threads could first both check whether they own the last instance
+    /// in `Arc::try_unwrap`, determine that they both do not, and then both
+    /// discard and drop their instance in the call to [`ok`][`Result::ok`].
+    /// In this scenario, the value inside the `Arc` is safely destroyed
+    /// by exactly one of the threads, but neither thread will ever be able
+    /// to use the value.
     ///
     /// # Examples
     ///