Rollup merge of rust-lang#81873 - mark-i-m:unlock, r=m-ou-se

Add Mutex::unlock

Tracking issue: rust-lang#81872

Discussion: rust-lang#79434 (comment)

r? `@m-ou-se`

library/std/src/sync/mutex.rs

@@ -219,6 +219,26 @@ impl<T> Mutex<T> {
             data: UnsafeCell::new(t),
         }
     }
+
+    /// Immediately drops the guard, and consequently unlocks the mutex.
+    ///
+    /// This function is equivalent to calling [`drop`] on the guard but is more self-documenting.
+    /// Alternately, the guard will be automatically dropped when it goes out of scope.
+    ///
+    /// ```
+    /// #![feature(mutex_unlock)]
+    ///
+    /// use std::sync::Mutex;
+    /// let mutex = Mutex::new(0);
+    ///
+    /// let mut guard = mutex.lock().unwrap();
+    /// *guard += 20;
+    /// Mutex::unlock(guard);
+    /// ```
+    #[unstable(feature = "mutex_unlock", issue = "81872")]
+    pub fn unlock(guard: MutexGuard<'_, T>) {
+        drop(guard);
+    }
 }
 
 impl<T: ?Sized> Mutex<T> {