Auto merge of #51529 - nodakai:improve-sys_common-mutex, r=oli-obk

libstd: add an RAII utility for sys_common::mutex::Mutex

It is indeed debatable whether or not we should introduce more sophistication like this to the lowest layer of a system library. In fact, `Drop::drop()` cannot be `unsafe` (IIRC there was a discussion on introducing an unsafe variant of `Drop` whose entire scope must be within `unsafe`)

1 files changed, 1 insertions, 4 deletions

diff --git a/src/libstd/thread/mod.rs b/src/libstd/thread/mod.rs
index 1b976b79b4c..1dacf99b64b 100644
--- a/src/libstd/thread/mod.rs
+++ b/src/libstd/thread/mod.rs
@@ -935,20 +935,17 @@ impl ThreadId {
         static mut COUNTER: u64 = 0;
 
         unsafe {
-            GUARD.lock();
+            let _guard = GUARD.lock();
 
             // If we somehow use up all our bits, panic so that we're not
             // covering up subtle bugs of IDs being reused.
             if COUNTER == ::u64::MAX {
-                GUARD.unlock();
                 panic!("failed to generate unique thread ID: bitspace exhausted");
             }
 
             let id = COUNTER;
             COUNTER += 1;
 
-            GUARD.unlock();
-
             ThreadId(id)
         }
     }