clarify partially initialized Mutex issues

1 files changed, 3 insertions, 0 deletions

diff --git a/src/libstd/sys_common/thread_local.rs b/src/libstd/sys_common/thread_local.rs
index 75f6b9ac7fd..2cc5372e268 100644
--- a/src/libstd/sys_common/thread_local.rs
+++ b/src/libstd/sys_common/thread_local.rs
@@ -161,6 +161,9 @@ impl StaticKey {
         // Additionally a 0-index of a tls key hasn't been seen on windows, so
         // we just simplify the whole branch.
         if imp::requires_synchronized_create() {
+            // `INIT_LOCK` is never initialized fully, so this mutex is reentrant!
+            // Do not use it in a way that might be reentrant, that could lead to
+            // aliasing `&mut`.
             static INIT_LOCK: Mutex = Mutex::new();
             let _guard = INIT_LOCK.lock();
             let mut key = self.key.load(Ordering::SeqCst);