I think we have to strengthen Mutex::init UB

src/libstd/sys_common/mutex.rs

@@ -32,7 +32,7 @@ pub const fn new() -> Mutex { Mutex(imp::Mutex::new()) }
     /// Prepare the mutex for use.
     ///
     /// This should be called once the mutex is at a stable memory address.
-    /// It must not be called concurrently with any other operation.
+    /// Behavior is undefined unless this is called before any other operation.
     #[inline]
     pub unsafe fn init(&mut self) { self.0.init() }