Disallow missing unsafe blocks in unsafe fn in panicking.rs

This adds SAFETY comments where necessary, explaining the preconditions
and how they are respected.

library/std/src/thread/local.rs

@@ -172,7 +172,11 @@ macro_rules! __thread_local_inner {
                 static __KEY: $crate::thread::__OsLocalKeyInner<$t> =
                     $crate::thread::__OsLocalKeyInner::new();
 
-                __KEY.get(__init)
+                // FIXME: remove the #[allow(...)] marker when macros don't
+                // raise warning for missing/extraneous unsafe blocks anymore.
+                // See https://github.com/rust-lang/rust/issues/74838.
+                #[allow(unused_unsafe)]
+                unsafe { __KEY.get(__init) }
             }
 
             unsafe {