Add SAFETY comments to the thread local implementation

Reduce `unsafe` block scope and add `SAFETY` comments.

library/std/src/thread/local.rs

@@ -281,11 +281,10 @@ impl<T: 'static> LocalKey<T> {
     where
         F: FnOnce(&T) -> R,
     {
-        unsafe {
+        // SAFETY: `inner` is safe to call within the lifetime of the thread
-            let thread_local = (self.inner)(None).ok_or(AccessError)?;
+        let thread_local = unsafe { (self.inner)(None).ok_or(AccessError)? };
         Ok(f(thread_local))
     }
-    }
 
     /// Acquires a reference to the value in this TLS key, initializing it with
     /// `init` if it wasn't already initialized on this thread.
@@ -303,16 +302,19 @@ impl<T: 'static> LocalKey<T> {
     where
         F: FnOnce(Option<T>, &T) -> R,
     {
-        unsafe {
         let mut init = Some(init);
-            let reference = (self.inner)(Some(&mut init)).expect(
+
+        // SAFETY: `inner` is safe to call within the lifetime of the thread
+        let reference = unsafe {
+            (self.inner)(Some(&mut init)).expect(
                 "cannot access a Thread Local Storage value \
                  during or after destruction",
-            );
+            )
+        };
+
         f(init, reference)
     }
 }
-}
 
 impl<T: 'static> LocalKey<Cell<T>> {
     /// Sets or initializes the contained value.