std::thread: add SAFETY comment

rust-lang · bors · Oct 27, 2023 · Oct 17, 2023 · Oct 26, 2023 · Oct 26, 2023
commit ccb36a688d2b67e8df05142a0f525882dae19709
diff --git a/library/std/src/thread/mod.rs b/library/std/src/thread/mod.rs
@@ -546,6 +546,8 @@ impl Builder {
         }
 
         let main = Box::new(main);
+        // SAFETY: dynamic size and alignment of the Box remain the same. See below for why the
+        // lifetime change is justified.
         #[cfg(bootstrap)]
         let main =
             unsafe { mem::transmute::<Box<dyn FnOnce() + 'a>, Box<dyn FnOnce() + 'static>>(main) };