Auto merge of #53108 - RalfJung:mutex, r=alexcrichton

clarify partially initialized Mutex issues

Using a `sys_common::mutex::Mutex` without calling `init` is dangerous, and yet there are some places that do this. I tried to find all of them and add an appropriate comment about reentrancy.

I found two places where (I think) reentrancy can actually occur, and was not able to come up with an argument for why this is okay. Someone who knows `io::lazy` and/or `sys_common::at_exit_imp` should have a careful look at this.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/libstd/thread/mod.rs b/src/libstd/thread/mod.rs
index bbe80df7e8b..61c6084a250 100644
--- a/src/libstd/thread/mod.rs
+++ b/src/libstd/thread/mod.rs
@@ -940,6 +940,8 @@ pub struct ThreadId(u64);
 impl ThreadId {
     // Generate a new unique thread ID.
     fn new() -> ThreadId {
+        // We never call `GUARD.init()`, so it is UB to attempt to
+        // acquire this mutex reentrantly!
         static GUARD: mutex::Mutex = mutex::Mutex::new();
         static mut COUNTER: u64 = 0;