> The parent comment correctly explained that it would be incorrect for a Mutex<A> to hand out unique references &mut A to multiple call frames simultaneously.

> The parent comment correctly explained that it would be incorrect for a Mutex<A> to hand out unique references &mut A to multiple call frames simultaneously. If it did, one of them could be passed to another thread and accessed concurrently with the other one to create race conditions. FWIW they wouldn't even need to be moved between threads. IIRC creating two independent &mut to the same object is one of the instant UBs, it already is an invalid program state.