Dependong on your API there may be headers that can help you.

This talk has some info on that if memory serves me well https://m.youtube.com/watch?v=BIguvia6AvM

Because when T is !Sized, the layout is different, it looks more like the layout of &[T].

This is outdatded. Mutexes don’t allocate anymore. That’s how Mutex::new can be const.