Any value behind a &mut reference can be replaced with a new one using mem::replace or mem::swap, so code shouldn't assume any reachable mutable references can't have their internals changed by replacing.
Rust doesn't guarantee destructors will run when a value is leaked (which can be done with mem::forget), so code should avoid relying on them for maintaining safety. Remember, everyone poops.
It's ok not to run a destructor when a value is leaked because its storage isn't deallocated or repurposed. If the storage is initialized and is being deallocated or repurposed then destructors need to be run first, because memory may be pinned. Having said that, there can still be exceptions for skipping destructors when deallocating if you can guarantee there's never pinning involved.
If there's a Drop impl involved, look out for possible soundness issues that could come from that destructor never running.