Comment by Panzerschrek

I am pretty skeptical about the whole idea about adding such exceptions to the single mutable reference rule. It may be safe to share references to simple structs in terms of raw bytes access, but as long as some non-trivial invariants are involved, it can be a source of nasty bugs.

In my programming language I generally don't allow having more than one mutable reference to a variable. The only exception is when two references point to different elements of structs/tuples. This gives some flexibility without sacrificing correctness.