What I mean is that unsafe code allows you to eg break std invariants (set invalid length to Vec, unlock a locked mutex, etc.) in ways that can be UB, which of

What I mean is that unsafe code allows you to eg break std invariants (set invalid length to Vec, unlock a locked mutex, etc.) in ways that can be UB, which of these are UB and which can be used "safely" is sort of unknown. For better or worse they are much more well understood in C as they are front and center of the language semantics