Add a macro for "unreachable" whose meaning differs in debug and release builds

[kmcallister]

In debug builds it means "I think we can't get here, please fail!() if we do", i.e. the same as the current unreachable!().

In release builds it means "I promise we can't get here, LLVM, you are free to do absolutely anything if we can", i.e. intrinsics::unreachable().

Improper use can totally break memory safety in release builds, so we should make it unsafe-only even in debug builds. For the same reason I don't think the current unreachable!() should change meaning to this. I don't have a great alternative name, which I think is the main obstacle with this ticket!

E-easy E-bikeshed :)

[vojtechkral]

OTOH, if unreachable!() were made unsafe, existing code using it (potentialy incorrectly) would not compile, right? The meaning could be changed and for the "original" purpose an assert could be created, eg. assert_unreachable!() ... which admitedly wouldn't be much different from assert!(false, "..."), but conveys the intent much more clearly.

[smaudet]

Anyone mind if I take a look at this?

[kmcallister]

Perhaps it should be debug_unreachable!() by analogy with debug_assert!() and the like. It has the same meaning of "unchecked in release builds".

[vojtechkral]

This doesn't require an RFC? @kmcallister

edit: @smaudet I'm having a look at it if that's alright by you??? 😁

[steveklabnik]

I'm pulling a massive triage effort to get us ready for 1.0. As part of this, I'm moving stuff that's wishlist-like to the RFCs repo, as that's where major new things should get discussed/prioritized.

This issue has been moved to the RFCs repo: rust-lang/rfcs#698