should get_unchecked and get_unchecked_mut be using unreachable!() ?

[oconnor663]

I think the bad branches in those functions are reachable, if the caller passes in a key that's been removed. Should those branches be ordinary panics? Or should those functions maybe return Option<&T> / Option<&mut T>, like the get and get_mut functions do? (Or is it possible these functions don't have any callers in the wild, and we can remove them?)

[carllerche]

If they are reachable, it should probably be changed to a panic instead. That said, unreachable is just a panic.

The functions are here to avoid any extra bounds checking.

[oconnor663]

I wonder if there's a way to be even less safe, by unwrapping the Option without a branch. Maybe we could construct a mem::uninitialized instance of Some(T) (or rather...a Some(mem::uninitialized())?), figure out the pointer offset for the T inside, and mem::forget the "temporary" instance, and the compiler would just optimize all that code away to a constant? I need to learn to read assembly...

[oconnor663]

Relevant: rust-lang/rfcs#1863

[tormol]

Making them less safe can now be done by using std::hint::unreachable_unchecked(), but that requires Rust 1.27.

Currently the compiler isn't inserting any trap (ud2) instructions either.

EDIT: I have a commit implementing this change, but I'm not opening a PR because I don't think this is worth making a breaking change for.