Make str::ends_with a const fn

[alex]

Currently it can't be called from a const fn, which is sad :-(

Ordinarily I'd take a pass at implementing this myself, but str::ends_with is slightly more complex than meets the eye because of the Pattern abstraction, and I'm not particularly sure how that's best handled.

[oli-obk]

This is very hard! We need if at the least (which has a merged RFC). But we also need rust-lang/rfcs#2345 to abort in the case of bad indices.

I don't think this can be done any time soon and keeping an extra issue open for it is not really much use.

There's also a chance we need a new RFC entirely to be able to do this (due to the internal indexing traits being involved)

Once all the prerequisites are fulfilled we'll definitely do this though.

[alex]

👍 thanks for the quick response!

[FalkWoldmann]

Hi @oli-obk, I hope you don’t mind me bringing this up again. Since the linked PR has been merged and a couple of years have passed, do you think it would now be possible to implement this feature?

[oli-obk]

The groundwork for this is happening right now under the const_traits feature gate: #67792