Non-'static Lifetimes in Const Generics

[yodaldevoid]

We currently don't correctly handle non-'static lifetimes in const generics. Until #74051 is merged we ICE when a non-'static lifetime is hit, as seen in #60814.

Many uses for const generics does not hit this limitation, but the following case has been brought up:

fn test<'a, const VALUE: std::mem::Discriminant<Enum<'a>>(v: Enum<'a>) -> bool {
     std::mem::discriminant(&v) == VALUE
}

as quoted from @lcnr on Zulip

mem::Discriminant is invariant so VALUE has to be mem::Discriminant of Enum<'a> and can't use Enum<'static>.

@varkor @eddyb @nikomatsakis

[nikomatsakis]

This is "just" an implementation limitation, correct?

[lcnr]

Mostly, it should be theoretically fine, but ends up with similar problems to what #74113 ended up solving.

It also has some open design questions IMO, which I don't want to care about for now 😆

[lcnr]

const generics triage

This is part of generic const parameter types. The design work for that is tracked in rust-lang/project-const-generics#28.