Permit unused generic const parameters in type definitions

[varkor]

#![feature(const_generics)]

struct A<const N: usize>;

should compile, as there are no variance issues with respect to const parameters. (See #44580 (comment).)

[cramertj]

Technically we could do this with 'static types as well, right? struct A<T: 'static>; also doesn't have any variance issues that I can think of.

[jonas-schievink]

Related: #59875

[Centril]

@cramertj We can do it sensibly for all type parameters by assuming that PhantomData<T> is there for all unused type parameters T (which gives rise to covariance & owned dropck behavior & ...). But that's a larger discussion.

[cramertj]

@Centril sorry, that's a different thing-- that's assuming a variance. I'm talking about allowing it where we don't have to assume a variance because variance can't matter.

[hanna-kruppe]

But variance is not the only thing determined by how the type parameter is used, it also affects dropck, auto traits, and possibly more that I'm forgetting right now. So T: 'static doesn't remove the need to "assume" things when T is unused.

[cramertj]

Good point! :)