GAT + enum: incorrect unused lifetime error

[dhardy]

This is a follow up on #67089, which now works. The following doesn't (play link):

#![feature(generic_associated_types)]

trait A {
    type B<'a>;
    
    fn make_b<'a>(&'a self) -> Self::B<'a>;
}

struct S {}

impl A for S {
    type B<'a> = &'a S;
    fn make_b<'a>(&'a self) -> &'a Self {
        self
    }
}

enum E<'a> {
    S(<S as A>::B<'a>),
}

Error is:

18 | enum E<'a> {
   |        ^^ unused parameter

however, this lifetime clearly is used and is required.

[Centril]

The variance code in src/librustc_typeck/variance/constraints.rs is:

            ty::Projection(ref data) => {
                let tcx = self.tcx();
                self.add_constraints_from_trait_ref(current, data.trait_ref(tcx), variance);
            }

It seems to me that this is disregarding the substitutions for ::B<'a> if I'm understanding what .trait_ref(tcx) does here wrt. the substitutions (this pretty poorly documented right now).

cc @matthewjasper

[dhardy]

BTW how long do we need to keep this warning around? I understand that this is basically a given when using nightly features. (It puts me off using it in my normal build path — but presumably that's exactly what is wanted, in order to detect bugs like the one reported here.)

warning: the feature `generic_associated_types` is incomplete and may cause the compiler to crash

[Centril]

We only use INCOMPLETE_FEATURES for those that have been ICE-prone in the past or are generally not in a "considered implemented modulo bugs" state. It's quite a small set of features that are incomplete like this. GATs specifically are to my knowledge nowhere near ready to be "implemented modulo bugs".

[dhardy]

On this topic, the following builds,

trait Foo {}
impl<'a, T: A> Foo for T
where <T as A>::B<'a>: std::fmt::Debug
{}

but the bound <T as A>::B<'_>: std::fmt::Debug fails with:

error[E0637]: `'_` cannot be used here

I *think* this should work?

[dhardy]

Also, the following fails:

trait Foo {}
impl<'a, T: A> Foo for T
where <T as A>::B<'a>: 'static
{}

with:

error[E0310]: the associated type `<T as A>::B<'_>` may not live long enough

This bound may not make much sense (since there is another lifetime parameter), but I believe it should be valid to specify here.

[Dirbaio]

I'm running into a similar issue which I believe is the same as this: (play)

trait Bar{ 
    type Foo<'a>: 'a;
}

struct What<'a, T: Bar> {
    foo: T::Foo<'a>,
}