Unconstrained lifetimes permitted in assoc. type as long as they are in a projection

[nikomatsakis]

This makes things unsound. Here is an example from @eddyb:

pub trait MakeRef<'a> {
    type Ref;
}
impl<'a, T: 'a> MakeRef<'a> for T {
    type Ref = &'a T;
}

pub trait MakeRef2 {
    type Ref2;
}
impl<'a, T> MakeRef2 for T {
    type Ref2 = <T as MakeRef<'a>>::Ref;
}

fn foo() -> <String as MakeRef2>::Ref2 { &String::from("foo") }

fn main() {
    println!("{}", foo());
}

This should be prohibited by https://github.com/rust-lang/rfcs/blob/master/text/0447-no-unused-impl-parameters.md, but it seems there is some bug.

[nikomatsakis]

triage: P-high

[eddyb]

cc @james-darkfox (who tripped on this and #29859 in his lense crate)

[arielb1]

This is just stupid. I made ctp::parameters_for_type return the set of parameters that are constrained by a type, but we are also interested in the basis of parameters that determine a type, and I missed that that logic also uses the method. I will issue a patch.

[arielb1]

I really should try to stop having my bugfixes having bugs.