Lifetime in trait method declaration affects RFC 1214 checks

[eefriedman]

trait Test {
    fn f<'a>(&self) where PartialEq<i32> : std::fmt::Display {}
}
trait Test2 {
    fn f<'a>(&self) where (PartialEq<i32> + 'a) : std::fmt::Display {}
}
fn main() {
}

Currently, Test1 triggers an "error", but Test2 doesn't. I would expect them to be treated the same way.

[arielb1]

this is because middle::traits::normalize_param_env both

• registers all predicates as obligations
• adds all non-global predicates (these that don't contain lifetimes or type-variables) to the param-env where-clauses list

for this reason, predicates that contain lifetimes or type parameters can be used to satisfy themselves, while global predicates don't enter the where-clause list, and therefore correctly cause a warning. This is only an RFC1214 error because you did it in a trait method.

Type-less predicates being allowed in trait/impl methods can indeed cause ICEs:

pub struct S;
impl S {
    pub fn f<'a>(&self, f: &'a f32) where u32: PartialEq<&'a f32> {
        4==f;
    }
}
fn main() {}

[steveklabnik]

/cc @rust-lang/lang

[nikomatsakis]

This has to do w/ the early- vs late-bound lifetime distinction. I'd like to ease and remove that distinction, but it'll take more effort to get there.

[arielb1]

@nikomatsakis

No it doesn't. Nothing changes if you force 'a to be early-bound - e.g. by adding &'a (): Sized. This is just an consequence of the global-predicate-ignoring restriction.

@eefriedman was surprised by the distinction in #28046 though.

[nikomatsakis]

@arielb1 ah I see, yes, I stand corrected.

[steveklabnik]

1. @arielb1 's example no longer ICEs.
2. Test2 now compiles, Test does not.

I don't know what this means regarding the bug.

[arielb1]

The ICE was fixed by me in #42797, the "global predicate" thing is still present.