Underscore lifetimes are incorrectly accepted as lifetime bounds in impl headers

[scottmcm]

This is currently accepted in 2018: https://play.rust-lang.org/?gist=7ba2dee1134f9848b54b7364ea9568aa&version=nightly&edition=2018

trait Foo<'a> {}
impl<'b: '_> Foo<'b> for i32 {}

AFAICT, it desugars to

impl<'b: 'c, 'c> Foo<'b> for i32 {}

So it's "fine", but useless.

It should error, as trying to do this does in other places, such as

struct Foo<'a: '_>(&'a i32);

Tracking issue for the impl_header_lifetime_elision feature: #15872

On the current beta this gives

error[E0688]: cannot mix in-band and explicit lifetime definitions

so this was at least exposed by #54458

[pnkfelix]

visited for triage. Agreed that P-high and RC2 milestones are appropriate. @nikomatsakis is going to work first on mentorship; if that fails to find a volunter, they will fallback to fixing it themself.

[nikomatsakis]

Actually, I'm not sure if this is an error or not. We currently permit '_ in other, similar contexts.

Example 1 (playground):

trait Foo<'a> { }

struct Bar<T> { t: T }

impl<T: Foo<'_>> Bar<T> { }

fn main() { }

and example 2 (playground):

trait Foo<'a> { }

struct Bar<T> { t: T }

impl<T> Bar<T> where T: Foo<'_> { }

fn main() { }

Are we comfortable with those, but just not this? (I guess because this is clearly useless, whereas those .. may be useful?)

[nikomatsakis]

For some reason, I thought maybe we were not going to permit '_ in those contexts, out of fear that the semantics were not what we might want. In particular, I can imagine that people might expect T: Foo<'_> to be equivalent to T: for<'a> Foo<'a>.