non_lifetime_binders falsely recognizes (and rejects) trait as lifetime

[LastExceed]

minimal repro:

#![allow(incomplete_features)]
#![feature(non_lifetime_binders)]

trait MyTrait1 {}

trait MyTrait2
    where for <T: MyTrait1> ():
{
    
}

output:

error: lifetime bounds cannot be used in this context
 --> src/lib.rs:7:19
  |
7 |     where for <T: MyTrait1> ():
  |                   ^^^^^^^^

use case:

[playground]
weird example, i know. its difficult to construct a proper example that isn't overly complex

workaround:

add the bound to all call sites that require it (e.g. add where Human: Consume<Food> to foo() in the above example)

rustc:

1.76.0-nightly (2023-12-17 6a62871)

[fmease]

Right, this is a diagnostic bug and a current limitation of non_lifetime_binders.

Obviously, we should say bounds cannot be used in this context instead (dropping the word lifetime). The diagnostic has simply never been updated after for<T> became syntactically valid and still assumes that for<'a: 'b> is the only possible way for a user to sneak in a higher-ranked bound.
Note to ppl interested in fixing this: Please add a regression test for the following code, too:

#![feature(closure_lifetime_binder)]
fn main() { let _ = for<T: Trait> || {}; }

Secondly, higher-ranked bounds aren't supported yet. There's a draft PR, namely #115362, which is blocked on a performance run.

[surechen]

@rustbot claim