Const closure weirdness

[DutchGhost]

The following compiles just fine:

#![feature(impl_trait_in_bindings)]
fn main() {
    let func: impl Fn(&str) -> usize = |s| s.parse().unwrap();

}

However this does not:

#![feature(impl_trait_in_bindings)]

fn main() {
    const func: impl Fn(&str) -> usize = |s| s.parse().unwrap();
}

It gives the following error message:

error: non-defining existential type use in defining scope
 --> src/main.rs:4:42
  |
4 |     const func: impl Fn(&str) -> usize = |s| s.parse().unwrap();
  |                                          ^^^^^^^^^^^^^^^^^^^^^^ lifetime `` is part of concrete type but not used in parameter list of existential type

error: aborting due to previous error

error: Could not compile `playground`.

Why is this an error? The closure does not capture anything, so it's lifetime is 'static, and const requires 'static.

[ExpHP]

Note also that the following works:

#![feature(impl_trait_in_bindings)]
fn main() {
    const func: impl Fn(String) -> usize = |s| s.parse().unwrap();
}

so it has something to do with the &str type parameter. But the trick that often fixes problems like this in other places in rust does not help here:

#![feature(impl_trait_in_bindings)]
fn main() {
    // explicitly annotate the closure arg as &_.
    // same error as OP
    const func: impl Fn(&str) -> usize = |s: &_| s.parse().unwrap();

    // not even this helps
    const func: impl for<'a> Fn(&'a str) -> usize = |s: &_| s.parse().unwrap();
}

[ilyvion]

Seems this feature doesn't even exist anymore.

[traviscross]

We can close this as the feature was pulled out in #86729 and what was added back in #134185 only supports local bindings.