impl Trait cannot work well with lifetime bounds

[oxalica]

Playground

pub struct S<'t>(usize, &'t str);

impl<'t> S<'t> {
    // error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
    pub fn foo<'s>(&'s mut self) -> impl FnMut() + 's {
        move || self.0 += 1
    }
}

fn main() {
    let mut s = S(0, "hello");
    s.foo()();
    println!("{}", s.0);
}

The self: &'s mut S<'t> implies 't: 's, so the signature should be correct. But compiler still complain that 't is captured but does not appear in impl bounds.

I checked the error description of E0700 and tried impl FnMut() + 's where 't: 's and impl FnMut() + 's + 't, while it still fails.

Note that when using dyn Trait as pub fn foo<'s>(&'s mut self) -> Box<dyn FnMut() + 's>, it compiles.

[kennytm]

You could workaround using the Captures trait.

pub trait Captures<U: ?Sized> {}
impl<U: ?Sized, T: ?Sized> Captures<U> for T {}

impl<'t> S<'t> {
    pub fn foo(&mut self) -> impl FnMut() + Captures<&'t str> + '_ {
        move || self.0 += 1
    }
}

[Nadrieril]

Could the Captures trait make it into std so that rustc can suggest it? It's otherwise impossible to discover this trick but it's a very useful one.