Rustc error with lambda's and type alias?

The following code gives an error about static lifetimes. Eliminating the type alias makes the error go away. Is this a rustc bug or am I misunderstanding something (maybe re: lifetime elision and type aliases?)?

// this fails:
//    error[E0477]: the type `[closure@compiler-error.rs:5:20: 5:43 g:&mut &mut std::ops::FnMut(u32) -> u32 + 'static]` does not fulfill the required lifetime
type F = FnMut(u32) -> u32;
fn apply_type(f :&mut F, x :u32) -> u32 { f(x) }
fn apply_type_one(g :&mut F, x :u32) -> u32 { 
        let mut adaptor = |x:u32| -> u32 { g(x) };
        apply_type(&mut adaptor,x)
}

// eliminating the type alias = successful compile.
fn apply_type_raw(f :&mut FnMut(u32)->u32, x :u32) -> u32 { f(x) }
fn apply_type_one_raw(g :&mut FnMut(u32)->u32, x :u32) -> u32 {
        let mut adaptor = |x:u32| -> u32 { g(x) };
        apply_type_raw(&mut adaptor,x)
}

fn main() {
}

Yes, this is due to default object bounds. More info here

The way to fix this is to parameterize the type alias with a lifetime and then let elision handle it at use sites:

type F<'a> = FnMut(u32) -> u32 + 'a;

Separately, wanted to point out (in case you’re not aware) that you’re using dynamic dispatch here by virtue of trait objects.

1 Like

Sweet. Yup, that did it. Shame rustc --explain doesn’t (appear to) have this explanation; would have saved time. Thanks.

The only downside to dynamic dispatch is speed, right?

In this particular example, yeah. The larger topic is generics vs trait objects, and each have their pros/cons (not just in performance, but expressivity and ergonomics as well).

Mostly, I don’t know your familiarity with Rust so wanted to confirm you were opting for what you wanted :slight_smile: