Error code E0491

A reference has a longer lifetime than the data it references.

Erroneous code example:

#![allow(unused)] fn main() { struct Foo<'a> { x: fn(&'a i32), } trait Trait<'a, 'b> { type Out; } impl<'a, 'b> Trait<'a, 'b> for usize { type Out = &'a Foo<'b>; // error! } }

Here, the problem is that the compiler cannot be sure that the 'b lifetime will live longer than 'a, which should be mandatory in order to be sure that Trait::Out will always have a reference pointing to an existing type. So in this case, we just need to tell the compiler than 'b must outlive 'a:

#![allow(unused)] fn main() { struct Foo<'a> { x: fn(&'a i32), } trait Trait<'a, 'b> { type Out; } impl<'a, 'b: 'a> Trait<'a, 'b> for usize { // we added the lifetime enforcement type Out = &'a Foo<'b>; // it now works! } }