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# 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!
    }
    }