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! } }