compiler/rustc_error_codes/src/error_codes/E0491.md

   1 A reference has a longer lifetime than the data it references.
   2
   3 Erroneous code example:
   4
   5 ```compile_fail,E0491
   6 struct Foo<'a> {
   7     x: fn(&'a i32),
   8 }
   9
  10 trait Trait<'a, 'b> {
  11     type Out;
  12 }
  13
  14 impl<'a, 'b> Trait<'a, 'b> for usize {
  15     type Out = &'a Foo<'b>; // error!
  16 }
  17 ```
  18
  19 Here, the problem is that the compiler cannot be sure that the `'b` lifetime
  20 will live longer than `'a`, which should be mandatory in order to be sure that
  21 `Trait::Out` will always have a reference pointing to an existing type. So in
  22 this case, we just need to tell the compiler than `'b` must outlive `'a`:
  23
  24 ```
  25 struct Foo<'a> {
  26     x: fn(&'a i32),
  27 }
  28
  29 trait Trait<'a, 'b> {
  30     type Out;
  31 }
  32
  33 impl<'a, 'b: 'a> Trait<'a, 'b> for usize { // we added the lifetime enforcement
  34     type Out = &'a Foo<'b>; // it now works!
  35 }
  36 ```