The type definition contains some field whose type requires an outlives annotation. Outlives annotations (e.g., T: 'a
) are used to guarantee that all the data in T is valid for at least the lifetime 'a
. This scenario most commonly arises when the type contains an associated type reference like <T as SomeTrait<'a>>::Output
, as shown in this example:
// This won't compile because the applicable impl of // `SomeTrait` (below) requires that `T: 'a`, but the struct does // not have a matching where-clause. struct Foo<'a, T> { foo: <T as SomeTrait<'a>>::Output, } trait SomeTrait<'a> { type Output; } impl<'a, T> SomeTrait<'a> for T where T: 'a, { type Output = u32; }
Here, the where clause T: 'a
that appears on the impl is not known to be satisfied on the struct. To make this example compile, you have to add a where-clause like T: 'a
to the struct definition:
struct Foo<'a, T> where T: 'a, { foo: <T as SomeTrait<'a>>::Output } trait SomeTrait<'a> { type Output; } impl<'a, T> SomeTrait<'a> for T where T: 'a, { type Output = u32; }