# Error code E0309

A parameter type is missing an explicit lifetime bound and may not live long enough.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    // 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;
    }
    }

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 the previous code.

There, 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:

    #![allow(unused)]
    fn main() {
    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;
    }
    }