Understanding Rust Error E0309: Missing Lifetime Bound

Overview of Error E0309

Rust error E0309 occurs when a parameter type is missing an explicit lifetime parameter. Lifetime parameters (e.g., T: 'a) ensure that the data in T remains valid for at least the lifetime 'a. This error often appears when the type contains an associated type reference like <T as SomeTrait<'a>>::Output.

Erroneous Code Example

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

In the erroneous example above, the struct definition has a field with the type <T as SomeTrait<'a>>::Output, but it is missing the necessary lifetime bound. This results in error E0309.

Solution: Add a Where-Clause with Lifetime Bound

To resolve error E0309, 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;
    }
}

By adding the T: 'a where-clause to the struct definition, you ensure that the lifetime bound requirement is explicitly defined, fixing error E0309.