Rust error E0582: binding for associated type `Output` references lifetime `'a`, which does not appear in the trait input types

The compiler team's explanation

A lifetime is only present in an associated-type binding, and not in the input types to the trait.

Erroneous code example:

fn bar<F>(t: F)
    // No type can satisfy this requirement, since `'a` does not
    // appear in any of the input types (here, `i32`):
    where F: for<'a> Fn(i32) -> Option<&'a i32>
{
}

fn main() { }

To fix this issue, either use the lifetime in the inputs, or use 'static. Example:

fn bar<F, G>(t: F, u: G)
    where F: for<'a> Fn(&'a i32) -> Option<&'a i32>,
          G: Fn(i32) -> Option<&'static i32>,
{
}

fn main() { }

This error also includes the use of associated types with lifetime parameters.

trait Foo {
    type Assoc<'a>;
}

struct Bar<X, F>
where
    X: Foo,
    F: for<'a> Fn(X::Assoc<'a>) -> &'a i32
{
    x: X,
    f: F
}

The latter scenario encounters this error because Foo::Assoc<'a> could be implemented by a type that does not use the 'a parameter, so there is no guarantee that X::Assoc<'a> actually uses 'a.

To fix this we can pass a dummy parameter:

struct Bar<X, F>
where
    X: Foo,
    F: for<'a> Fn(X::Assoc<'a>, /* dummy */ &'a ()) -> &'a i32
{
    x: X,
    f: F
}

Note: The examples above used to be (erroneously) accepted by the compiler, but this was since corrected. See issue #33685 for more details.

From rustc --explain E0582 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.