Rust error E0700: hidden type for `impl Trait<'y>` captures lifetime that does not appear in bounds

The compiler team's explanation

The impl Trait return type captures lifetime parameters that do not appear within the impl Trait itself.

Erroneous code example:

use std::cell::Cell;

trait Trait<'a> { }

impl<'a, 'b> Trait<'b> for Cell<&'a u32> { }

fn foo<'x, 'y>(x: Cell<&'x u32>) -> impl Trait<'y>
where 'x: 'y
{
    x
}

Here, the function foo returns a value of type Cell<&'x u32>, which references the lifetime 'x. However, the return type is declared as impl Trait<'y> -- this indicates that foo returns "some type that implements Trait<'y>", but it also indicates that the return type **only captures data referencing the lifetime 'y**. In this case, though, we are referencing data with lifetime 'x, so this function is in error.

To fix this, you must reference the lifetime 'x from the return type. For example, changing the return type to impl Trait<'y> + 'x would work:

use std::cell::Cell;

trait Trait<'a> { }

impl<'a,'b> Trait<'b> for Cell<&'a u32> { }

fn foo<'x, 'y>(x: Cell<&'x u32>) -> impl Trait<'y> + 'x
where 'x: 'y
{
    x
}

From rustc --explain E0700 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.