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
}