A 'static requirement in a return type involving a trait is not fulfilled.

Erroneous code examples:

use std::fmt::Debug;

fn foo(x: &i32) -> impl Debug {
    x
}

# use std::fmt::Debug;
fn bar(x: &i32) -> Box<dyn Debug> {
    Box::new(x)
}

These examples have the same semantics as the following:

# use std::fmt::Debug;
fn foo(x: &i32) -> impl Debug + 'static {
    x
}

# use std::fmt::Debug;
fn bar(x: &i32) -> Box<dyn Debug + 'static> {
    Box::new(x)
}

Both dyn Trait and impl Trait in return types have a an implicit 'static requirement, meaning that the value implementing them that is being returned has to be either a 'static borrow or an owned value.

In order to change the requirement from 'static to be a lifetime derived from its arguments, you can add an explicit bound, either to an anonymous lifetime '_ or some appropriate named lifetime.

# use std::fmt::Debug;
fn foo(x: &i32) -> impl Debug + '_ {
    x
}
fn bar(x: &i32) -> Box<dyn Debug + '_> {
    Box::new(x)
}

These are equivalent to the following explicit lifetime annotations:

# use std::fmt::Debug;
fn foo<'a>(x: &'a i32) -> impl Debug + 'a {
    x
}
fn bar<'a>(x: &'a i32) -> Box<dyn Debug + 'a> {
    Box::new(x)
}