Error code E0759

Note: this error code is no longer emitted by the compiler.

Return type involving a trait did not require 'static lifetime.

Erroneous code examples:

#![allow(unused)]
fn main() {
use std::fmt::Debug;

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

fn bar(x: &i32) -> Box<dyn Debug> { // error!
    Box::new(x)
}
}
ⓘ

Add 'static requirement to fix them:

#![allow(unused)]
fn main() {
use std::fmt::Debug;
fn foo(x: &'static i32) -> impl Debug + 'static { // ok!
    x
}

fn bar(x: &'static i32) -> Box<dyn Debug + 'static> { // ok!
    Box::new(x)
}
}

Both dyn Trait and impl Trait in return types have 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.

#![allow(unused)]
fn main() {
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:

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