# 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)
    }
    }