Rust Error E0759: Fixing Lifetime Issues with Traits

Understanding Rust Error E0759

Rust Error E0759 occurs when a function's return type involving a trait does not require a 'static lifetime. The Rust compiler previously emitted this error code. It's important to resolve this error as it can lead to incorrect or unstable code.

Here's an example of erroneous code:

use std::fmt::Debug;

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

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

Fixing Rust Error E0759

To fix the Rust Error E0759, you'll need to add a 'static requirement to the return type of your function. Here's an example of how to do this:

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. This means the value implementing them that is being returned must be either a 'static borrow or an owned value.

Changing the Lifetime Requirements

To change the 'static lifetime requirement to a lifetime derived from the function's arguments, you can add an explicit lifetime bound (either to an anonymous lifetime '_' or a named lifetime).

Here's an example with 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)
}

In conclusion, Rust Error E0759 arises when the return type involving a trait doesn't have the required 'static lifetime. Fixing this issue involves adding a 'static requirement to the return type or changing the lifetime requirement.