Understanding Rust Error E0482: Lifetime of Returned Value

Overview of Rust Error E0482

Rust Error E0482 occurs when the lifetime of a returned value does not outlive the function call. This error is commonly related to the impl Trait feature that uses an implicit 'static lifetime restriction in the returned type. To resolve this issue, it is necessary to make the lifetime of the returned value explicit.

Erroneous code example:

#![allow(unused)]
fn main() {
fn prefix<'a>(
    words: impl Iterator<Item = &'a str>
) -> impl Iterator<Item = String> { // error!
    words.map(|v| format!("foo-{}", v))
}
}

Fixing the error

To fix Rust Error E0482, add lifetime bound to both the function argument and the return value. This ensures that the values inside the iterator are not dropped when the function goes out of scope.

Corrected code example:

#![allow(unused)]
fn main() {
fn prefix<'a>(
    words: impl Iterator<Item = &'a str> + 'a
) -> impl Iterator<Item = String> + 'a { // ok!
    words.map(|v| format!("foo-{}", v))
}
}

Alternative solution

Another solution for Rust Error E0482 is to guarantee that the Item references in the iterator are alive for the whole lifetime of the program. This can be achieved by using an 'static lifetime.

Alternative corrected code example:

#![allow(unused)]
fn main() {
fn prefix(
    words: impl Iterator<Item = &'static str>
) -> impl Iterator<Item = String> {  // ok!
    words.map(|v| format!("foo-{}", v))
}
}

Lifetime issues with closures

Rust Error E0482 might also arise when returning closures. To resolve this issue, use explicit return lifetime and move the ownership of the variable to the closure.

Closure code example:

#![allow(unused)]
fn main() {
fn foo<'a>(
    x: &'a mut Vec<i32>
) -> impl FnMut(&mut Vec<i32>) -> &[i32] + 'a { // ok!
    move |y| {
        y.append(x);
        y
    }
}
}