Understanding Rust Error E0621

Rust Error E0621 Explained

Rust Error E0621 occurs when there is a mismatch between the lifetimes specified in the function signature, including the parameter types and the return type, and the data flow found within the function body.

Example with Incorrect Code

#![allow(unused)]
fn main() {
fn foo<'a>(x: &'a i32, y: &i32) -> &'a i32 { // error: explicit lifetime
                                             //        required in the type of
                                             //        `y`
    if x > y { x } else { y }
}
}

In the erroneous code snippet above, the function is intended to return data borrowed from either x or y. However, the 'a lifetime annotation indicates that it is only returning data from x. This creates a conflict between the function signature and the actual data flow within the function body.

Fixing Rust Error E0621

To resolve Rust Error E0621, you need to update either the function signature or the function body. Here are two ways to fix the error:

1. Update the Function Signature

One way to solve Error E0621 is by updating the function signature to better match the data flow in the function body. In this case, change the type of y to &'a i32.

Example with Updated Function Signature

#![allow(unused)]
fn main() {
fn foo<'a>(x: &'a i32, y: &'a i32) -> &'a i32 {
    if x > y { x } else { y }
}
}

Now, the function signature indicates that the function returns data borrowed from either x or y, resolving the mismatch.

2. Update the Function Body

Alternatively, you can update the function body so that it doesn't return data from y. Here's an example of how to do that:

Example with Updated Function Body

#![allow(unused)]
fn main() {
fn foo<'a>(x: &'a i32, y: &i32) -> &'a i32 {
    x
}
}

In this updated example, the function body now only returns data borrowed from x, which is in line with the original function signature.