Understanding Rust Error E0478: Lifetime Bound Not Satisfied

What causes Rust Error E0478?

Rust Error E0478 is caused by a lifetime bound not being satisfied. In Rust, lifetimes are a way to ensure that a reference to an object outlives the object itself. This error occurs when a lifetime specified for a type parameter doesn't satisfy the required lifetime bounds.

Example of code triggering Rust Error E0478

#![allow(unused)]
fn main() {
    trait Wedding<'t>: 't { }

    struct Prince<'kiss, 'SnowWhite> {
        child: Box<Wedding<'kiss> + 'SnowWhite>,
        // error: lifetime bound not satisfied
    }
}

The code above results in Rust Error E0478 because the 'SnowWhite lifetime should outlive the 'kiss lifetime, but the declaration of the Prince struct doesn't enforce it. To fix this issue, you need to specify the required lifetime bounds.

How to fix Rust Error E0478?

To fix Rust Error E0478, you should specify the appropriate lifetime bounds for the type parameters. In this case, you can enforce the 'SnowWhite lifetime to outlive the 'kiss lifetime by modifying the Prince struct as shown in the corrected example below.

Corrected code example:

#![allow(unused)]
fn main() {
    trait Wedding<'t>: 't { }

    struct Prince<'kiss, 'SnowWhite: 'kiss> { // 'SnowWhite must live longer than 'kiss
        child: Box<Wedding<'kiss> + 'SnowWhite>, // And now it's all good!
    }
}

In the corrected code, the 'SnowWhite lifetime is explicitly defined to outlive the 'kiss lifetime by using the syntax 'SnowWhite: 'kiss. This enforces the required lifetime bound, and the code no longer generates Rust Error E0478.