Rust Error E0226: Fixing Multiple Lifetime Bounds on Trait Objects

Understanding Rust Error E0226

Rust Error E0226 occurs when more than one explicit lifetime bound is used with a trait object. To ensure memory safety and prevent dangling references, Rust utilizes lifetimes to express the valid scope of references. However, trait objects are only allowed to have a single lifetime bound.

Example of Erroneous Code

#![allow(unused)]
fn main() {
    trait Foo {}

    type T<'a, 'b> = dyn Foo + 'a + 'b; // error: Trait object `arg` has two
                                        //        lifetime bound, 'a and 'b.
}

In this example, a trait object `T` is created with two explicit lifetime bounds, 'a and 'b. The code raises E0226, indicating that only one lifetime bound can be used with a trait object.

Fixing Rust Error E0226

To fix Rust Error E0226, remove one of the explicit lifetime bounds from the trait object. This will leave the trait object with only a single lifetime bound, as required.

Example of Correct Code

#![allow(unused)]
fn main() {
    trait Foo {}

    type T<'a> = dyn Foo + 'a;
}

In this corrected example, the lifetime bound 'b has been removed, leaving the trait object `T` with a single lifetime bound 'a. The revised code now complies with Rust's requirements for trait objects and will no longer raise Error E0226.

Conclusion