lotsoftools

Understanding Rust Error E0228: Undeducible Lifetime Bound

Introduction to Rust Error E0228

Rust Error E0228 is encountered when the compiler is unable to deduce the lifetime bound of an object type from the context in which it is used. This error typically occurs when a trait object is utilized as a type argument in a generic type with multiple lifetime bounds.

Erroneous Code Example

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

    struct TwoBounds<'a, 'b, T: Sized + 'a + 'b> {
        x: &'a i32,
        y: &'b i32,
        z: T,
    }

    type Foo<'a, 'b> = TwoBounds<'a, 'b, dyn Trait>;
}

In this example, the struct TwoBounds has two lifetime parameters 'a and 'b, as well as a generic type parameter T. The type Foo is an alias for the TwoBounds struct, using a trait object (dyn Trait) as its type argument. The Rust compiler is unable to infer the trait object's lifetime in this scenario, resulting in Error E0228.

Solving Error E0228

To fix this error, you can either reduce the number of lifetime bounds to one, or make the trait object lifetime explicit. Here's a corrected version of the code:

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

    struct TwoBounds<'a, 'b, T: Sized + 'a + 'b> {
        x: &'a i32,
        y: &'b i32,
        z: T,
    }

    type Foo<'a, 'b> = TwoBounds<'a, 'b, dyn Trait + 'b>;
}

This revised code specifies the trait object's lifetime as 'b, allowing the Rust compiler to correctly infer the lifetime bound.

Relevant Resources

For more information on lifetimes and trait objects, refer to The Rustonomicon's chapter on Lifetimes, as well as RFC 599 and its amendment RFC 1156.

Recommended Reading