Understanding Rust Error E0222: Constraint on Associated Type

Introduction to Rust Error E0222

Rust Error E0222 occurs when an attempt is made to place a constraint on an associated type that's inherited from both of its supertraits. This error arises when combining two traits that have an associated type with the same name into a third trait. In this article, we'll discuss what causes Rust Error E0222, walk through the erroneous code example from the official documentation, and then demonstrate how to fix the error by using a where clause and a new type parameter.

Code Example with Rust Error E0222

#![allow(unused)]
fn main() {
pub trait Vehicle {
    type Color;
}

pub trait Box {
    type Color;
}

pub trait BoxCar : Box + Vehicle {}

fn dent_object<COLOR>(c: dyn BoxCar<Color=COLOR>) {} // Invalid constraint
}

In the code above, two traits, Vehicle and Box, are created, both with an associated type named Color. Then, BoxCar is created as a third trait, inheriting from both Vehicle and Box. The next line contains a function declaration, dent_object, which attempts to place a constraint on the associated type inherited from both supertraits. Due to this constraint attempt, Rust Error E0222 is triggered.

Fixing Rust Error E0222 with a where Clause and a New Type Parameter

To fix Rust Error E0222, we need to rewrite the function signature using a where clause and introduce a new type parameter. The where clause can be used to encode the desired constraint, while the new type parameter helps to specify the intended associated type use. Here's the updated code:

#![allow(unused)]
fn main() {
pub trait Vehicle {
    type Color;
}

pub trait Box {
    type Color;
}

pub trait BoxCar : Box + Vehicle {}

// Introduce a new `CAR` type parameter
fn foo<CAR, COLOR>(
    c: CAR,
) where
    // Bind the type parameter `CAR` to the trait `BoxCar`
    CAR: BoxCar,
    // Further restrict `<BoxCar as Vehicle>::Color` to be the same as the
    // type parameter `COLOR`
    CAR: Vehicle<Color = COLOR>,
    // We can also simultaneously restrict the other trait's associated type
    CAR: Box<Color = COLOR>
{}
}

In this example, we have introduced a new CAR type parameter and used where clauses to bind CAR to the BoxCar trait as well as restrict both associated types from Vehicle and Box to match the COLOR type parameter. This allows us to work with associated types without triggering Rust Error E0222.

Conclusion

Rust Error E0222 is caused by placing constraints on associated types that are inherited from multiple supertraits. By introducing a new type parameter and using where clauses, this error can be resolved. Keep this in mind whenever you're working with associated types and inheritances in Rust to avoid encountering E0222.