Understanding Rust Error E0229

Introduction to Rust Error E0229

Rust Error E0229 occurs when an associated type binding is incorrectly placed outside the type parameter declaration or a where clause. This article will help you understand the error better, see examples of erroneous code, and learn how to fix it.

Erroneous Code Example

Let's take a look at the erroneous code example:

#![allow(unused)]
fn main() {
    pub trait Foo {
        type A;
        fn boo(&self) -> <Self as Foo>::A;
    }

    struct Bar;

    impl Foo for isize {
        type A = usize;
        fn boo(&self) -> usize { 42 }
    }

    fn baz<I>(x: &<I as Foo<A=Bar>>::A) {}
    // error: associated type bindings are not allowed here
}

In this example, the associated type binding 'A=Bar' in the 'baz' function definition is declared outside of the type parameter declaration and where clause, causing Rust Error E0229.

Solution

To fix this error, you need to move the type binding either into the type parameter declaration or the where clause. Here are two possible solutions:

1. Move type binding to the type parameter declaration:

#![allow(unused)]
fn main() {
    struct Bar;
    trait Foo { type A; }
    fn baz<I: Foo<A=Bar>>(x: &<I as Foo>::A) {} // ok!
}

2. Move type binding to the where clause:

#![allow(unused)]
fn main() {
    struct Bar;
    trait Foo { type A; }
    fn baz<I>(x: &<I as Foo>::A) where I: Foo<A=Bar> {} // ok!
}

Both solutions ensure that the 'A=Bar' type binding is placed correctly, eliminating the E0229 error. In general, remember to declare associated type bindings within the type parameter declaration or where clause to avoid this error.