Rust Error E0643: Mismatch Between Generic and impl Trait Parameters

Understanding Rust Error E0643

Rust Error E0643 occurs when there is a mismatch between generic parameters and impl Trait parameters in a trait declaration versus its impl. This is often due to using different syntax or not properly accounting for the relationships between the generic parameters and impl Trait parameters. In this article, we'll break down the cause of this error and show you how to fix it.

Example: Triggering Error E0643

Consider the following code snippet illustrating Rust Error E0643:

trait Foo {
    fn foo(&self, _: &impl Iterator);
}

impl Foo for () {
    fn foo<U: Iterator>(&self, _: &U) { } // error method `foo` has incompatible signature for trait
}

In this example, the Foo trait has a method foo accepting a single argument of type &impl Iterator. The implementation of Foo for the unit type () uses a different syntax, relying on a generic parameter U instead of the impl Trait syntax. This causes Rust Error E0643 due to the mismatch of syntax and parameters in the trait and its impl.

Fixing Error E0643

To fix Error E0643, you'll need to align the trait and impl declarations so that they use the same syntax and parameters. Here's the corrected code snippet:

trait Foo {
    fn foo<U: Iterator>(&self, _: &U);
}

impl Foo for () {
    fn foo<U: Iterator>(&self, _: &U) { }
}

By changing the trait's method declaration to use a generic parameter with the same bound, the error is resolved. Alternatively, you could also change both declarations to use the impl Trait syntax:

trait Foo {
    fn foo(&self, _: &impl Iterator);
}

impl Foo for () {
    fn foo(&self, _: &impl Iterator) { }
}

Both of these adjustments ensure consistency between the trait declaration and the impl, eliminating Rust Error E0643.