Understanding Rust Error E0185

Introduction to Rust Error E0185

Rust Error E0185 occurs when an associated function for a trait is defined as static, but the implementation of the same trait declares the function to be a method with a self parameter. In this article, we will explain the error in detail and provide a correct implementation to fix the issue.

Erroneous Code Example

Consider the following erroneous Rust code example:


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

    struct Bar;

    impl Foo for Bar {
        // error, method `foo` has a `&self` declaration in the impl, but not in
        // the trait
        fn foo(&self) {}
    }
}

In this example, the trait Foo has an associated function foo which is static (no self parameter). However, when the trait is implemented for the struct Bar, the function foo is incorrectly declared as a method with a self parameter, causing the Rust E0185 error.

Fixing Rust Error E0185

To fix Rust Error E0185, the function signature in the trait and its implementation must match. In this case, the function foo in the implementation for Bar must also be static, without a self parameter.

Corrected Code Example

Here is the corrected Rust code example:


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

    struct Bar;

    impl Foo for Bar {
        fn foo() {} // ok!
    }
}

This corrected code now properly matches the function signature of the trait Foo, with the function foo being static in both the trait and its implementation for Bar. This resolves Rust Error E0185.

Conclusion