Understanding Rust Error E0186

Introduction to Rust Error E0186

Rust Error E0186 occurs when there is a mismatch between a trait's associated function and its implementation in a struct or enum. Specifically, the error happens when the trait defines the function as a method (taking a self parameter), but the implementation defines the function as static (without a self parameter).

Examining the Erroneous Code Example

Consider the following code snippet that demonstrates the occurrence of Rust Error E0186:

trait Foo {
    fn foo(&self);
}

struct Bar;

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

Here, the trait Foo defines an associated function foo() with a &self parameter. However, when implementing Foo for the struct Bar, the foo() function is incorrectly defined without a &self parameter, causing Error E0186.

Fixing Rust Error E0186

To resolve Rust Error E0186, the implementation must match the trait's associated function signature. This means that the implementation of foo() for Bar should include the &self parameter:

trait Foo {
    fn foo(&self);
}

struct Bar;

impl Foo for Bar {
    fn foo(&self) {} // Fixed Error E0186
}

By ensuring that the function signatures in the trait and its implementation match, the error is resolved.

Key Takeaways

Remember these critical aspects when dealing with Rust Error E0186: 1. Ensure that the associated function signature in the trait matches its implementation in the struct or enum. 2. Be aware of whether the function requires a self parameter or not and adjust the implementation accordingly. By adhering to these guidelines, you can avoid encountering Rust Error E0186.