Understanding Rust Error E0399

Overview of Rust Error E0399

Rust Error E0399 occurs when a trait is implemented that overrides one or more of its associated types, but the default methods of the trait are not overridden. It is crucial to re-implement the default methods for the override to work properly.

Error E0399 Example

Consider the following erroneous code as an example:

#![allow(unused)]
#![feature(associated_type_defaults)]

fn main() {
  pub trait Foo {
      type Assoc = u8;
      fn bar(&self) {}
  }

  impl Foo for i32 {
      // error - the following trait items need to be reimplemented as
      //         `Assoc` was overridden: `bar`
      type Assoc = i32;
  }
}

This code will result in Rust Error E0399, as it does not provide an implementation for the `bar` method after overridding the `Assoc` associated type.

How to Fix Error E0399

To fix Error E0399, you need to reimplement the default methods from the trait, like in the following example:

#![allow(unused)]
#![feature(associated_type_defaults)]

fn main() {
  pub trait Foo {
      type Assoc = u8;
      fn bar(&self) {}
  }

  impl Foo for i32 {
      type Assoc = i32;
      fn bar(&self) {} // ok!
  }
}

In this fixed example, the implementation of the `bar` method is provided after overriding the `Assoc` associated type. This will not produce Error E0399 and will compile successfully.

Additional Tips

It is essential to be careful when implementing traits and overriding associated types. Remember to always reimplement the default methods of the trait when an associated type is overridden. By doing so, you'll avoid running into Rust Error E0399.