Understanding Rust Error E0116

Introduction to Rust Error E0116

Rust Error E0116 occurs when an inherent implementation is defined for a type outside the current crate. In this article, we'll discuss why this error occurs and provide solutions to fix it.

Understanding the Error

First, let's examine the erroneous code example: impl Vec<u8> { } // error Here, we're trying to implement inherent methods directly on the Vec<u8> type. However, Vec is defined in the standard library, not in the current crate. In Rust, inherent implementations can only be defined for types within the same crate.


To resolve Rust Error E0116, you can choose one of the following approaches:

1. Implement a Trait

Define a trait with the desired associated functions, types, or constants, and then implement the trait for the type in question. Here's an example:

trait CustomMethods {
    fn custom_method(&self);

impl CustomMethods for Vec<u8> {
    fn custom_method(&self) {
        // Implementation here

2. Create a Newtype

Define a new type wrapping the original type and implement the inherent methods on the new type. Example:

struct MyVec(Vec<u8>);

impl MyVec {
    fn custom_method(&self) {
        // Implementation here

Avoiding the 'type' Keyword

Note that using the 'type' keyword to create a type alias is not a valid solution, as it only introduces a new name for an existing type without creating a genuine new type. The following code will still produce error E0116:

type Bytes = Vec<u8>;

impl Bytes { } // error, same as above

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