Understanding Rust Error E0230

Overview

Rust Error E0230 occurs when using the #[rustc_on_unimplemented] attribute for providing a custom error message for a trait that isn't implemented on a specific type. This error is triggered when the curly braces in the custom error message contain an identifier that does not match any type parameters or the string Self.

Explaining Custom Error Messages

The #[rustc_on_unimplemented] attribute allows you to create more descriptive error messages when a particular trait isn't implemented on a type. You can include type parameters and Self in the custom error message to make it more informative. The type parameters can be specified in curly braces and will be substituted with the actual types in the error message. The {Self} keyword represents the type that we tried to use.

Example

Here's an example that demonstrates the usage of #[rustc_on_unimplemented] attribute and how Error E0230 occurs:

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

fn main() {
#[rustc_on_unimplemented = "error on `{Self}` with params `<{A},{B}>`"]
trait BadAnnotation<A> {}
}

Addressing Error E0230

To fix Rust Error E0230, ensure that the identifiers within the curly braces match the type parameters or the string Self. If you need to include literal curly braces in the custom error message, use double curly braces like {{ or }} to escape them.

Example of Correct Usage

Let's correct the previous example to avoid Error E0230:

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

fn main() {
#[rustc_on_unimplemented = "error on `{Self}` with params `<{{A}},{{B}}>`"]
trait GoodAnnotation<A, B> {}
}

This code now correctly uses double curly braces to escape the literal curly braces in the custom error message, which prevents Error E0230 from occurring.