Rust Error E0231 Explained
Understanding Rust Error E0231
Rust Error E0231 occurs when the #[rustc_on_unimplemented] attribute is used incorrectly. This attribute allows specifying custom error messages when a specific trait is not implemented on a type placed in a position that requires the trait.
How to use #[rustc_on_unimplemented] correctly
With the #[rustc_on_unimplemented] attribute, you can specify a custom error message to be displayed when a type does not implement the required trait. This attribute should be applied to the trait definition itself. You can substitute actual types for the type parameters in the error message using the curly braces ({}).
Here's an example of properly utilizing the #[rustc_on_unimplemented] attribute:
#![allow(unused)]
#![feature(rustc_attrs)]
fn main() {
#[rustc_on_unimplemented = "error on `{Self}` with params `<{A},{}>`"]
trait MyTrait<A> {}
struct MyType;
impl MyTrait<u8> for MyType {}
}Resolving Error E0231
Rust Error E0231 is shown when the curly braces do not contain an identifier within the #[rustc_on_unimplemented] attribute. To resolve this error, ensure that the curly braces contain an identifier with the same name as a type parameter. If you intend to use literal curly braces, escape them with double curly braces ({{ and }}).
Here's an example that causes Error E0231:
#![allow(unused)]
#![feature(rustc_attrs)]
fn main() {
#[rustc_on_unimplemented = "error on `{Self}` with params `<{},{}>`"] // error!
trait BadAnnotation<A> {}
}To fix this error, add the identifier (in this case, A) inside the curly braces:
#![allow(unused)]
#![feature(rustc_attrs)]
fn main() {
#[rustc_on_unimplemented = "error on `{Self}` with params `<{A},{}>`"]
trait GoodAnnotation<A> {}
}