Error codes

We generally try to assign each error message a unique code like E0123. These codes are defined in the compiler in the diagnostics.rs files found in each crate, which basically consist of macros. All error codes have an associated explanation: new error codes must include them. Note that not all historical (no longer emitted) error codes have explanations.

Error explanations

The explanations are written in Markdown (see the CommonMark Spec for specifics around syntax), and all of them are linked in the rustc_error_codes crate. Please read RFC 1567 for details on how to format and write long error codes. As of February 2023, there is an effort¹ to replace this largely outdated RFC with a new more flexible standard.

Error explanations should expand on the error message and provide details about why the error occurs. It is not helpful for users to copy-paste a quick fix; explanations should help users understand why their code cannot be accepted by the compiler. Rust prides itself on helpful error messages and long-form explanations are no exception. However, before error explanations are overhauled¹ it is a bit open as to how exactly they should be written, as always: ask your reviewer or ask around on the Rust Discord or Zulip.

Allocating a fresh code

Error codes are stored in compiler/rustc_error_codes.

To create a new error, you first need to find the next available code. You can find it with tidy:

./x test tidy

This will invoke the tidy script, which generally checks that your code obeys our coding conventions. Some of these jobs check error codes and ensure that there aren't duplicates, etc (the tidy check is defined in src/tools/tidy/src/error_codes.rs). Once it is finished with that, tidy will print out the highest used error code:

...
tidy check
Found 505 error codes
Highest error code: `E0591`
...

Here we see the highest error code in use is E0591, so we probably want E0592. To be sure, run rg E0592 and check, you should see no references.

You will have to write an extended description for your error, which will go in rustc_error_codes/src/error_codes/E0592.md. To register the error, open rustc_error_codes/src/error_codes.rs and add the code (in its proper numerical order) into register_diagnostics! macro, like this:

#![allow(unused)]
fn main() {
register_diagnostics! {
    ...
    E0592: include_str!("./error_codes/E0592.md"),
}
}

To actually issue the error, you can use the struct_span_code_err! macro:

#![allow(unused)]
fn main() {
struct_span_code_err!(self.dcx(), // some path to the `DiagCtxt` here
                 span, // whatever span in the source you want
                 E0592, // your new error code
                 fluent::example::an_error_message)
    .emit() // actually issue the error
}

If you want to add notes or other snippets, you can invoke methods before you call .emit():

#![allow(unused)]
fn main() {
struct_span_code_err!(...)
    .span_label(another_span, fluent::example::example_label)
    .span_note(another_span, fluent::example::separate_note)
    .emit()
}

For an example of a PR adding an error code, see #76143.