We generally try 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. The codes come in two varieties: those that have an extended write-up, and those that do not. Whenever possible, if you are making a new code, you should write an extended write-up.

Allocating a fresh code

If you want to create a new error, you first need to find the next available code. This is a bit tricky since the codes are defined in various crates. To do it, run this obscure command:

./x.py test --stage 0 src/tools/tidy

This will invoke the tidy script, which generally checks that your code obeys our coding conventions. One of those jobs is to check that diagnostic codes are indeed unique. Once it is finished with that, tidy will print out the lowest unused code:

...
tidy check (x86_64-apple-darwin)
* 470 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.

Next, open src/{crate}/diagnostics.rs within the crate where you wish to issue the error (e.g., src/librustc_typeck/diagnostics.rs). Ideally, you will add the code (in its proper numerical order) into the register_long_diagnostics! macro, sort of like this:

register_long_diagnostics! {
    ...
    E0592: r##"
Your extended error text goes here!
"##,
}

But you can also add it without an extended description:

register_diagnostics! {
    ...
    E0592, // put a description here
}

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

struct_span_err!(self.tcx.sess, // some path to the session here
                 span, // whatever span in the source you want
                 E0592, // your new error code
                 &format!("text of the error"))
    .emit() // actually issue the error

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

struct_span_err!(...)
    .span_label(another_span, "something to label in the source")
    .span_note(another_span, "some separate note, probably avoid these")
    .emit_()