Part of #59346

It would be nice to have a UI test for the annotate-snippet emitter, that includes annotations that span multiple files. The goal of this issue is to find and add such a test.

Instructions

A simple way to look for this, would be to add this code:

if annotated_files.len() > 1 {
    let filename = primary_lo.file.name.to_string();
    eprintln!("annotated_files > 1 in {}", filename);
}

to the top of fn slices_for_files and then check the failing UI tests.

If there are failing UI tests, it means that those UI tests include a diagnostic that probably spans multiple files.

The next step, is to copy that file (and possible auxiliary files) over into src/test/ui/annotate-snippet/ and add // compile-flags: --error-format human-annotate-rs to the top of the UI test.

Once that's done, be sure to run the added test and open a PR including the blessed stderr files.