Add span_lint_note and span_lint_help to the LintContext

[nxnfufunezn]

Fixes #19668

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Aatch (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[nxnfufunezn]

r? @Manishearth

[Manishearth]

This part shouldn't be here, it's clippy-specific 😄

[Manishearth]

Good start! The code from clippy isn't exactly what we want though, just the first bits.

(Sorry, I had to go out, so I couldn't finish explaining in IRC). span_lint will emit a warning, error, or nothing, depending on the "lint level" of the code it originated from. When the lint level is allow (i.e. there's an #[allow(name_of_lint)] somewhere enclosing the code), the lint isn't shown. Often, one wants to show a note with the lint -- but it shouldn't be shown if the lint itself isn't shown. So we check the lint level before emitting the note. This is a task which probably is required often, so we should have a help method to do it.

You should also update

rust/src/librustc_lint/builtin.rs

Line 1258 in 2939666

ctx.span_lint(DROP_WITH_REPR_EXTERN,

to use this code.

To test, make check-stage2-cfail -j4 should work. If you want it to be faster and only run the specific test corresponding to that span_lint, try TESTNAME=lint-no-drop-on-repr-extern make check-stage2-cfail -j4