Add child diagnostics

[TedDriggs]

Fixes #224

Open Items

• Update release notes
• Add unit tests (details)
• Update CI to run diagnostics unit tests

[AlisCode]

This seems to do the work, good job! I've put some feedback, feel free to take it or not :)

[TedDriggs]

@AlisCode thanks for the great review so far! The area that I would most appreciate help with on this is coming up with some unit tests; these would serve as a sanity check for the functionality and catch ergonomic issues like the one about being able to pass string literals without needing &. The main challenges are:

• ChildDiagnostic::append_to cannot be called from the unit test, or it will panic. I've filed Convert from proc_macro2::Span to proc_macro::Span without panic dtolnay/proc-macro2#365 to see if that can be addressed
• Setting up CI to make sure that the test can run without failing due to a bad nightly build. I think your idea of pinning a specific nightly build for the diagnostics unit tests makes sense, so the GitHub workflows will need to be updated to do that.

[AlisCode]

I'll see what I can do for the unit-test coverage of this, although I don't immediately see a good solution to the panic.
I'll try to update the GH workflow to pin the version of nightly like I suggested, and cross my fingers I don't break anything doing so :)

Thanks for the reactivity and the good work !

[TedDriggs]

@AlisCode Rather than always pinning nightly, I'd suggest adding a specific new job that tests diagnostics with that feature enabled and the nightly version pinned. That'll avoid changing the current test slate which runs against the latest nightly.

I think the best solution for the panic issue is to manually test append_to now and then just hope for the best going forward 😅 The unit tests can focus on making sure the ergonomics of adding these messages to an error work well.

Thanks for the reactivity and the good work !

Thanks for the great feature request!

[TedDriggs]

@AlisCode the doc tests I added seem to do a good enough job of making sure the happy-path compiles, so I'm going to declare this "good enough for a feature-flagged feature" and merge.