We should have a tool that finds unused diagnostic IDs

[beccadax]

While working on #67075, I found several stale diagnostics that are declared in Diagnostics*.def files, but are never used anywhere in the compiler. I deleted most of the ones I found, but I’m sure there are many more.

We should have a tool that scans lib/ and include/ for uses of diagnostic IDs and flags any that are never actually used. This could perhaps be a test that would fail if there are unused diagnostics.

This might be a good first issue for a new contributor who has reasonably strong Python or Swift skills but doesn’t have any experience with compilers yet.

[ED-1-0]

Can I take this issue? This is my first time contributing to open source project.

[AnthonyLatsis]

Sure, go ahead.

[ED-1-0]

Just to clarify, am I supposed to build diagnostic tools that finds unused diagnostic ID? In addition, Any suggestion on where should I start?

[xedin]

You'd need to load include/swift/AST/DiagnosticsAll.def and for each diagnostic ID in there check whether diag::<diagnostic-id> appears anywhere in lib/ or include/ excluding *.def files.

[XaurDesu]

This could perhaps be a test that would fail if there are unused diagnostics.

Would https://github.com/apple/swift/tree/main/test/diagnostics be a good location for it? I was thinking we could probably add it as a new file, given how generally files on this folder seem to be designed to just check for a specific problem and i'd imagine this would probably fit under that description more or less. Something along the lines of no-unused-diagnostic-ids.swift.

[mininny]

It seems like there hasn't been much activity in this issue, so I went ahead and created the script. #68902

If you run the script right now, there are about 90 unused diagnostics.
I could go ahead and remove all of the unused diagnostics in the same(or different) PR, but I also think they were created with specific intentions, so maybe we should check if we can find appropriate places for all of the diagnostics.

[tbkka]

Using git blame, you should be able to find out when/why that diagnostic was added, then trace to see whether it was ever used or whether it was used and then deleted. That might help you identify which ones can safely be deleted now.