Case split plugin

[Aster89]

This PR is for (re-¹)introducing the so called case-split plugin, as requested in #5013, and it is part of my project for GSoC 2026.

To see the full summary of the project, click here. The summary also contains a list of stretch goals, some of which have been addressed already.

The work I've done and that I plan to submit via the present pull request, has consisted mainly of:

• implementing the case-split plugin by incrementally focusing on several use-cases from less to more complex,
• making use of ghc-exactprint for correctly layout out the patterns to be inserted,
• contextually writing tests for guarding such use-cases as I enabled them,
• finally, striving to address some stretch goals.

The result of such work is a plugin providing a code action for adding missing patterns to a case/\case expression that has a non-exhaustive list of patterns.

For instance, the plugin turns this

data X = A
       | B
       | C Int
       | D Int Int
       | E Int Int Int Int
       | F

foo :: X -> Int
foo x = case x of
          A -> 3;
                B -> 4;
              C _ -> 5

into this

data X = -- same as above

foo :: X -> Int
foo x = case x of
          A -> 3;
                B -> 4;
              C _ -> 5
          D _ _ -> _
          E {} -> _
          F -> _

---

¹ Earlier attempts

Indeed, there were some earlier attempts. See the summary for some pointers.

[dschrempf]

Everybody is eagerly anticipating this change, thank you!

Given the many WIP commits, I just wanted to ask if you could sqaush or reword them. I guess you had this in mind anyways.

[Aster89]

Everybody is eagerly anticipating this change, thank you!

Glad to hear that!

Given the many WIP commits, I just wanted to ask if you could sqaush or reword them. I guess you had this in mind anyways.

Yep, see 3rd paragraph at the top :P

[dschrempf]

Ah, the second sub-clause :-P my attention had already shifted before that one, I apologize :-)! Thanks!

[MangoIV]

First pass. I think after you clean up the main logic a bit more and add some documentation there, I can take another look. :)

Very good work, looking forward to having this in HLS!

[MangoIV]

another small round :)

[fendor]

Very nice, this looks like it is making great progress!

I reviewed the plugin, I have mostly comments regarding readability and suggestions for improvements that hopefully improve the maintainability in the long run.

Some things are nitpicks, and you should feel free to ignore them.

I think we need one Note that outlines what the implementation strategy of the case split plugin is (e.g., reads the structured error message, etc...) so that it is easier to tell what the individual steps for case splitting are.