handle trailing comma in import list properly

[kokobd]

fix #2958

[kokobd]

Strange. I can not reproduce this test failure locally. Both running tests and manually trying this feature give successful results.

extend multi line import with trailing comma:                                                     FAIL (1.67s)
          test/exe/Main.hs:[208](https://github.com/haskell/haskell-language-server/runs/7295318339?check_suite_focus=true#step:8:209)3:
          expected: "module ModuleB where\nimport ModuleA (stuffB, stuffA,\n               )\nmain = print (stuffA, stuffB)\n"
           but got: "module ModuleB where\nimport ModuleA (stuffB, stuffA\n               )\nmain = print (stuffA, stuffB)\n"
          Use -p '/without checkAll.extend multi line import with trailing comma/' to rerun this test only.

[kokobd]

I can not reproduce this test failure locally.

Okay, that's because ghc-exactprint behaves differently in GHC 9.0 (not 9.2!) and GHC 8.10.7, where the API is the same. But in GHC 9.0, an extra comma is attached to the "list", besides the one attached to the "list item". According to the docs, that behavior might be wrong.

Anyway, I finally got all compatibility issues handled.

[georgefst]

Works for me on GHC 9.2.3, thanks! (but I won't approve this because I haven't looked at the code)

[kokobd]

Works for me on GHC 9.2.3

@georgefst Thanks for your testing!

[fendor]

LGTM!

[fendor]

nitpick: can we align this or remove the superfuous space?

[kokobd]

Aha, my mistake. btw, stylish-haskell refuses to run on this file, because there are too many CPP, making it fail.