Add Weeder

[ulysses4ever]

Mikolaj says:

PR that removes dead code (after asking around if it's not used in a weird way in some test script) is ... [very] valuable (and there are tools for that, e.g, weeder).

So, I'm opening a ticket about Weeder (didn't find any old one). It looks like the repo still has remnants of a past Weeder setup even:

• https://github.com/haskell/cabal/blob/c1659a3f43d57a3e15a32e248b444cbfd069ded0/weeder.dhall
• https://github.com/haskell/cabal/blob/c1659a3f43d57a3e15a32e248b444cbfd069ded0/cabal.project.weeder

I think it could be added to CI, perhaps?

[ffaf1]

If we run weeder now, does it find dead code?

The only thing I am worried about is false positives.

[philderbeast]

There's a weeder gh-action.

[philderbeast]

Weeder has switched to using TOML for configuration. Also the .hie files in cabal-testsuite cannot yet be ignored by configuration and must be deleted (as a workaround) ocharles/weeder#56 (comment).

$ weeder --version
weeder version 2.7.0
hie version 9063
$ weeder --write-default-config
Did not find config: wrote default config to ./weeder.toml
incompatible hie file: /.../cabal-testsuite/PackageTests/CopyHie/setup.dist/work/dist/build/extra-compilation-artifacts/hie/HieLocal.hie
    this version of weeder was compiled with GHC version 9063
    the hie files in this project were generated with GHC version 9047
    weeder must be built with the same GHC version as the project it is used on

[philderbeast]

From the Cabal-3.4.0.0 release notes and cabal-install-3.4.0.0 release notes:

• Add and use weeder !6779 !6790

[Mikolaj]

I wonder if we can make CI run weeder, to avoid it bit-rotting so quickly (3 years) again? But it's tricky, because false positive can annoy users and also a temporarily unused code while developing should not cancel CI runs.