Add “Add default-language” suggestion

[ffaf1]

Closes #9620

Add a warning ("Hackage would refuse this package") when default-language is missing. Done to prepare the user to GHC language editions.

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** QA notes **

1. cabal init a package, choose cabal-version 3.4
2. run cabal check
3. old cabal: it will not complain about missing default-language
4. new cabal: it wil complain about missing default-language

[adamgundry]

Thanks for tackling this @ffaf1!

I think we should make Hackage reject packages with no default-language. Otherwise it's very confusing that such packages will end up using Haskell98 (or maybe Haskell2010 in some cases) due to a Cabal default that differs from GHC's default. See https://github.com/ghc-proposals/ghc-proposals/pull/613#issuecomment-1879581188 where @gbaz expresses the same opinion.

[ffaf1]

Sure @adamgundry, the warning is now hard ("Hackage would reject this package").

[jasagredo]

Confirmed working on Windows in today's master

[Kleidukos]

@mergifyIO backport 3.12

[mergify[bot]]

backport 3.12

✅ Backports have been created

• #10023 Add “Add default-language” suggestion (backport #9766) has been created for branch 3.12

[Bodigrim]

Just a reminder that adding new constructors such as CVDefaultLanguageComponentSoft and CICVDefaultLanguageComponentSoft is a breaking change and mandates a major version bump:

• https://hackage.haskell.org/package/Cabal-3.12.0.0/docs/Distribution-PackageDescription-Check.html#t:CheckExplanation
• https://hackage.haskell.org/package/Cabal-3.12.1.0/docs/Distribution-PackageDescription-Check.html#t:CheckExplanation

[geekosaur]

Rushing 3.12.0.0 out before we were really ready is looking more and more like Bad Plan….

[Bodigrim]

Well, Cabal-3.12.0.0 had to match GHC 9.10 release.

[Mikolaj]

Rushing 3.12.0.0 out before we were really ready is looking more and more like Bad Plan….

I don't follow. Cabal 3.12.0.0 did not have this issue and I don't see how we were not ready. @geekosaur: would you like to explain the link in #10160?