Cabal Plugin is too strict for the newest cabal versions

[fendor]

Generating a new package with cabal 3.12 via

$ mkdir test-hls-cabal
$ cd test-hls-cabal
$ cabal init --lib -n

Generates a .cabal file with cabal-version: 3.12, which haskell-language-server-2.9.0.1 built with ghc-9.6.5, which is linked against Cabal-3.10.3.0. Consequentially, HLS highlights the whole .cabal file as incorrect, as it doesn't recognise 3.12 as a valid cabal-version.

This is bad user-experience.

I am not sure whether we can fix that for good, as older HLS versions will never get linked against a newer Cabal version.
However, what we could do is to be more lenient when it comes to parsing .cabal files.
We can scan the .cabal file for the line cabal-version: (scanSpecVersion) and if the version is not known to us, we parse the cabal file with the newest CabalSpecVersion available to the HLS binary.
The cabal features may still break if users actually need new cabal-the-spec-features, but for many future changes, we likely only display warnings and many other IDE features keep working.

The only real disadvantage is that we can't reliably report incorrect cabal-version numbers any more. Also, the proposed behaviour might be unexpected to users.

The last resort I can think of, is to simply not provide any IDE features if the cabal-version is seemingly too new for the Cabal version linked into HLS.

[fendor]

It seems to be more difficult than hoped to parse a .cabal file with a newer cabal-version in a lenient way. The cabal-version is parsed in the FieldGrammar, so we would have to basically copy-paste major parts of Cabal-syntax to not verify the provided CabalSpecVersion is not equal to the actual parsed ones.

Thus, the only way I can think of now is to basically overwrite the cabal-version field in-memory.

Ignoring .cabal files with a too new version seems to be the less hacky approach.

[michaelpj]

Yeah, I think just emitting a diagnostic saying that we don't know how to parse it is best? So pretty much the current behaviour, but with a nicer diagnostic that doesn't cover the whole file 😅

[fridewald]

I'll give it a try

[fridewald]

There was an issue with handling errors in the cabal file if the error is in the first line. Cabal normally uses 1-based line numbering, but for the first line it can also return the line 0. Created a PR to safely fix this.

[fendor]

Fixed by #4425