Removal of sub-component targets breaks HLS/hie-bios: we need regression test

[TeofilC]

UPDATE: the change was reverted but let's leave the ticket open until we have a regression test.

Describe the bug

#8966 removes cabal-install's support for sub-module targets.

But this (mis-)feature is used by HLS via hie-bios to determine the flags for a component.

In particular it makes a call to cabal v2-repl <path to a .hs in the component>

To Reproduce
Steps to reproduce the behavior:

Have a prerelease of cabal-install on your PATH

$ cabal repl <path to a .hs file>
Warning: this is a debug build of cabal-install with assertions enabled.
Error: [Cabal-7121]
Failed extracting script block: `{- cabal:` start marker not found

or

$ haskell-language-server <path to .hs file or a component target>
...
Loading the module 'so-and-so.hs' failed.

  It may not be listed in your .cabal file!
  Perhaps you need to add `so-and-so` to other-modules or exposed-modules.

Expected behavior
HLS should at least work.
Perhaps some work needs to be done to migrate HLS/hie-bios off this feature.
Alternatively the MR could be reverted.

System information
tested using cabal-head

[TeofilC]

Issue on the hie-bios tracker: haskell/hie-bios#427

[fendor]

Until a proper interface is given by cabal to find compilation options given a FilePath, the patch #8966 must not get released into the wild. It also breaks HLS for all users that are using cabal nightlies.

Likely, the patch #8966 should be reverted. If the target interface is to be redesigned, I think there should be a design phase with a clear picture, what is supported and what isn't supported.

[andreabedini]

🤬 ok, fair call to revert in the short term.

[andreabedini]

Can someone contribute an integration test for what hls relies upon?

[fendor]

What would an integration test look like? Just a bunch of tests with the description: Features HLS relies on

I will take care of writing such a test.

[ulysses4ever]

I think it'd be really nice to add a test. We were close to releasing it and get a full release of Cabal breaking HLS. We should try to make sure we avoid it in the future.