haskell/cabal

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

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#9,688 opened on Feb 2, 2024

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cabal-testsuitegood first issuenewcomertype: enhancement

Description

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

Describe the bug

https://github.com/haskell/cabal/pull/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

Contributor guide

Tech stack
None
Domain
toolingbuild system
Issue type
test
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
knowledge of cabalbasic testing
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70
Research direction
Investigate the reverted PR #8966 to understand the sub component targets feature that was removed. Write a regression test in cabal install's test suite that calls `cabal repl` with a file path and verifies it works correctly. The test should ensure similar breakage is caught in the future.