Fix PATH changes not triggering REPL reconfiguration (#2015)

[mpickering]

When a user's PATH environment variable changes between Cabal commands, the environment in the REPL becomes incorrect. This occurs because during initial package configuration, the current PATH is captured via programSearchPathAsPATHVar and then stored in the programOverrideEnv field of a ConfiguredProgram. These ConfiguredPrograms are subsequently serialized into the setup-config file, effectively baking in the PATH environment from the time of configuration.

The issue manifests when PATH is updated after initial configuration. Although the solver re-runs when detecting PATH changes, the dryRunLocalPkg function examines ElaboratedConfiguredPackage and incorrectly determines that nothing has changed that would require reconfiguration. This decision is incorrect because setup-config now contains a stale reference to the original PATH value, which no longer matches the current environment.

To fix this problem, we need to store the ConfiguredPrograms directly in ElaboratedConfiguredPackage. This approach will ensure that when PATH changes, the ConfiguredPrograms will also change, properly triggering reconfiguration. While this solution will cause more recompilations when PATH changes, it guarantees that the correct environment is always used during builds and REPL sessions.

An alternative approach would be to stop baking the PATH variable into the environment of programs, but this would require more substantial changes to how Cabal manages environment variables.

Fixes #2015

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[Mikolaj]

Is this ready for review? If so, could you please add the label?

[mpickering]

Any reviewers? @ulysses4ever ?

1 week without movement, so I am happy to just merge it if no-one wishes to review.

[ulysses4ever]

Sorry for slow progress here: everyone was busy with the release. I'll try to take a look today.

I don't think it's possible to merge anything without reviews. Unless you're going to change repository settings, which is a big change for the project and should be discussed on a meeting.

[ulysses4ever]

I'd suggest that you mention in the commit message why adding a new field, seemingly unused anywhere, changes the behaviour (it's because of the Generic business afaiu).

[ulysses4ever]

This text, including the title, feels a little repetitive. I suggest dropping the first sentence in the description and make it into one paragraph:

Suggested change

	When modifying your PATH environment variable between Cabal commands, REPL
	sessions will now correctly use the updated environment. Previously, if you
	changed your PATH after initial configuration, Cabal would continue using the
	old PATH settings in REPL sessions without warning.
	With this fix, Cabal properly detects PATH changes and reconfigures the REPL
	accordingly, ensuring tools and libraries in your current PATH are available.
	Previously, if you changed your PATH after initial configuration, Cabal would
	continue using the old PATH settings in REPL sessions without warning.
	With this fix, Cabal properly detects PATH changes and reconfigures the REPL
	accordingly, ensuring tools and libraries in your current PATH are available.

Again, consider that it appears right after the title ("synopsis" field). And that the former third sentence basically repeated the former first one that I suggest to drop.