`--haddock-executables` index does not distinguish `Main` modules

[mpilgrem]

Version 2.16.0, Git revision 1817292f22987cc764a72146cf707f028c575bf5 (10654 commits) PRE-RELEASE x86_64 has --haddock-executables (built on top of Cabal (the library)'s haddock --executables), but there are problems with the index.

Consider a two-package project (packageA, packageB) each with two executable components (eg packageA-exe1, packageA-exe2). The index generated for project packages has four links to Main but all of them are to the Main of packageB-exe1.

Using haddock --show-interface reveals that all four *.haddock files commence:

{"link_env":{"$main$Main$main":"Main", ...

That can be contrasted with the two *.haddock files for the main library:

{"link_env":{"$packageA-0.1.0.0-ErEp6QMCDA5L7wclG18d4W$Lib$someFunc":"Lib"}, ...

and

{"link_env":{"$packageB-0.1.0.0-LJmkqLQIxknEQ7YIpA65Ah$Lib$someFunc":"Lib"}, ...

--cabal-verbosity 3 reveals that, with the main library, Cabal (the library) invokes Haddock with options that include:

packageA> --optghc=-this-unit-id
packageA> --optghc=packageA-0.1.0.0-ErEp6QMCDA5L7wclG18d4W

packageB> --optghc=-this-unit-id
packageB> --optghc=packageB-0.1.0.0-LJmkqLQIxknEQ7YIpA65Ah

There is no equivalent for when Cabal invokes Haddock for the executable components.

[mpilgrem]

@mikolaj has suggested as reading: https://github.com/haskell/cabal/pulls?q=is%3Apr+author%3Acoot authored by @coot.

[mpilgrem]

If I am correct about the -this-unit-id difference, it has it origins in Distribution.Simple.Haddock.componentGhcOptions and Distribution.Simple.GHC.Internal.componentGhcOptions.

The master branch of Cabal has (extract):

componentGhcOptions verbosity lbi bi clbi odir =
  let implInfo = getImplInfo $ compiler lbi
   in mempty
        { -- Respect -v0, but don't crank up verbosity on GHC if
          -- Cabal verbosity is requested. For that, use --ghc-option=-v instead!
          ghcOptVerbosity = toFlag (min verbosity normal)
        , ghcOptCabal = toFlag True
        , ghcOptThisUnitId = case clbi of
            LibComponentLocalBuildInfo{componentCompatPackageKey = pk} ->
              toFlag pk
            _ | not (unitIdForExes implInfo) -> mempty
            ExeComponentLocalBuildInfo{componentUnitId = uid} ->
              toFlag (unUnitId uid)
            ...

but unitIdForExes is not in any released version (up to Cabal-3.10.2.1 - GHC 9.8.2 comes with Cabal-3.10.2.0.). It was added in https://github.com/haskell/cabal/commit/8d402c7b2900ebeb2c320e40cf4c16338232eb43