build-tool-depends entries don't get added to 'depends', and thus won't be in shellFor

[ocharles]

It seems that the correct way to use tasty-discover is to add it to a test-suites build-tool-depends section, rather than just build-depends. build-tool-depends guarantees that it's on $PATH at build time, which is needed for it to work as it's a GHC preprocessor. However, if we do this then it correctly builds as a component, but the derived shellFor doesn't include tasty-discover. Looking at the output of cabal-to-nix, we see:

      tests = {
        "tests" = {
          depends = [
            ...
            (hsPkgs."tasty" or (errorHandler.buildDepError "tasty"))
            (hsPkgs."tasty-hedgehog" or (errorHandler.buildDepError "tasty-hedgehog"))
            (hsPkgs."tasty-hspec" or (errorHandler.buildDepError "tasty-hspec"))
            ];
          build-tools = [
            (hsPkgs.buildPackages.tasty-discover or (pkgs.buildPackages.tasty-discover or (errorHandler.buildToolDepError "tasty-discover")))
            ];
          buildable = true;
          };
        };

Note that tasty-discover is not in depends. But in shellFor, we have:

  packageInputs = removeSelectedPackages (lib.concatMap (cfg: cfg.depends) selectedConfigs) ++ additionalPackages;

which only looks at depends.

[ocharles]

Perhaps the other fix is for shellFor to also add build-tools?

[ocharles]

Closing as I think input-output-hk/haskell.nix#839 is really where this issue belongs.