re-enable nix ci tests

1. we don't need to import diffutils in nix, they come with stdenv

2. cabal2nix doesn't seem to pick up build-tool-depends, so we add agda2hs
   to $PATH manually in the nix build

Author: liesnikov

.github/workflows/nix-ci.yml

@@ -61,6 +61,15 @@ jobs:
       with:
         primary-key: nix-devshell-${{ hashFiles('**/*.nix', '**/flake.lock') }}
     - name: drop into the devshell and run cabal
-      # for some reason this doesn't work without update
-      # even though it doesn't actually download anything
+      # we're using v1 command due to a lonstanding cabal-nix bug
+      #
+      # explainer: https://gist.github.com/ScottFreeCode/ef9f254e2dd91544bba4a068852fc81f
+      #
+      # bugs:
+      # https://github.com/haskell/cabal/issues/8434
+      # https://github.com/NixOS/nixpkgs/issues/130556
+      # https://github.com/NixOS/nixpkgs/issues/176887
+      #
+      # finally, v1-build doesn't work without update for some reason
+      # even though the update doesn't actually download anything
       run: nix develop -Lv -c bash -c "cabal update; cabal v1-build"

nix/agda2hs.nix

@@ -12,7 +12,6 @@
   makeWrapper,
   writeText,
   ghcWithPackages,
-  diffutils,
 }:
 
 let
@@ -53,7 +52,6 @@ let
       {
         inherit pname version;
         nativeBuildInputs = [
-          diffutils
           makeWrapper
         ];
         passthru = {

nix/lib.nix

@@ -10,10 +10,18 @@ let
       src = ../.;
       extraCabal2nixOptions = options; # "--jailbreak"
     };
-  # Disable tests in nix build - the testsuite requires the agda2hs binary
-  # to be in PATH, which is a circular dependency. Tests should be run
-  # via `cabal test` in a development environment instead.
-  hpkg = pkgs.haskell.lib.dontCheck (pkgs.haskellPackages.callPackage (hsrc "") { });
+  hpkg = (pkgs.haskellPackages.callPackage (hsrc "") {}).overrideAttrs (
+    finAttr: preAttr: {
+      # make executables executable and add ./dist/build to PATH
+      # cabal2nix doesn't pick up agda2hs from build-tool-depends of the test suite
+      # my (@liesnikov) guess is that since the intermediate binary
+      # is not a separate derivation it's hard to form a fixpoint on a derivation level
+      preCheck = ''
+        find dist/build -exec chmod u+X {} +
+        export PATH=$(pwd)/dist/build/:$PATH
+      '';
+    }
+  );
   expr = import ./agda2hs.nix;
   agda2hs = pkgs.lib.makeScope pkgs.newScope (
     self:

nix/shell.nix

@@ -24,7 +24,5 @@ pkgs.haskellPackages.shellFor {
     pkgs.nixfmt-rfc-style
     cabal2nix
     pkgs.haskellPackages.fix-whitespace
-    # The testsuite uses diff, so ensure `diff` is available in the shell/tests
-    pkgs.diffutils
   ];
 }