flake.nix: Do check

flake.nix

@@ -2,10 +2,13 @@
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
 
+    # must match golden-tests/derivations/complex/new/flake.lock
+    nixpkgs-golden.url = "github:NixOS/nixpkgs/5ca8e2e9e1fa5e66a749b39261ad6bd0e07bc87f";
+
     utils.url = "github:numtide/flake-utils";
   };
 
-  outputs = { nixpkgs, utils, ... }:
+  outputs = inputs@{ nixpkgs, utils, ... }:
     let
       config = { };
 
@@ -22,7 +25,32 @@
               # Golden tests require write access to the /nix/var
               # and read access to the test data into /nix/store
               # So we can't run these tests in build time
-              pkgsNew.haskell.lib.dontCheck
+              pkgsNew.haskell.lib.compose.overrideCabal
+                (old: {
+                  testToolDepends = old.testToolDepends or [] ++ [
+                    pkgsNew.nix
+                  ];
+                  # For testing, we use a relocated Nix store, because the
+                  # real /nix/store in the sandbox is only accessible as files;
+                  # not a store objects (https://nixos.org/manual/nix/stable/glossary.html#gloss-store-object)
+                  # A relocated store lets us use /nix/store as the logical
+                  # storeDir, while using an arbitrary directory for storage.
+                  preCheck = ''
+                    ${old.preCheck or ""}
+                    TESTROOT="$(mktemp --tmpdir -d test-state-XXXXXX)"
+                    export NIX_BUILD_HOOK=
+                    export NIX_CONF_DIR=$TEST_ROOT/etc
+                    export NIX_LOCALSTATE_DIR=$TEST_ROOT/var
+                    export NIX_LOG_DIR=$TEST_ROOT/var/log/nix
+                    export NIX_STATE_DIR=$TEST_ROOT/var/nix
+                    export NIX_REMOTE="$TESTROOT/store"
+                    export HOME="$TESTROOT/home"
+                    mkdir -p $HOME
+                    echo "Copying pinned Nixpkgs for golden tests to relocated store $NIX_REMOTE"
+                    nix eval --expr 'builtins.path { path = ${inputs.nixpkgs-golden}; name = "source"; }' \
+                      --impure --extra-experimental-features nix-command --offline
+                  '';
+                })
                 (self.callCabal2nix "nix-diff" ./. { });
           };
         });