nix: separate derivations for build, check, test

flake.nix

@@ -26,7 +26,7 @@
               pname = "tomono";
               version = "2.0.0";
               src = ./.;
-              outputs = [ "out" "doc" ];
+              outputs = [ "out" ];
               buildPhase = ''
                 # Remove the stale VCS copy
                 rm tomono
@@ -59,45 +59,37 @@
               outputs = [ "out" ];
             });
           };
-          # Build a whole new test derivation and use the final, baked, built
-          # derivation as input so we really fully test the actual final
-          # released program. Rather than overriding attrs, which would rebuild
-          # a copy of the program, and might test something different
-          # (particularly with differing phases or hooks that accidentally do or
-          # dont get called). This is a more reliable way of testing in Nix,
-          # imo. Unfortunately, the test code itself is also built by the
-          # derivation, so while it looks like overriding here, I’m only doing
-          # that to build the /test/ code, not the actual binary that will be
-          # tested. But if e.g. the test script was checked into git, I’d just
-          # create a completely separate derivation and call that.
-          checks.default = self.packages.${system}.default.overrideAttrs (_: {
-            pname = "tomono-test";
-            version = "1.0";
-            env = {
-              GIT_AUTHOR_NAME = "Test";
-              GIT_AUTHOR_EMAIL = "test@test.com";
-              GIT_COMMITTER_NAME = "Test";
-              GIT_COMMITTER_EMAIL = "test@test.com";
-            };
-            outputs = [ "out" ];
-            nativeBuildInputs = [
-              # The actual code being tested
-              self.packages.${system}.default
-              pkgs.makeWrapper
-            ];
-            doCheck = true;
-            checkPhase = ''
-              # Ensure we're testing the pre-built tomono derivation, not the new one
-              rm -f tomono
-              # I could add pkgs.git to nativeBuildInputs but I'd be polluting
-              # the PATH for the tomono script itself as well, which means I
-              # could miss a misconfiguration if that derivation doesn't
-              # properly supply its own git path somehow.
-              wrapProgram test --suffix PATH : "${pkgs.git}/bin"
-              ./test
-            '';
-            installPhase = "echo done > $out";
-            dontFixup = true;
-          });
+          checks.default =
+            let
+              # Create an entirely separate derivation for the test script
+              # alone. This can reuse the git path baking and the shebang
+              # patching of the main derivation, so I can just immediately call
+              # it.
+              tomono-test = self.packages.${system}.default.overrideAttrs (_: {
+                pname = "tomono-test";
+                installPhase = "mkdir -p $out/bin; cp test $out/bin/tomono-test";
+              });
+            in
+              pkgs.stdenv.mkDerivation (_: {
+                pname = "tomono-check";
+                version = "1.0";
+                env = {
+                  GIT_AUTHOR_NAME = "Test";
+                  GIT_AUTHOR_EMAIL = "test@test.com";
+                  GIT_COMMITTER_NAME = "Test";
+                  GIT_COMMITTER_EMAIL = "test@test.com";
+                };
+                nativeBuildInputs = [
+                  # The actual code being tested. Must be in PATH.
+                  self.packages.${system}.default
+                  pkgs.makeWrapper
+                ];
+                dontUnpack = true;
+                buildPhase = ''
+                  ${tomono-test}/bin/tomono-test
+                '';
+                # To keep Nix happy
+                installPhase = "echo done > $out";
+              });
         });
   }