Extract Zig package builder

Consider moving to NUR or upstream?

flake.nix

@@ -12,6 +12,10 @@
         inherit (nixpkgs) lib;
         pkgs = import nixpkgs { inherit system; };
         zig = pkgs.zig_0_11;
+        buildZigPackage = import ./nix/build-zig-package.nix {
+          inherit (pkgs) stdenv;
+          inherit zig;
+        };
       in
       {
         devShells = {
@@ -23,39 +27,9 @@
           };
         };
 
-        packages.default =
+        packages.default = buildZigPackage {
-          let
+          name = "aoc23";
-            optimize = "ReleaseSafe";
+          src = lib.cleanSource ./.;
-          in
+        };
-          pkgs.stdenv.mkDerivation {
-            name = "aoc23";
-            src = lib.cleanSource ./.;
-
-            buildInputs = [ zig ];
-
-            # https://github.com/ziglang/zig/issues/6810
-            XDG_CACHE_HOME = "xdg_cache";
-
-            zigFlags = [ "-Doptimize=${optimize}" ];
-
-            buildPhase = ''
-              runHook preBuild
-              zig build $zigFlags
-              runHook postBuild
-            '';
-
-            doCheck = true;
-            checkPhase = ''
-              runHook preCheck
-              zig build test $zigFlags
-              runHook postCheck
-            '';
-
-            installPhase = ''
-              runHook preInstall
-              zig build install --prefix $out $zigFlags
-              runHook postInstall
-            '';
-          };
       });
 }

nix/build-zig-package.nix

@@ -0,0 +1,57 @@
+{ stdenv
+, zig
+}:
+
+{ buildInputs ? [ ]
+, nativeBuildInputs ? [ ]
+, optimize ? "ReleaseSafe"
+, meta ? { }
+, ...
+}@args:
+
+stdenv.mkDerivation (args // {
+  nativeBuildInputs = nativeBuildInputs ++ [
+    zig
+  ];
+
+  buildInputs = buildInputs;
+
+  strictDeps = true;
+
+  configurePhase = args.configurePhase or ''
+    runHook preConfigure
+    runHook postConfigure
+  '';
+
+  ZIGFLAGS = args.ZIGFLAGS or [
+    "-Doptimize=${optimize}"
+  ];
+
+  # https://github.com/ziglang/zig/issues/6810 requires setting XDG_CACHE_HOME
+  # in all zig build phases. --cache-dir and --global-cache-dir do not prevent
+  # the builder from attempting to create the XDG cache.
+
+  buildPhase = args.buildPhase or ''
+    runHook preBuild
+    XDG_CACHE_HOME=_cache zig build $ZIGFLAGS
+    runHook postBuild
+  '';
+
+  doCheck = args.doCheck or true;
+  checkPhase = args.checkPhase or ''
+    runHook preCheck
+    XDG_CACHE_HOME=_cache zig build test $ZIGFLAGS
+    runHook postCheck
+  '';
+
+  installPhase = args.installPhase or ''
+    runHook preInstall
+    XDG_CACHE_HOME=_cache zig build install --prefix $out $ZIGFLAGS
+    runHook postInstall
+  '';
+
+  meta = {
+    # default to Zig's platforms
+    platforms = zig.meta.platforms;
+  } // meta;
+})