zig: add fetcher

pkgs/development/compilers/zig/fetcher.nix

@@ -0,0 +1,32 @@
+{
+  lib,
+  zig,
+  runCommand,
+}:
+{
+  pname,
+  version,
+  name ? "${pname}-${version}",
+  src,
+  hash ? lib.fakeHash,
+}@args:
+runCommand "${name}-zig-deps"
+  {
+    inherit (args) src;
+
+    nativeBuildInputs = [ zig ];
+
+    outputHashAlgo = null;
+    outputHashMode = "recursive";
+    outputHash = hash;
+  }
+  ''
+    export ZIG_GLOBAL_CACHE_DIR=$(mktemp -d)
+
+    runHook unpackPhase
+
+    cd $sourceRoot
+    zig build --fetch
+
+    mv $ZIG_GLOBAL_CACHE_DIR/p $out
+  ''

pkgs/development/compilers/zig/passthru.nix

@@ -30,4 +30,6 @@
   };
 
   stdenv = overrideCC stdenv zig.cc;
+
+  fetchDeps = callPackage ./fetcher.nix { inherit zig; };
 }