Include pure Swift version of swiftformat

This should probably be upstreamed... eventually.

pkgs/swiftformat.nix

@@ -0,0 +1,39 @@
+{ stdenv, lib, fetchFromGitHub, swift, swiftpm, swiftPackages }:
+
+# This derivation is impure: it relies on an Xcode toolchain being installed
+# and available in the expected place. The values of sandboxProfile and
+# hydraPlatforms are copied pretty directly from the MacVim derivation, which
+# is also impure.
+
+stdenv.mkDerivation rec {
+  pname = "swiftformat";
+  version = "0.51.13";
+
+  src = fetchFromGitHub {
+    owner = "nicklockwood";
+    repo = "SwiftFormat";
+    rev = version;
+    sha256 = "sha256-8AvPA8KzejU4BI/BAX+gCjwPi4Xv0jEQbLHj9OCtasw";
+  };
+
+  nativeBuildInputs = [ swift swiftpm ];
+  buildInputs = [ swiftPackages.Foundation ];
+
+  # We only install the swiftformat binary, so don't need the other products.
+  swiftpmFlags = [ "--product swiftformat" ];
+
+  installPhase = ''
+    binPath="$(swiftpmBinPath)"
+    mkdir -p $out/bin
+    cp $binPath/swiftformat $out/bin/
+  '';
+
+  meta = with lib; {
+    description = "A code formatting and linting tool for Swift";
+    homepage = "https://github.com/nicklockwood/SwiftFormat";
+    license = licenses.mit;
+    maintainers = [ maintainers.bdesham ];
+    platforms = with lib.platforms; linux ++ darwin;
+    hydraPlatforms = [];
+  };
+}