feat(nix/buildTypedGo): Add a //nix/buildGo wrapper for typed Go

These functions work like buildGo.program & buildGo.package, but run
the .go2 sources through go2go first before passing them to the
//nix/buildGo equivalents.

Change-Id: Id1ebab6085c390d6986387370181377b9f5d39e8

nix/buildTypedGo/default.nix

@@ -0,0 +1,33 @@
+# SPDX-License-Identifier: Apache-2.0
+#
+# A crude wrapper around //nix/buildGo that supports the Go 2 alpha.
+#
+# The way the alpha is implemented is via a transpiler from typed to
+# untyped Go.
+{ depot, pkgs, ... }:
+
+let
+  inherit (builtins)
+    stringLength
+    substring;
+
+  inherit (depot.nix.buildGo) gpackage program;
+
+  go2goext = file: substring 0 ((stringLength file) - 1) file;
+  go2go = file: pkgs.runCommandNoCC "${go2goext (toString file)}" {} ''
+    cp ${file} .
+    ${pkgs.go}/bin/go tool go2go translate *.go2
+    mv *.go $out
+  '';
+
+in rec {
+  program = { name, srcs, deps ? [], x_defs ? {} }: depot.nix.buildGo.program {
+    inherit name deps x_defs;
+    srcs = map go2go srcs;
+  };
+
+  package = { name, srcs, deps ? [], x_defs ? {}, sfiles ? [] }: depot.nix.buildGo.package {
+    inherit name deps x_defs sfiles;
+    srcs = map go2go srcs;
+  };
+}

nix/buildTypedGo/example/default.nix

@@ -0,0 +1,8 @@
+{ depot, ... }:
+
+depot.nix.buildTypedGo.program {
+  name = "example";
+  srcs = [
+    ./main.go2
+  ];
+}

nix/buildTypedGo/example/main.go2

@@ -0,0 +1,15 @@
+package main
+
+import (
+	"fmt"
+)
+
+func Print(type T)(s []T) {
+	for _, v := range s {
+		fmt.Print(v)
+	}
+}
+
+func main() {
+	Print([]string{"Hello, ", "TVL\n"})
+}