go_proto_compiler: remove dependence on C/C++ toolchain

The C compiler was used to set PATH. No idea why that was useful though.

Fixes bazel-contrib#2089

go/tools/builders/protoc.go

@@ -53,7 +53,6 @@ func run(args []string) error {
 	outPath := flags.String("out_path", "", "The base output path to write to.")
 	plugin := flags.String("plugin", "", "The go plugin to use.")
 	importpath := flags.String("importpath", "", "The importpath for the generated sources.")
-	compilerPath := flags.String("compiler_path", "", "The value for PATH.")
 	flags.Var(&options, "option", "The plugin options.")
 	flags.Var(&descriptors, "descriptor_set", "The descriptor set to read.")
 	flags.Var(&expected, "expected", "The expected output files.")
@@ -85,7 +84,6 @@ func run(args []string) error {
 	}
 	protoc_args = append(protoc_args, flags.Args()...)
 	cmd := exec.Command(*protoc, protoc_args...)
-	cmd.Env = append(os.Environ(), fmt.Sprintf("PATH=%s", *compilerPath))
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr
 	if err := cmd.Run(); err != nil {

proto/compiler.bzl

@@ -69,7 +69,6 @@ def go_proto_compile(go, compiler, protos, imports, importpath):
     args.add("-plugin", compiler.plugin)
 
     # TODO(jayconrod): can we just use go.env instead?
-    args.add("-compiler_path", go.cgo_tools.c_compiler_path.rpartition("/")[0])
     args.add_all(compiler.options, before_each = "-option")
     if compiler.import_path_option:
         args.add_all([importpath], before_each = "-option", format_each = "import_path=%s")

tests/core/cross/BUILD.bazel

@@ -1,4 +1,5 @@
 load("@io_bazel_rules_go//go:def.bzl", "go_binary", "go_library", "go_source", "go_test")
+load("@io_bazel_rules_go//proto:def.bzl", "go_proto_library")
 load("@io_bazel_rules_go//tests:bazel_tests.bzl", "bazel_test")
 
 test_suite(
@@ -80,3 +81,23 @@ go_library(
     importpath = "github.com/bazelbuild/rules_go/tests/core/cross/ios_lib",
     tags = ["manual"],
 )
+
+bazel_test(
+    name = "proto_test",
+    args = ["--platforms=@io_bazel_rules_go//go/toolchain:ios_amd64"],
+    command = "build",
+    targets = [":cross_go_proto"],
+)
+
+proto_library(
+    name = "cross_proto",
+    srcs = ["cross.proto"],
+    tags = ["manual"],
+)
+
+go_proto_library(
+    name = "cross_go_proto",
+    importpath = "github.com/bazelbuild/rules_go/tests/core/cross",
+    protos = [":cross_proto"],
+    tags = ["manual"],
+)

tests/core/cross/README.rst

@@ -25,3 +25,7 @@ a dependency using ``@io_bazel_rules_go//go/platform:darwin``, which is true
 when building for iOS (tested by ``ios_select_test``) and macOS
 (tested by ``use_ios_lib``).
 
+proto_test
+----------
+
+Tests that a ``go_proto_library`` can be cross-compiled with ``--platforms``.

tests/core/cross/cross.proto

@@ -0,0 +1,9 @@
+syntax = "proto3";
+
+package cross;
+
+option go_package = "github.com/bazelbuild/rules_go/tests/core/cross";
+
+message Foo {
+  int64 x = 1;
+}