Add basic bzlmod setup

This commit adds a MODULE.bazel that makes rules_go usable as a bzlmod
module for basic use cases.

The new module definition is verified by a test module that will be used
in the presubmit tests of the Bazel Central Registry (BCR).

The following features require more thought and/or work and are not yet
supported:
* SDK rules other than go_host_sdk and go_download_sdk.
* non-no-op nogo
* go_proto_library

.bazelci/presubmit.yml

@@ -23,6 +23,20 @@ tasks:
     - "//..."
     test_targets:
     - "//..."
+  ubuntu2004_bcr_tests:
+    name: BCR test module
+    platform: ubuntu2004
+    bazel: 5.2.0
+    working_directory: bcr_tests
+    build_flags:
+      - "--experimental_enable_bzlmod"
+    test_flags:
+      - "--experimental_enable_bzlmod"
+    build_targets:
+      - "//..."
+      - "@go_sdk//..."
+    test_targets:
+      - "//..."
   macos:
     shell_commands:
       - tests/core/cgo/generate_imported_dylib.sh

.bazelignore

@@ -0,0 +1 @@
+bcr_tests

.gitignore

@@ -1,3 +1,4 @@
 /bazel-*
 /tests/core/cgo/libimported.*
 /tests/core/cgo/libversioned.*
+/bcr_tests/bazel-*

MODULE.bazel

@@ -0,0 +1,21 @@
+module(
+    name = "io_bazel_rules_go",
+    version = "0.33.0",
+    compatibility_level = 1,
+    toolchains_to_register = [
+        "@go_default_sdk//:all",
+    ],
+)
+
+bazel_dep(name = "bazel_skylib", version = "1.2.0")
+bazel_dep(name = "platforms", version = "0.0.4")
+
+non_module_dependencies = use_extension("@io_bazel_rules_go//go/private:extensions.bzl", "non_module_dependencies")
+use_repo(
+    non_module_dependencies,
+    "io_bazel_rules_nogo",
+)
+
+go_sdk = use_extension("@io_bazel_rules_go//go:extensions.bzl", "go_sdk")
+go_sdk.download(name = "go_default_sdk", version = "1.18.3")
+use_repo(go_sdk, "go_default_sdk")

bcr_tests/.bazelrc

@@ -0,0 +1 @@
+build --experimental_enable_bzlmod

bcr_tests/BUILD.bazel

@@ -0,0 +1,19 @@
+load("@rules_go//go:def.bzl", "go_binary", "go_library", "go_test")
+
+go_library(
+    name = "lib",
+    srcs = ["lib.go"],
+    importpath = "example.com/lib",
+)
+
+go_binary(
+    name = "main",
+    srcs = ["main.go"],
+    deps = [":lib"],
+)
+
+go_test(
+    name = "test",
+    srcs = ["test.go"],
+    embed = [":lib"],
+)

bcr_tests/MODULE.bazel

@@ -0,0 +1,15 @@
+module(
+    name = "rules_go_bcr_tests",
+    # Test that the default SDK is registered by not registering one from the test module.
+)
+
+bazel_dep(name = "io_bazel_rules_go", version = "", repo_name = "rules_go")
+local_path_override(
+    module_name = "io_bazel_rules_go",
+    path = "..",
+)
+
+# Test that this correctly downloads the SDK by requesting it from the commandline (see presubmit.yml).
+go_sdk = use_extension("@rules_go//go:extensions.bzl", "go_sdk")
+go_sdk.download(name = "go_sdk", version = "1.17.5")
+use_repo(go_sdk, "go_sdk")

bcr_tests/lib.go

@@ -0,0 +1,5 @@
+package lib
+
+func Name() string {
+	return "bzlmod"
+}

bcr_tests/main.go

@@ -0,0 +1,11 @@
+package main
+
+import (
+	"fmt"
+
+	"example.com/lib"
+)
+
+func main() {
+	fmt.Printf("Hello %s!", lib.Name())
+}

bcr_tests/test.go

@@ -0,0 +1,11 @@
+package lib
+
+import (
+	"testing"
+)
+
+func TestName(t *testing.T) {
+	if Name() != "bzlmod" {
+		t.Fail()
+	}
+}

go/extensions.bzl

@@ -0,0 +1,3 @@
+load("//go/private:extensions.bzl", _go_sdk = "go_sdk")
+
+go_sdk = _go_sdk

go/private/extensions.bzl

@@ -0,0 +1,53 @@
+load("//go/private:sdk.bzl", "go_download_sdk", "go_host_sdk")
+load("//go/private:repositories.bzl", "go_rules_dependencies")
+
+_download_tag = tag_class(
+    attrs = {
+        "name": attr.string(mandatory = True),
+        "goos": attr.string(),
+        "goarch": attr.string(),
+        "sdks": attr.string_list_dict(),
+        "urls": attr.string_list(default = ["https://dl.google.com/go/{}"]),
+        "version": attr.string(),
+        "strip_prefix": attr.string(default = "go"),
+    },
+)
+
+_host_tag = tag_class(
+    attrs = {
+        "name": attr.string(mandatory = True),
+    },
+)
+
+def _go_sdk_impl(ctx):
+    for mod in ctx.modules:
+        for download_tag in mod.tags.download:
+            go_download_sdk(
+                name = download_tag.name,
+                goos = download_tag.goos,
+                goarch = download_tag.goarch,
+                sdks = download_tag.sdks,
+                urls = download_tag.urls,
+                version = download_tag.version,
+                register_toolchains = False,
+            )
+        for host_tag in mod.tags.host:
+            go_host_sdk(
+                name = host_tag.name,
+                register_toolchains = False,
+            )
+
+go_sdk = module_extension(
+    implementation = _go_sdk_impl,
+    tag_classes = {
+        "download": _download_tag,
+        "host": _host_tag,
+    },
+)
+
+def _non_module_dependencies_impl(ctx):
+    go_rules_dependencies(force = True)
+
+non_module_dependencies = module_extension(
+    implementation = _non_module_dependencies_impl,
+)