Add automatic platform detection from inbound crosstool_top and cpu

go/platform/crosstool.bzl

@@ -0,0 +1,58 @@
+# Copyright 2020 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+def _match_apple(crosstool_top, cpu):
+    """_match_apple will try to detect wether the inbound crosstool/cpu is
+    targeting the Apple ecosystem. Apple crosstool CPUs are prefixed, so
+    matching is easy."""
+    platform = {
+        "darwin_x86_64": "darwin_amd64",
+        "ios_arm64": "ios_arm64",
+        "ios_armv7": "ios_arm",
+        "ios_i386": "ios_386",
+        "ios_x86_64": "ios_amd64",
+    }.get(cpu)
+    if platform:
+        return "{}_cgo".format(platform)
+    return None
+
+def _match_android(crosstool_top, cpu):
+    """_match_android will try to detect wether the inbound crosstool is the
+    Android NDK toolchain. It can either be `//external:android/crosstool` or be
+    part of the `@androidndk` workspace. After that, translate Android CPUs to
+    Go CPUs."""
+    if str(crosstool_top) == "//external:android/crosstool" or \
+       crosstool_top.workspace_name == "androidndk":
+        platform_cpu = {
+            "arm64-v8a": "arm64",
+            "armeabi-v7a": "arm",
+            "x86": "386",
+            "x86_64": "amd64",
+        }.get(cpu)
+        if platform_cpu:
+            return "android_{}_cgo".format(platform_cpu)
+    return None
+
+def platform_from_crosstool(crosstool_top, cpu):
+    """platform_from_crosstool runs matchers against the crosstool_top/cpu pair
+    to automatically infer the target platform."""
+    matchers = [
+        _match_apple,
+        _match_android,
+    ]
+    for matcher in matchers:
+        platform = matcher(crosstool_top, cpu)
+        if platform:
+            return "@io_bazel_rules_go//go/toolchain:{}".format(platform)
+    return None

go/private/rules/transition.bzl

@@ -25,6 +25,10 @@ load(
     "@io_bazel_rules_go_name_hack//:def.bzl",
     "IS_RULES_GO",
 )
+load(
+    "@io_bazel_rules_go//go/platform:crosstool.bzl",
+    "platform_from_crosstool",
+)
 
 def _filter_transition_label(label):
     """Transforms transition labels for the current workspace.
@@ -106,6 +110,8 @@ def _go_transition_impl(settings, attr):
     goarch = getattr(attr, "goarch", "auto")
     pure = getattr(attr, "pure", "auto")
     _check_ternary("pure", pure)
+    crosstool_top = settings.pop("//command_line_option:crosstool_top")
+    cpu = settings.pop("//command_line_option:cpu")
     if goos != "auto" or goarch != "auto":
         if goos == "auto":
             fail("goos must be set if goarch is set")
@@ -118,6 +124,11 @@ def _go_transition_impl(settings, attr):
             fail('pure is "off" but cgo is not supported on {} {}'.format(goos, goarch))
         platform = "@io_bazel_rules_go//go/toolchain:{}_{}{}".format(goos, goarch, "_cgo" if cgo else "")
         settings["//command_line_option:platforms"] = platform
+    else:
+        # If not auto, try to detect the platform the inbound crosstool/cpu.
+        platform = platform_from_crosstool(crosstool_top, cpu)
+        if platform:
+            settings["//command_line_option:platforms"] = platform
     if pure != "auto":
         pure_label = _filter_transition_label("@io_bazel_rules_go//go/config:pure")
         settings[pure_label] = pure == "on"
@@ -138,6 +149,8 @@ def _go_transition_impl(settings, attr):
 go_transition = transition(
     implementation = _go_transition_impl,
     inputs = [_filter_transition_label(label) for label in [
+        "//command_line_option:cpu",
+        "//command_line_option:crosstool_top",
         "//command_line_option:platforms",
         "@io_bazel_rules_go//go/config:static",
         "@io_bazel_rules_go//go/config:msan",