Windows: genenrate a fake CROSSTOOL when VC not found

When Visual C++ build tools are not found on Windows, we generating a
fake CROSSTOOL so that users can still build for other languages.
In the fake CROSSTOOL, we point all tools to a batch script which prints
error messages to notify users when they try to build any cc targets.

Also, added an extra default_toolchain specification for x64_windows cpu
value, so that users can use this cpu in select on non-Windows
platforms.

Fixed bazelbuild#3853

Change-Id: Ib4d7a38fd4d98075f6b73ce310c28822e58af4e2
PiperOrigin-RevId: 172341985

tools/cpp/CROSSTOOL.tpl

@@ -12,6 +12,11 @@ default_toolchain {
   toolchain_identifier: "stub_armeabi-v7a"
 }
 
+default_toolchain {
+  cpu: "x64_windows"
+  toolchain_identifier: "msvc_x64"
+}
+
 default_toolchain {
   cpu: "x64_windows_msvc"
   toolchain_identifier: "msvc_x64"

tools/cpp/vc_path_not_found.bat

@@ -0,0 +1,24 @@
+:: Copyright 2017 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.
+
+:: Invoke the python script under pydir with the same basename
+@echo OFF
+
+echo. 1>&2
+echo The target you are compiling requires Visual C++ build tools. 1>&2
+echo Bazel couldn't find Visual C++ build tools on your machine. 1>&2
+echo Please check your installation following https://docs.bazel.build/versions/master/windows.html#using 1>&2
+echo. 1>&2
+
+exit -1

tools/cpp/windows_cc_configure.bzl

@@ -162,7 +162,7 @@ def _find_vc_path(repository_ctx):
             vc_dir = line[line.find("REG_SZ") + len("REG_SZ"):].strip() + suffix
 
   if not vc_dir:
-    auto_configure_fail("Visual C++ build tools not found on your machine.")
+    return "visual-studio-not-found"
   auto_configure_warning("Visual C++ build tools found at %s" % vc_dir)
   return vc_dir
 
@@ -288,6 +288,29 @@ def configure_windows_toolchain(repository_ctx):
   repository_ctx.symlink(Label("@bazel_tools//tools/cpp:BUILD.static"), "BUILD")
 
   vc_path = _find_vc_path(repository_ctx)
+  if vc_path == "visual-studio-not-found":
+    vc_path_error_script = "vc_path_not_found.bat"
+    repository_ctx.symlink(Label("@bazel_tools//tools/cpp:vc_path_not_found.bat"), vc_path_error_script)
+    tpl(repository_ctx, "CROSSTOOL", {
+        "%{cpu}": "x64_windows",
+        "%{default_toolchain_name}": "msvc_x64",
+        "%{toolchain_name}": "msys_x64",
+        "%{msvc_env_tmp}": "",
+        "%{msvc_env_path}": "",
+        "%{msvc_env_include}": "",
+        "%{msvc_env_lib}": "",
+        "%{msvc_cl_path}": vc_path_error_script,
+        "%{msvc_link_path}": vc_path_error_script,
+        "%{msvc_lib_path}": vc_path_error_script,
+        "%{compilation_mode_content}": "",
+        "%{content}": _get_escaped_windows_msys_crosstool_content(repository_ctx),
+        "%{opt_content}": "",
+        "%{dbg_content}": "",
+        "%{cxx_builtin_include_directory}": "",
+        "%{coverage}": "",
+    })
+    return
+
   env = _find_env_vars(repository_ctx, vc_path)
   escaped_paths = escape_string(env["PATH"])
   escaped_include_paths = escape_string(env["INCLUDE"])