commit 12ebd7bb4905f0fdabdaa6db4a22dab7fd95dea8
author Yun Peng <pcloudy@google.com> Fri Jul 19 07:05:16 2019 -0700
committer Copybara-Service <copybara-worker@google.com> Fri Jul 19 07:06:10 2019 -0700
tree 54010dcba5b0b44a14bb3eecf43d373deffe648a
parent d26a3c1d4df959156cf3dff543e5d20c21802840

Add toolchain support for clang-cl on Windows

Previously we [added clang-cl.exe support](https://github.com/bazelbuild/bazel/pull/6553) through `USE_CLANG_CL` environment variable. That is a "hack" on the existing MSVC toolchain to make it use Clang tools. This change add proper toolchain support for `clang-cl.exe` on Windows.
Because `clang-cl.exe` is MSVC compatible, we can just reuse the MSVC toolchain definition.

After this change, to select the clang-cl toolchain on Windows, you can use:

-  Before Bazel 1.0, specify build flag `--compiler=clang-cl`, this is deprecated and will be disabled in Bazel 1.0.
-  From Bazel 1.0 (or with `--incompatible_enable_cc_toolchain_resolution` flag). You have to add a platform target to your build file (eg. the top level BUILD file):
```
platform(
  name = "windows-clang-cl",
  constraint_values = [
    "@platforms//cpu:x86_64",
    "@platforms//os:windows",
    "@bazel_tools//tools/cpp:clang-cl",
  ]
)
```
Then you can tell Bazel to use the clang-cl toolchain by specifying build flags
```
--extra_toolchains=@local_config_cc//:cc-toolchain-x64_windows-clang-cl --extra_execution_platforms=//:windows-clang-cl
```
or registering the platform and toolchain in your WORKSPACE file:
```
register_execution_platforms(
  ":windows-clang-cl"
)

register_toolchains(
    "@local_config_cc//:cc-toolchain-x64_windows-clang-cl",
)
```
Related issue: [incompatible_enable_cc_toolchain_resolution: Turn on toolchain resolution for cc rules](https://github.com/bazelbuild/bazel/issues/7260)

Closes #8917.

PiperOrigin-RevId: 258961022