Config setting with msvc compiler constraint appears to be never selected

[oakad]

Tested with bazel 0.23.1

I'm trying to use the following construct to select compiler specific code:

config_setting(
	name = "windows_msvc",
	constraint_values = [
		"@bazel_tools//platforms:windows",
		"@bazel_tools//tools/cpp:msvc",
	],
	visibility = ["//visibility:public"],
)

Unfortunately, it appears that select clauses predicated with ":windows_msvc" are never selected. No errors are being reported and I have tried several compiler/cpu command line flag combinations as well as some custom ...cpp:cc_compiler constraint values to no avail.

If I comment out the "msvc" line everything works as expected, apart from the fact that the build becomes unsuitable for msys.

It appears to me that a minor bug of some sort may exist within the msvc toolchain definitions.

[laszlocsomor]

@katre : Could you please help triaging?

[katre]

The config_setting.constraint_values are matched against the target platform, not the selected toolchain. I suspect that this is happening because no platform is setting the :msvc constraint.

How are you selecting the msvc compiler in the first place? If you are using the --compiler flag, then your constraint_setting should also be using that.

[oakad]

@katre Of course I tested various variants.

bazel build --compiler msvc ...
in cc_toolchain_suite rule @local_config_cc//:toolchain: cc_toolchain_suite '@local_config_cc//:toolchain' does not contain a toolchain for cpu 'x64_windows' and compiler 'msvc'.

Ok, another try:

build:msvc --compiler msvc-cl ...

Works, but there's no constraint to select msvc-cl.

Let's try to create a custom constraint value:

constraint_value(
	name = "msvc-cl",
	constraint_setting = "@bazel_tools//tools/cpp:cc_compiler",
)

config_setting(
	name = "windows_msvc",
	constraint_values = [
		"@bazel_tools//platforms:windows",
		#"@bazel_tools//tools/cpp:msvc",
		":msvc-cl",
	],
	visibility = ["//visibility:public"],
)

No errors reported, but config is not selected - default select clause gets triggered.

Which brings me to a more general question: is there a setting or aspect to make bazel print the state of constraints and selected configs?

[katre]

I am sorry that I was not clear: do not use the msvc constraint value as part of your config_setting.

Since you are setting the flag --compiler, use that in the config_setting:

config_setting(
    name = "windows_msvc",
    values = {
        "compiler": "msvc",
    },
)

This config_setting will match whenever the build has the flag --compiler msvc.

[laszlocsomor]

@katre : Thanks for answering! Is there a Bazel bug here?

[katre]

No, there is no bug.

I should probably work with @hlopko to remove the confusing compiler constraint_setting, however.

[laszlocsomor]

Thanks!

[oakad]

If there's no bug, then the whole thing should be branded as "confusing feature implementation".

Allow me to repeat:

bazel build --compiler msvc ...
in cc_toolchain_suite rule @local_config_cc//:toolchain: cc_toolchain_suite '@local_config_cc//:toolchain' does not contain a toolchain for cpu 'x64_windows' and compiler 'msvc'.

But it does appears to work if compiler is specified as "msvc-cl" (this should be at least documented).

In general, though, the diagnostics offered by the Bazel regarding selected config values must be somehow improved (or documented!).

[katre]

Re-opening and assigning to @laszlocsomor to figure out why the --compiler flag isn't working as expected.

[hlopko]

@oakad I think you have to use bazel build --compiler msvc-cl, that's the compiler used by our autoconfigured toolchain.

If you want to decouple your select from whether we use --crosstool_top, --cpu, and --compiler to select the C++ toolchain, or platforms/toolchains, you can use https://source.bazel.build/bazel/+/fc69ecd1824d1f318a7a18d5a955040eed23c6ea:tools/cpp/compiler_flag.bzl.

Does that help?

[laszlocsomor]

@hlopko : Please help triaging this bug. Shall we improve documentation? If so, how?

[laszlocsomor]

@hlopko, What shall we do here? Is "type: documentation" accurate?

[meisterT]

@oakad anything needs to be done here?

[oakad]

Don't know. I'm avoiding msvc now (clang on msys will be much welcome, though, as mentioned in my other ticket).