CGO constraint does nothing

[alercah]

Platform: Windows 11 x64

I cannot get cgo to work, and this is one of the ways I tried that is not being respected.

Reproduction steps

Add the following to BUILD.bazel:

platform(
    name = "x64_windows-clang-cl",
    constraint_values = [
        "@platforms//cpu:x86_64",
        "@platforms//os:windows",
        "@bazel_tools//tools/cpp:clang-cl",
        "@rules_go//go/toolchain:cgo_on",
    ],
)

Run bazel with --platforms=//tools:x64_windows-clang-cl --host_platform=//tools:x64_windows-clang-cl and compile a cgo rule with -s.

Expected behaviour

Either the rule is compiled with CGO_ENABLED=1, or an error occurs because the toolchain is not compatible with the cgo_on constraint.

Actual behaviour

The rule is compiled with CGO_ENABLED=0

What should happen

Either the cgo_on constraint should only be supported by toolchains that actually support cgo, or it should be removed.

[bdleitner]

I've had many cgo issues (see #4180). With this issue plus some help from gemini I was able to run
bazel build --host_platform=@rules_go//go/toolchain:windows_amd64_cgo --platforms=@rules_go//go/toolchain:windows_amd64_cgo --incompatible_enable_cc_toolchain_resolution --toolchain_resolution_debug=.* //ocr:ocr_lib > full_build_debug_output.txt 2>&1
Which gave full_build_debug_output.txt

Getting the name of the toolchain that was selected: @@rules_go++go_sdk+go_default_sdk//:go_windows_amd64-impl I was able to see:

> bazel query '@@rules_go++go_sdk+go_default_sdk//:go_windows_amd64-impl' --output=build                                                                                                                                               
# D:/_bazel_out/uni3i7pb/external/rules_go++go_sdk+go_default_sdk/BUILD.bazel:100:22                                                                                                                                                                                    
go_toolchain(
  name = "go_windows_amd64-impl",
  visibility = ["//visibility:public"],
  tags = ["manual"],
  generator_name = "go_windows_amd64-impl",
  generator_function = "declare_go_toolchains",
  generator_location = "D:/_bazel_out/uni3i7pb/external/rules_go++go_sdk+go_default_sdk/BUILD.bazel:100:22",
  builder = "@@rules_go++go_sdk+go_default_sdk//:builder_reset",
  goos = "windows",
  goarch = "amd64",
  sdk = "@@rules_go++go_sdk+go_default_sdk//:go_sdk",
  link_flags = [],
  cgo_link_flags = [],
)

So, indeed, even through the platforms have _cgo, it's choosing a toolchain without cgo enabled.
In fact, a query for all go_toolchains in that repo suggests that none of them have cgo enabled, so perhaps I'm misunderstanding things. But this does seem to be the source of my cgo problems as well.