Using @platforms//os:windows to assume MSVC won't work with MinGW

[jesseschalken]

I don't have a specific reproducible error but I just noticed that the select()s that assume that @platforms//os:windows means MSVC are most likely wrong, such as

rules_boost/BUILD.zlib

Line 78 in 5be5ea4

"//conditions:default": ["-Wno-shift-negative-value"],

and

rules_boost/BUILD.boost

Line 281 in 5be5ea4

"/D_WIN32_WINNT=0x0601",

I use MinGW to cross compile to Windows from Linux, so @platforms//os:windows is active but the compiler wont understand something like /D_WIN32_WINNT=0x0601.

Bazel doesn't document how to select() on the chosen compiler in a way that doesn't require the compiler to be specified as a constraint in target platform and available toolchains, but Abseil has the correct way in its sources here:

https://github.com/abseil/abseil-cpp/blob/46d939a918092ef94498a126dc1244b437100b31/absl/BUILD.bazel#L28

Note that both msvc and clang-cl should be effectively treated as MSVC, so I typically define:

config_setting(
    name = "is_msvc",
    flag_values = {"@bazel_tools//tools/cpp:compiler": "msvc-cl"},
)

config_setting(
    name = "is_clang_cl",
    flag_values = {"@bazel_tools//tools/cpp:compiler": "clang-cl"},
)

selects.config_setting_group(
    name = "is_msvc_like",
    match_any = [":is_msvc", ":is_clang_cl"],
)

And then use:

select({
    ":is_msvc_like": "I'm msvc or clang-cl",
    "//conditions:default": "I'm probably gcc or gcc-compatible",
})

The values for @bazel_tools//tools/cpp:compiler come from the compiler attribute on the cc_toolchain_config. When targeting Windows the possible values are mingw-gcc, msys-gcc, msvc-cl and clang-cl, defined in this file:

https://github.com/bazelbuild/bazel/blob/36116ce73f97a7d2ac528dca07cc77bc772c9a22/tools/cpp/BUILD.windows.tpl#L132