use -msvc variant on Windows

[dae]

use -msvc variant on Windows

Both the standard Rust installer, and rust_repositories() in this repo
default to the -msvc platform. Since it is the default on Windows,
this change comments out the -gnu variant instead.

This change (along with some fixes in cargo raze) allows crates with
msvc-specific build deps to compile. I do not know enough about Bazel
to say if it will have any unintended side effects, but it seems to
work for me, and allows a moderate sized project with about 250
crate dependencies to compile using the default toolchain.

[dae]

I experimented with enabling both -gnu and -msvc variants, and using a constraint setting to switch between them. This avoids the "Illegal ambiguous match on configurable attribute deps" errors that arise when both the -gnu variant and -msvc variant are listed in a crate's dependencies. I wasn't able to figure out how this could be made a default though, and if that's not possible, it means Windows users would need to define the environment they wish to use in their BUILD file, eg

platform(
    name = "x86_64-windows-gnu",
    constraint_values = [
        "@platforms//cpu:x86_64",
        "@platforms//os:windows",
        "@io_bazel_rules_rust//rust/platform/env:gnu",
    ],
)

and then specify the platform when compiling:

bazel build //some:target --platforms=x86_64-windows-gnu --host_platform=x86-windows-gnu

This is a bit of a pain, but then it's already necessary to pass -enable_runfiles in on Windows in order to get crate buildscripts to compile. Thoughts?

[damienmg]

I think commenting out the gnu version is preferable.

[dae]

Sorry, I had thought I'd commented out the other set already - corrected.

[damienmg]

Thanks!

[UebelAndre]

@damienmg @dae Hey, this seems to have broken CI. Would one of you guys mind taking a look?

[UebelAndre]

It looks like cargo-raze will need to be updated and these outputs will need to be regenerated 😞 or a list of targets will need to be added to exclude the windows -gnu platform declarations.

[UebelAndre]

I have a fix in #450

[UebelAndre]

Just an FYI: bazelbuild/platforms#38 may potentially lead to support for both -msvc and -gnu