Add constraint values for the Rust release channel.

[jmillikin]

This allows users to define separate toolchains and platforms for stable/beta/nightly, and to select() based on which channel is being used in a build.

In the upcoming Bazel v4.0, the target_compatible_with target attr will allow targets to be skipped based on channel compatibility.

[mfarrugi]

This could use an example. Is CI setup to run 4.0 yet?

[mfarrugi]

I assume this is not a global name, right? I am not familiar w/ defining constraints.

[jmillikin]

This is a target name, it's referenced as @io_bazel_rules_rust//rust/platform/channel and the constraint values are referenced like @io_bazel_rules_rust//rust/platform/channel:nightly.

[jmillikin]

This could use an example. Is CI setup to run 4.0 yet?

For an example it might be better to use toolchains, since those are better known and have been around for a while.

Given the following BUILD targets:

platform(
    name = "linux-armv7-stable",
    constraint_values = [
        "@platforms//os:linux",
        "@platforms//cpu:arm",
        "@io_bazel_rules_rust//rust/platform/channel:stable",
    ],
)

platform(
    name = "linux-armv7-nightly",
    constraint_values = [
        "@platforms//os:linux",
        "@platforms//cpu:arm",
        "@io_bazel_rules_rust//rust/platform/channel:nightly",
    ],
)


toolchain(
    name = "rust_darwin-to-linux-armv7-stable",
    exec_compatible_with = [
        "@platforms//cpu:x86_64",
        "@platforms//os:macos",
    ],
    target_compatible_with = [
        "@platforms//cpu:arm",
        "@platforms//os:linux",
        "@io_bazel_rules_rust//rust/platform/channel:stable",
    ],
    toolchain = "@rust_darwin-to-linux-armv7-stable//:toolchain_for_arm-unknown-linux-musleabihf_impl",
    toolchain_type = "@io_bazel_rules_rust//rust:toolchain",
)

toolchain(
    name = "rust_darwin-to-linux-armv7-nightly",
    exec_compatible_with = [
        "@platforms//cpu:x86_64",
        "@platforms//os:macos",
    ],
    target_compatible_with = [
        "@platforms//cpu:arm",
        "@platforms//os:linux",
        "@io_bazel_rules_rust//rust/platform/channel:nightly",
    ],
    toolchain = "@rust_darwin-to-linux-armv7-nightly//:toolchain_for_arm-unknown-linux-musleabihf_impl",
    toolchain_type = "@io_bazel_rules_rust//rust:toolchain",
)

I can use now select whether I want to build with the stable or nightly toolchain using --platforms=@my_repo//:linux-armv7-stable or @my_repo//:linux-armv7-nightly flags, similar to a combination of Cargo's options +stable / +nightly and --target.

[hlopko]

I'll go ahead and merge this PR, IMHO it's useful and uncontroversial. Thank you @jmillikin!