Make platforms targets publicly visible

[dududko]

Make targets in the platforms package publicly visible so they could be used in the attributes of other targets.
Example:

with_platform(
    name = "linux",
    platform = "@com_grail_bazel_toolchain//platforms:linux-x86_64",
    tool = ":binary",
    visibility = ["//visibility:public"],
)

[rrbutani]

Thanks for the PR!

Can you tell me a little bit more about your use case? //platforms really only exists for our own tests; I'm a bit wary to have users depend on the stuff there.

In my mind, users would either just use the auto-generated @local_config_platform//:host as their host platform (and let toolchain resolution pick the appropriate toolchain) or define their own platform when cross-compiling or doing something a bit more special.

In your example, what does with_platform do?

[dududko]

There are two way to do cross-compilation:

• by specifying target platform with --platform option
• using transition

There is a third option in rules_go, where you can specify goos/goarch attributes in go_binary, but this approach is actually based on transitions.

In my case I need to build binary for multiple platforms (darwin/linux) and publish build artifacts. I prefer doing this in a single bazel executable target without specifying --platform option on each build, thus I use transitions. A have a very simple rule for this which is called with_platform, and all this rules does is apply a target platform in the build configuration of a tool attribute, which can be any binary target.

I use this approach when build a custom gazelle gazelle_binary for multiple platforms. This target does not support goos/goarch anymore, so I had to implement a custom transition. And I use the same transition for building go_binary with cgo for multiple platforms using the toolchains that you provide.

I'm a bit wary to have users depend on the stuff there.

In rules_go platforms are also publicly visible, and users depend on those targets. I do no see the reason why not to make your platform targets visible as well.

[rrbutani]

In rules_go platforms are also publicly visible, and users depend on those targets. I do no see the reason why not to make your platform targets visible as well.

My hesitation mostly comes from the fact that while we have a small list of supported target/host platforms at the moment, we eventually hope to support arbitrary target triples in which case we'd want to generate the platforms (as #85 does with constraints). Under that scenario it's potentially problematic to have users depend on the generated platforms since they'd be tied to the value of extra_targets that's provided (I'd hope that, for example, @llvm_toolchain//platforms:wasi only being present if wasm32-unknown-wasi is specified as an extra target is expected and reasonable behavior but I'm not sure this is the case; I think it'd be easy to accidentally assume the generated platforms are always present and to use them for things that have nothing to do with this toolchain).

That said, I see the utility in providing these and I think it makes sense to expose these platforms for now with the caveat that they may move to the generated toolchain repo in a future release (0.7+).