go: deprecate the go_rule wrapper

[jayconrod]

See bazel-contrib/bazel-gazelle#676.

A Bazel incompatibility flag (possibly --incompatible_remap_main_repo, see bazelbuild/bazel#7130) causes default labels in attributes to be resolved in @io_bazel_rules_go//go/private (where go_rule is defined) instead of wherever the rule should actually have been defined.

We use go_rule as a wrapper around rule to add hidden attributes and a toolchain dependency. Instead, we should move hidden attributes to the toolchain or provide a set of common attributes. The toolchain dependency can be explicit.

[jmhodges]

I see the deprecation commit made it in and would love to hear what the migration should look like! (bazel_gomock is still using go_rule, so it's relevant to my interests)

[jayconrod]

@jmhodges I apologize for creating a classic Google "old thing is deprecated, new thing is not ready yet" dilemma. I thought it was important to mark this is deprecated, given that there are potential problems caused by Bazel.

The change I would like to make is for a skeleton Go rule to look something like this:

go_example = rule(
    implementation = _go_example_impl,
    attrs = {
        "_go_context": attr.label(default = "@io_bazel_rules_go//:go_context_data"),
    },
    toolchains = ["@io_bazel_rules_go//go:toolchain"],
)

The rule would get nearly all of the information it needs from the toolchain. There are a few things that must be configured for the target platform (C toolchain, flags), and toolchains are always configured for the host platform, so we still need go_context_data to provide those.

After this is ready and rules_go is migrated, go_rule would continue to be available for at least a full release cycle (about a quarter). It may be reduced to just adding the _go_context attribute and the toolchain.