Tweak how we pass GOEXPERIMENT to actions

[dzbarsky]

What type of PR is this?
Small cleanup. We can just encode the string in the repo rule instead of creating it for every go action.

What does this PR do? Why is it needed?

Which issues(s) does this PR fix?

Fixes #

Other notes for review

[jtacoma]

Is this the right place to ask for suggestions about how to deal with this change? Asking because it is a backwards-incompatible change. I understand this is allowed between releases that share the same major version number if that number is zero, as it was in the previous v0.50.1 and following v0.51.0 releases. For leaf monorepos this might not be a real problem because they can switch from list-of-strings to comma-delimited-string in the same commit that upgrades from v0.50.1 to v0.51.0 (or later). Unfortunately, that's not an option for repos that (like this one) are used in many other repos.

For example, I'm trying out Bazel + Nix + Go in a hobby project where I ran into this issue when using rules_nixpkgs because its toolchains/go/go.bzl uses experiments. I considered sending a patch to update to the new API before realizing that this would force all users of rules_nixpkgs to upgrade rules_go to v0.51.0 or later. Any idea how to bridge this gap with minimal churn?

[fmeum]

@jtacoma Thanks for raising this. What's the part of this PR that is backwards incompatible? My understanding is that it only modifies how attributes are passed around internally.

[jtacoma]

I was a little uncertain since I noticed go_sdk is defined and used only within the "private" subdirectory of rules_go. However, it is also public in go/def.bzl. I ran into this because rules_nixpkgs loads go_sdk from @rules_go//go:def.bzl.

[dzbarsky]

I was a little uncertain since I noticed go_sdk is defined and used only within the "private" subdirectory of rules_go. However, it is also public in go/def.bzl. I ran into this because rules_nixpkgs loads go_sdk from @rules_go//go:def.bzl.

Ah, we didn't realize that was exported. We should probably turn that export into a macro that rewrites the list into a comma-separated string to keep it backwards compatible.

[fmeum]

@dzbarsky We could also handle the conversion in the go_sdk rule and don't change its attributes.

[dzbarsky]

Yep, makes sense. I can send a PR Thursday to restore the interface