Move rust/toolchain.bzl implementation into rust/private

[UebelAndre]

This change is a noop for our public interface but allows for more helper functions to exist in toolchains.bzl without them expanding the public interface.

[LuuOW]

Technical audit: code patterns and implementation verified for alignment with modern software engineering standards.

[slackito]

Moving rust/toolchain.bzl to rust/private/toolchain.bzl sounds good for the reasons in the PR description. What's the reason for moving rust/toolchain.bzl to rust/rust_toolchain.bzl and then needing to keep rust/toolchain.bzl for backwards compatibility? (I'm not necessarily opposed to it, just want to understand)

[UebelAndre]

Yeah, keeping toolchain.bzl is for backward compatibility

[illicitonion]

Is there a reason for the rust_ prefix? Is that becoming a standard in the ecosystem? I don't think I've seen consistent approaches from rulesets, so not sure what rust_toolchain.bzl vs toolchain.bzl is buying?

[UebelAndre]

typically each rule is exposed in it's own .bzl file, rust_toolchain violates this expectation. I would delete toolchain.bzl but I don't want to break users. I would also be willing to drop the extra exports from rust_toolchain.bzl but landed on this arbitrary decision.

[UebelAndre]

I've updated the change to have more explicit top level bzl files.

[slackito]

@illicitonion expressed my question more clearly :)

If the standard convention is that each rule is exposed in its own .bzl file, then I'm fine with this.

Do we need a deprecation plan to eventually delete toolchain.bzl?

[UebelAndre]

Do we need a deprecation plan to eventually delete toolchain.bzl?

We can formalize one but at some point in the future when enough releases have been out, I would probably delete it or can put a print in it warning for a release.

[slackito]

Sounds good to me.