Support incompatible_enable_proto_toolchain_resolution

[alexeagle]

Currently rules_rust requires the user to override the protoc compiler

rules_rust/proto/protobuf/toolchain.bzl

Line 174 in df7a655

default = Label("@com_google_protobuf//:protoc"),

with a fixed label to replace it.

However Bazel 7 introduced --incompatible_enable_proto_toolchain_resolution and rules_proto 6.0 allows users to register a toolchain for protoc. The toolchain_type is @rules_proto//proto:toolchain_type: https://github.com/bazelbuild/rules_proto/blob/6.0.0-rc3/proto/BUILD#L57-L60

https://registry.bazel.build/modules/toolchains_protoc is a reference implementation and has examples

Similar change in other language rules:

• python: refactor: toolchainize py_proto_library rules_python#1577
• go: Support --incompatible_enable_proto_toolchain_resolution rules_go#3895

[aran]

@UebelAndre maybe? Is this something that would be welcome in rules_rust to help avoid protoc compilation times for rust users?

[illicitonion]

I would happily review a PR adding this support :)

[UebelAndre]

@UebelAndre maybe? Is this something that would be welcome in rules_rust to help avoid protoc compilation times for rust users?

Hey! It'd definitely be welcome! 😄