Allow bzlmod users to inhibit toolchain registration

[konkers]

Partial fix for #2818

[krasimirgg]

Why not modify the implementation of rust_register_toolchains to not register any toolchains when passed versions = []? This way we avoid the extra check and _empty_repository setup.

[konkers]

There did not seem to be a clean way to get an empty list passed as toolchain_names to toolchain_repository_hub here (https://github.com/bazelbuild/rules_rust/blob/main/rust/repositories.bzl#L252) without nesting most of that function under an if len(versions) > 0:. versions is not considered until inside calls to rust_repository_set and _get_toolchain_repositories. I could add the following at the beginning of rust_register_toolchains to avoid having to declare _empty_repository:

if len(versions) == 0:
    toolchain_repository_hub(
        name = "rust_toolchains",
        toolchain_names = [],
        toolchain_labels = {},
        toolchain_types = {},
        exec_compatible_with = {},
        target_compatible_with = {},
    )

Other refactor suggestions are welcome.

[krasimirgg]

Thanks! I think what you have now it cleaner than the toolchain_repository_hub approach.

[krasimirgg]

Thanks! I think what you have now it cleaner than the toolchain_repository_hub approach.