Don't register rust_host_tools twice

[hauserx]

The bazel-lsp uses rust_host_tools to perform some actions by using:

rust_host_tools = use_extension("@rules_rust//rust:extensions.bzl", "rust_host_tools") 
use_repo(rust_host_tools, "rust_host_tools")

See: https://github.com/cameron-martin/bazel-lsp/blob/19b799e528c884be6efeb108ed1840a4edff1330/MODULE.bazel#L14

It started failing when using rules_rust from HEAD after #3148 because rules_rust's MODULE.bazel registers toolchain with the same name:

rust_host_tools.host_tools(
    name = "rust_host_tools",
)

See:

rules_rust/MODULE.bazel

Line 70 in aae84e9

rust_host_tools = use_extension("//rust:extensions.bzl", "rust_host_tools")

[hauserx]

FYI @cameron-martin (in case what bazel-lsp is doing is not correct).

[UebelAndre]

Should this instead be called if no module defines host_tools?

[hauserx]

rules_rust's MODULE.bazel defines host tools already, so it will be always defined. Or I am missing something and it may not be defined in some other way?

[UebelAndre]

It just seems brittle to me for folks to be depending on the name of a particular dependency in rules_rust's MODULE.bazel file. That's why I tried to have the module extension itself always provide the repository and encode the consistent name.

[hauserx]

Just for reference, without this change currently one gets following error if one does not specify host_tools tag:

ERROR: (...)/external/rules_rust+/rust/extensions.bzl:262:44: Traceback (most recent call last):
        File "(...)/external/rules_rust+/rust/extensions.bzl", line 262, column 44, in _rust_host_tools_impl
                rust_toolchain_tools_repository(
Error in repository_rule: A repo named rust_host_tools is already generated by this module extension at (...)/external/rules_rust+/rust/extensions.bzl:274:44
ERROR: Analysis of target '//:generate_rustc_env_file' failed; build aborted: error evaluating module extension @@rules_rust+//rust:extensions.bzl%rust_host_tools

I guess for bazel-lsp workaround would be simply to specify some host_tool with unique name (like bazel_lsp_rust_host_tools). Other solution would be in rules_rust's MODULE to change name to smthg like rules_rust_internal_host_tools, which will require users to use some name, or roll back this change, but move this condition outside of loop as you mentioned.

Wonder whether the best solution could be to expose those toolchain files directly in @rules_rust, like @rules_rust//bin:cargo, something similar as you can do with go by running bazel run @io_bazel_rules_go//go. This way it would use binaries for configured toolchain.

[cameron-martin]

FYI @cameron-martin (in case what bazel-lsp is doing is not correct).

We use rust_host_tools to invoke cargo to read the version in the Cargo.toml. Not sure if this is the best way of doing it, but I don't know of a better way.