Support rustfmt toolchains for bzlmod

[stephencpope]

When running under bazel 6.4.0 using --enable_blzmod, the rustfmt toolchain doesn't get wired up as it should. A simple no-op bazel module fails to support the @rules_rust//:rustfmt target like this:

$ bazel run --enable_bzlmod @rules_rust//:rustfmt
Starting local Bazel server and connecting to it...
DEBUG: @@rules_rust~override//:MODULE.bazel:6:6: WARNING: The rules_rust Bazel module is still highly experimental and subject to change at any time. Only use it to try out bzlmod for now.
ERROR: .../external/rules_rust~override/rust/toolchain/BUILD.bazel:40:26: While resolving toolchains for target @rules_rust~override//rust/toolchain:current_rustfmt_toolchain: No matching toolchains found for types @rules_rust~override//rust/rustfmt:toolchain_type.
To debug, rerun with --toolchain_resolution_debug='@rules_rust~override//rust/rustfmt:toolchain_type'
If platforms or toolchains are a new concept for you, we'd encourage reading https://bazel.build/concepts/platforms-intro.
ERROR: Analysis of target '@rules_rust~override//:rustfmt' failed; build aborted: 
INFO: Elapsed time: 13.555s
INFO: 0 processes.
FAILED: Build did NOT complete successfully (42 packages loaded, 168 targets configured)
    currently loading: @rules_rust~override//tools/runfiles
ERROR: Build failed. Not running target

when using this MODULE.bazel:

bazel_dep(name = "rules_rust", version = "0.30.0")
git_override(
    module_name = "rules_rust",
    remote = "https://github.com/bazelbuild/rules_rust",
    commit = "54bef51d8e3458ec60ea2b6a27e15b62502c673d",
)

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

rust.toolchain()

use_repo(rust, "rust_toolchains")

register_toolchains(
    "@rust_toolchains//:all",
    dev_dependency = True,
)

The problem can also be seen in the output of

$ bazel query --enable_bzlmod @rust_toolchains//:\*
@rust_toolchains//:BUILD.bazel
@rust_toolchains//:rust_analyzer_1.73.0
@rust_toolchains//:rust_darwin_aarch64__aarch64-apple-darwin__nightly
@rust_toolchains//:rust_darwin_aarch64__aarch64-apple-darwin__stable
...

where there are no rustfmt toolchains.

Unfortunately at the moment I cannot sign a CLA and thus cannot submit a PR, but the following fixes the problem:

--- rust/repositories.bzl
+++ rust/repositories.bzl
@@ -250,6 +250,14 @@ def rust_register_toolchains(
             exec_triple = exec_triple,
         )
 
+        toolchain_names.append(rustfmt_repo_name)
+        toolchain_labels[rustfmt_repo_name] = "@{}_tools//:rustfmt_toolchain".format(
+            rustfmt_repo_name,
+        )
+        exec_compatible_with_by_toolchain[rustfmt_repo_name] = []
+        target_compatible_with_by_toolchain[rustfmt_repo_name] = []
+        toolchain_types[rustfmt_repo_name] = "@rules_rust//rust/rustfmt:toolchain_type"
+
         if register_toolchains:
             native.register_toolchains("@{}//:toolchain".format(
                 rustfmt_repo_name,

The problem is that although this isn't necessary when working in WORKSPACE mode when native.register_toolchains is invoked directly, without it the rustfmt toolchains are not added to the toolchain repository hub, and so are not picked up later in MODULE.bazel when register_toolchains is called. And as far as I can tell it is not harmful to have this added code when working in WORKSPACE mode.

I note that this problem not only affects the direct use of rustfmt, but also is a problem for rust_bindgen (which isn't yet supported for bzlmod but I am working on that!)

Disclaimer: I am no expert in toolchains; I am not sure if the target_compatible_with_by_toolchain setting is correct; I just copied it from the similar for for rust_analyzer farther above.