Add rustfmt_toolchain

This makes the nixpkgs_rust_configure repo rule add a rustfmt_toolchain
alongside rust_toolchain, using the same rustfmt binary as the
former. Support for a separate toolchain for rustfmt was added in
bazelbuild/rules_rust#1719.

testing/rust/MODULE.bazel

@@ -43,7 +43,7 @@ rust.toolchain(
 non_module_deps = use_extension("//:non_module_deps.bzl", "non_module_deps")
 use_repo(non_module_deps, "nixpkgs")
 use_repo(non_module_deps, "nixpkgs_config_rust_toolchain")
-register_toolchains("@nixpkgs_config_rust_toolchain//:rust_nix")
+register_toolchains("@nixpkgs_config_rust_toolchain//:all")
 
 use_repo(non_module_deps, "nixpkgs_config_cc")
 use_repo(non_module_deps, "nixpkgs_config_cc_info")

toolchains/rust/README.md

@@ -2,7 +2,7 @@
 
 <!-- Edit the docstring in `toolchains/rust/rust.bzl` and run `bazel run //docs:update-README.md` to change this repository's `README.md`. -->
 
-Rules for importing a Rust toolchain from Nixpkgs.
+Rules for importing a Rust and rustfmt toolchain from Nixpkgs.
 
 # Rules
 

toolchains/rust/rust.bzl

@@ -1,6 +1,6 @@
 """<!-- Edit the docstring in `toolchains/rust/rust.bzl` and run `bazel run //docs:update-README.md` to change this repository's `README.md`. -->
 
-Rules for importing a Rust toolchain from Nixpkgs.
+Rules for importing a Rust and rustfmt toolchain from Nixpkgs.
 
 # Rules
 
@@ -108,7 +108,7 @@ pkgs.buildEnv {{
             visibility = ["//visibility:public"],
         )
 
-        load('@rules_rust//rust:toolchain.bzl', 'rust_toolchain')
+        load('@rules_rust//rust:toolchain.bzl', 'rust_toolchain', 'rustfmt_toolchain')
         rust_toolchain(
             name = "rust_nix_impl",
             rust_doc = ":rust_doc",
@@ -127,6 +127,12 @@ pkgs.buildEnv {{
             stdlib_linkflags = {stdlib_linkflags},
             visibility = ["//visibility:public"],
         )
+
+        rustfmt_toolchain(
+            name = "rustfmt_nix_impl",
+            rustfmt = ":rustfmt",
+            visibility = ["//visibility:public"],
+        )
         EOF
     '';
 }}
@@ -140,6 +146,14 @@ toolchain(
     exec_compatible_with = {exec_constraints},
     target_compatible_with = {target_constraints},
 )
+
+toolchain(
+    name = "rustfmt_nix",
+    toolchain = "@{toolchain_repo}//:rustfmt_nix_impl",
+    toolchain_type = "@rules_rust//rust/rustfmt:toolchain_type",
+    exec_compatible_with = {exec_constraints},
+    target_compatible_with = {target_constraints},
+)
 """
 
 def _nixpkgs_rust_toolchain_impl(repository_ctx):