Depend on a copy of rustfmt for the target

rust/private/toolchain_utils.bzl

@@ -53,7 +53,7 @@ def _toolchain_files_impl(ctx):
     )]
 
 toolchain_files = rule(
-    doc = "A rule for fetching files from a rust toolchain.",
+    doc = "A rule for fetching files from a rust toolchain for the exec platform.",
     implementation = _toolchain_files_impl,
     attrs = {
         "tool": attr.string(
@@ -95,3 +95,32 @@ current_rust_toolchain = rule(
         str(Label("@rules_rust//rust:toolchain_type")),
     ],
 )
+
+def _transition_to_target_impl(settings, _attr):
+    return {
+        # String conversion is needed to prevent a crash with Bazel 6.x.
+        "//command_line_option:extra_execution_platforms": [
+            str(platform)
+            for platform in settings["//command_line_option:platforms"]
+        ],
+    }
+
+_transition_to_target = transition(
+    implementation = _transition_to_target_impl,
+    inputs = ["//command_line_option:platforms"],
+    outputs = ["//command_line_option:extra_execution_platforms"],
+)
+
+def _toolchain_files_for_target_impl(ctx):
+    return [ctx.attr.toolchain_files[0][DefaultInfo]]
+
+toolchain_files_for_target = rule(
+    doc = "A rule for fetching files from a rust toolchain for the target platform.",
+    implementation = _toolchain_files_for_target_impl,
+    attrs = {
+        "toolchain_files": attr.label(cfg = _transition_to_target, mandatory = True),
+        "_allowlist_function_transition": attr.label(
+            default = "@bazel_tools//tools/allowlists/function_transition_allowlist",
+        ),
+    },
+)

rust/toolchain/BUILD.bazel

@@ -1,5 +1,5 @@
 load("//rust/private:rustfmt.bzl", "current_rustfmt_toolchain")
-load("//rust/private:toolchain_utils.bzl", "current_rust_toolchain", "toolchain_files")
+load("//rust/private:toolchain_utils.bzl", "current_rust_toolchain", "toolchain_files", "toolchain_files_for_target")
 
 package(default_visibility = ["//visibility:public"])
 
@@ -41,6 +41,12 @@ current_rustfmt_toolchain(
     name = "current_rustfmt_toolchain",
 )
 
+toolchain_files_for_target(
+    name = "current_rustfmt_toolchain_for_target",
+    toolchain_files = ":current_rustfmt_toolchain",
+    visibility = ["//:__subpackages__"],
+)
+
 alias(
     name = "current_rustfmt_files",
     actual = "current_rustfmt_toolchain",

tools/rustfmt/BUILD.bazel

@@ -20,11 +20,11 @@ rust_library(
     ),
     data = [
         "//:rustfmt.toml",
-        "//rust/toolchain:current_rustfmt_toolchain",
+        "//rust/toolchain:current_rustfmt_toolchain_for_target",
     ],
     edition = "2018",
     rustc_env = {
-        "RUSTFMT": "$(rlocationpath //rust/toolchain:current_rustfmt_toolchain)",
+        "RUSTFMT": "$(rlocationpath //rust/toolchain:current_rustfmt_toolchain_for_target)",
         "RUSTFMT_CONFIG": "$(rlocationpath //:rustfmt.toml)",
     },
     deps = [
@@ -55,10 +55,7 @@ rust_binary(
         "ASPECT_REPOSITORY": aspect_repository(),
         "RUST_DEFAULT_EDITION": "$(RUST_DEFAULT_EDITION)",
     },
-    toolchains = [
-        "@rules_rust//rust/toolchain:current_rust_toolchain",
-        "@rules_rust//rust/toolchain:current_rustfmt_toolchain",
-    ],
+    toolchains = ["@rules_rust//rust/toolchain:current_rust_toolchain"],
     visibility = ["//visibility:public"],
     deps = [
         ":rustfmt_lib",
@@ -94,15 +91,12 @@ rust_binary(
     srcs = [
         "src/upstream_rustfmt_wrapper.rs",
     ],
-    data = ["//rust/toolchain:current_rustfmt_toolchain"],
+    data = ["//rust/toolchain:current_rustfmt_toolchain_for_target"],
     edition = "2018",
     rustc_env = {
-        "RUSTFMT": "$(rlocationpath //rust/toolchain:current_rustfmt_toolchain)",
+        "RUSTFMT": "$(rlocationpath //rust/toolchain:current_rustfmt_toolchain_for_target)",
     },
-    toolchains = [
-        "@rules_rust//rust/toolchain:current_rust_toolchain",
-        "@rules_rust//rust/toolchain:current_rustfmt_toolchain",
-    ],
+    toolchains = ["@rules_rust//rust/toolchain:current_rust_toolchain"],
     visibility = ["//visibility:public"],
     deps = [
         "//tools/runfiles",