Depend on a copy of rustfmt for the target

The toolchain_files() and current_rust_toolchain() rules return files
for the current 'exec' platform. This is perfecly reasonable if you want
to use these tools as part of ctx.actions.run(). But if you want to use
these as part of 'data = []' (runfiles), they must be built for the
target.

This change adds a helper rule, toolchain_files_for_target(), which can
take the aforementioned rules as an input, transitioning it so that it
yields files for the target platform. I'm not happy with how this rule
is implemented, but there is little we can do about that until
bazelbuild/bazel#19645 is addressed.

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",
+        ),
+    },
+)

tools/rustfmt/BUILD.bazel

@@ -1,4 +1,5 @@
 load("//rust:defs.bzl", "rust_binary", "rust_clippy", "rust_library")
+load("//rust/private:toolchain_utils.bzl", "toolchain_files_for_target")
 load("//tools:tool_utils.bzl", "aspect_repository")
 
 exports_files(
@@ -9,6 +10,11 @@ exports_files(
     visibility = ["//visibility:public"],
 )
 
+toolchain_files_for_target(
+    name = "current_rustfmt_toolchain_for_target",
+    toolchain_files = "//rust/toolchain:current_rustfmt_toolchain",
+)
+
 rust_library(
     name = "rustfmt_lib",
     srcs = glob(
@@ -20,11 +26,11 @@ rust_library(
     ),
     data = [
         "//:rustfmt.toml",
-        "//rust/toolchain:current_rustfmt_toolchain",
+        ":current_rustfmt_toolchain_for_target",
     ],
     edition = "2018",
     rustc_env = {
-        "RUSTFMT": "$(rlocationpath //rust/toolchain:current_rustfmt_toolchain)",
+        "RUSTFMT": "$(rlocationpath :current_rustfmt_toolchain_for_target)",
         "RUSTFMT_CONFIG": "$(rlocationpath //:rustfmt.toml)",
     },
     deps = [
@@ -91,10 +97,10 @@ rust_binary(
     srcs = [
         "src/upstream_rustfmt_wrapper.rs",
     ],
-    data = ["//rust/toolchain:current_rustfmt_toolchain"],
+    data = [":current_rustfmt_toolchain_for_target"],
     edition = "2018",
     rustc_env = {
-        "RUSTFMT": "$(rlocationpath //rust/toolchain:current_rustfmt_toolchain)",
+        "RUSTFMT": "$(rlocationpath :current_rustfmt_toolchain_for_target)",
     },
     toolchains = ["@rules_rust//rust/toolchain:current_rust_toolchain"],
     visibility = ["//visibility:public"],