Depend on a copy of rustfmt for the target (#2685)

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.

Fixes: #2684

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",