Add support for optional cc_toolchain

Author: UebelAndre

.bazelrc

@@ -46,6 +46,10 @@ build:unpretty --output_groups=+rust_unpretty
 # https://github.com/rust-lang/rust/issues/43364
 build:unpretty --config=nightly
 
+# Disable cc toolchains to test rust targets can be built without one.
+build:no_cc_toolchain --repo_env=BAZEL_DO_NOT_DETECT_CPP_TOOLCHAIN=1
+build:no_cc_toolchain --repo_env=BAZEL_NO_APPLE_CPP_TOOLCHAIN=1
+
 ###############################################################################
 ## Incompatibility flags
 ###############################################################################

cargo/private/BUILD.bazel

@@ -8,6 +8,36 @@ rust_binary(
     visibility = ["//visibility:public"],
 )
 
+rust_binary(
+    name = "no_ar",
+    srcs = ["no_binary.rs"],
+    edition = "2021",
+    rustc_env = {
+        "BINARY_ENV": "AR",
+    },
+    visibility = ["//visibility:public"],
+)
+
+rust_binary(
+    name = "no_cc",
+    srcs = ["no_binary.rs"],
+    edition = "2021",
+    rustc_env = {
+        "BINARY_ENV": "CC",
+    },
+    visibility = ["//visibility:public"],
+)
+
+rust_binary(
+    name = "no_cxx",
+    srcs = ["no_binary.rs"],
+    edition = "2021",
+    rustc_env = {
+        "BINARY_ENV": "CXX",
+    },
+    visibility = ["//visibility:public"],
+)
+
 bzl_library(
     name = "bzl_lib",
     srcs = glob(["**/*.bzl"]),

cargo/private/cargo_build_script.bzl

@@ -3,7 +3,6 @@
 load("@bazel_skylib//lib:paths.bzl", "paths")
 load("@bazel_skylib//rules:common_settings.bzl", "BuildSettingInfo")
 load("@rules_cc//cc:action_names.bzl", "ACTION_NAMES")
-load("@rules_cc//cc:find_cc_toolchain.bzl", find_cpp_toolchain = "find_cc_toolchain")
 load("@rules_cc//cc/common:cc_common.bzl", "cc_common")
 load("//rust:defs.bzl", "rust_common")
 load("//rust:rust_common.bzl", "BuildInfo", "CrateGroupInfo", "DepInfo")
@@ -367,8 +366,6 @@ def _cargo_build_script_impl(ctx):
 
     toolchain_tools = [toolchain.all_files]
 
-    cc_toolchain = find_cpp_toolchain(ctx)
-
     env = {}
 
     if ctx.attr.use_default_shell_env == -1:
@@ -417,18 +414,25 @@ def _cargo_build_script_impl(ctx):
     # Pull in env vars which may be required for the cc_toolchain to work (e.g. on OSX, the SDK version).
     # We hope that the linker env is sufficient for the whole cc_toolchain.
     cc_toolchain, feature_configuration = find_cc_toolchain(ctx)
-    linker, link_args, linker_env = get_linker_and_args(ctx, "bin", cc_toolchain, feature_configuration, None)
+    linker, _, link_args, linker_env = get_linker_and_args(ctx, "bin", toolchain, cc_toolchain, feature_configuration, None)
     env.update(**linker_env)
     env["LD"] = linker
     env["LDFLAGS"] = " ".join(_pwd_flags(link_args))
 
-    # MSVC requires INCLUDE to be set
-    cc_c_args, cc_cxx_args, cc_env = get_cc_compile_args_and_env(cc_toolchain, feature_configuration)
-    include = cc_env.get("INCLUDE")
-    if include:
-        env["INCLUDE"] = include
+    # Defaults for cxx flags.
+    env["CC"] = "${{pwd}}/{}".format(ctx.executable._fallback_cc.path)
+    env["CXX"] = "${{pwd}}/{}".format(ctx.executable._fallback_cxx.path)
+    env["AR"] = "${{pwd}}/{}".format(ctx.executable._fallback_ar.path)
+    env["CFLAGS"] = ""
+    env["CXXFLAGS"] = ""
 
     if cc_toolchain:
+        # MSVC requires INCLUDE to be set
+        cc_c_args, cc_cxx_args, cc_env = get_cc_compile_args_and_env(cc_toolchain, feature_configuration)
+        include = cc_env.get("INCLUDE")
+        if include:
+            env["INCLUDE"] = include
+
         toolchain_tools.append(cc_toolchain.all_files)
 
         env["CC"] = cc_common.get_tool_for_action(
@@ -503,6 +507,9 @@ def _cargo_build_script_impl(ctx):
         direct = [
             script,
             ctx.executable._cargo_build_script_runner,
+            ctx.executable._fallback_cc,
+            ctx.executable._fallback_cxx,
+            ctx.executable._fallback_ar,
         ] + ([toolchain.target_json] if toolchain.target_json else []),
         transitive = script_data + script_tools + toolchain_tools,
     )
@@ -725,14 +732,29 @@ cargo_build_script = rule(
         "_experimental_symlink_execroot": attr.label(
             default = Label("//cargo/settings:experimental_symlink_execroot"),
         ),
+        "_fallback_ar": attr.label(
+            cfg = "exec",
+            executable = True,
+            default = Label("//cargo/private:no_ar"),
+        ),
+        "_fallback_cc": attr.label(
+            cfg = "exec",
+            executable = True,
+            default = Label("//cargo/private:no_cc"),
+        ),
+        "_fallback_cxx": attr.label(
+            cfg = "exec",
+            executable = True,
+            default = Label("//cargo/private:no_cxx"),
+        ),
         "_incompatible_runfiles_cargo_manifest_dir": attr.label(
             default = Label("//cargo/settings:incompatible_runfiles_cargo_manifest_dir"),
         ),
     },
     fragments = ["cpp"],
     toolchains = [
         str(Label("//rust:toolchain_type")),
-        "@bazel_tools//tools/cpp:toolchain_type",
+        config_common.toolchain_type("@bazel_tools//tools/cpp:toolchain_type", mandatory = False),
     ],
 )
 

cargo/private/no_binary.rs

@@ -0,0 +1,6 @@
+//! A cross platform implementation of `/bin/false`
+
+fn main() {
+    eprintln!(concat!("No binary provided for ", env!("BINARY_ENV")));
+    std::process::exit(1);
+}

extensions/prost/private/prost.bzl

@@ -392,7 +392,7 @@ rust_prost_aspect = aspect(
     fragments = ["cpp"],
     toolchains = [
         TOOLCHAIN_TYPE,
-        "@bazel_tools//tools/cpp:toolchain_type",
+        config_common.toolchain_type("@bazel_tools//tools/cpp:toolchain_type", mandatory = False),
         "@rules_rust//rust:toolchain_type",
         "@rules_rust//rust/rustfmt:toolchain_type",
     ],

extensions/protobuf/proto.bzl

@@ -343,7 +343,7 @@ rust_proto_library = rule(
     toolchains = [
         str(Label("//:toolchain_type")),
         str(Label("@rules_rust//rust:toolchain_type")),
-        "@bazel_tools//tools/cpp:toolchain_type",
+        config_common.toolchain_type("@bazel_tools//tools/cpp:toolchain_type", mandatory = False),
     ],
     doc = """\
 Builds a Rust library crate from a set of `proto_library`s.
@@ -435,7 +435,7 @@ rust_grpc_library = rule(
     toolchains = [
         str(Label("//:toolchain_type")),
         str(Label("@rules_rust//rust:toolchain_type")),
-        "@bazel_tools//tools/cpp:toolchain_type",
+        config_common.toolchain_type("@bazel_tools//tools/cpp:toolchain_type", mandatory = False),
     ],
     doc = """\
 Builds a Rust library crate from a set of `proto_library`s suitable for gRPC.

extensions/wasm_bindgen/private/wasm_bindgen_test.bzl

@@ -8,9 +8,6 @@ load("@rules_rust//rust/private:rust.bzl", "RUSTC_ATTRS", "get_rust_test_flags")
 # buildifier: disable=bzl-visibility
 load("@rules_rust//rust/private:rustc.bzl", "rustc_compile_action")
 
-# buildifier: disable=bzl-visibility
-# load("@rules_rust//rust/private:toolchain_utils.bzl", "get_coverage_env")
-
 # buildifier: disable=bzl-visibility
 load(
     "@rules_rust//rust/private:utils.bzl",
@@ -334,7 +331,7 @@ rust_wasm_bindgen_test = rule(
     toolchains = [
         str(Label("//:toolchain_type")),
         "@rules_rust//rust:toolchain_type",
-        "@bazel_tools//tools/cpp:toolchain_type",
+        config_common.toolchain_type("@bazel_tools//tools/cpp:toolchain_type", mandatory = False),
     ],
     test = True,
 )

ffi/rs/BUILD.bazel

@@ -1,7 +1,7 @@
 load("@rules_cc//cc:cc_library.bzl", "cc_library")
 
 # buildifier: disable=bzl-visibility
-load("@rules_rust//rust/private:rust.bzl", "rust_allocator_libraries")
+load("@rules_rust//rust/private:rust_allocator_libraries.bzl", "rust_allocator_libraries")
 
 rust_allocator_libraries(
     name = "allocator_libraries_with_mangling_support",

rust/platform/BUILD.bazel

@@ -21,6 +21,22 @@ constraint_value(
     constraint_setting = ":wasi_version",
 )
 
+# ABI constraint settings
+constraint_setting(
+    name = "abi",
+    default_constraint_value = ":gnu",
+)
+
+constraint_value(
+    name = "gnu",
+    constraint_setting = ":abi",
+)
+
+constraint_value(
+    name = "musl",
+    constraint_setting = ":abi",
+)
+
 package_group(
     name = "function_transition_allowlist",
     packages = [

rust/platform/triple_mappings.bzl

@@ -69,6 +69,7 @@ SUPPORTED_T2_PLATFORM_TRIPLES = {
     "x86_64-linux-android": _support(std = True, host_tools = False),
     "x86_64-unknown-freebsd": _support(std = True, host_tools = True),
     "x86_64-unknown-fuchsia": _support(std = True, host_tools = False),
+    "x86_64-unknown-linux-musl": _support(std = True, host_tools = True),
     "x86_64-unknown-none": _support(std = True, host_tools = False),
     "x86_64-unknown-uefi": _support(std = True, host_tools = False),
 }