[build] Rely on CMake for compiler introspection

Amend 97465f7 with an approach that also works for externals using
`@rules_cc//cc/compiler`, instead of a custom Drake flag, by writing a
symlink in the build directory to the underlying compiler with a
filename expected by Bazel.

Author: tyler-yankee

CMakeLists.txt

@@ -291,23 +291,39 @@ endif()
 set(BAZEL_REPO_ENV)
 
 if(NOT APPLE)
+  # Determine the real underlying compiler, so that compiler- and version-
+  # specific warning logic is enforced throughout the build (even for
+  # externals). Since rules_cc only identifies the compiler based on the
+  # filename (rather than actually running it, as CMake does), create a local
+  # symlink to the compiler with the expected name.
+  # TODO(tyler-yankee): Ideally, rules_cc would perform proper compiler
+  # introspection so that this workaround is not needed, but that doesn't seem
+  # to exist yet.
+
+  # Base case; use whatever we were given. (If the filenames already look like
+  # gcc or clang, or if we fail to parse, we'll fall back to this.)
+  set(DRAKE_C_COMPILER "${CMAKE_C_COMPILER}")
+  set(DRAKE_CXX_COMPILER "${CMAKE_CXX_COMPILER}")
+
+  cmake_path(GET CMAKE_C_COMPILER FILENAME C_COMPILER_BASENAME)
+  if (NOT C_COMPILER_BASENAME MATCHES "(gcc|clang)(-\\d+)?")
+    if(CMAKE_C_COMPILER_ID STREQUAL Clang)
+      set(DRAKE_C_COMPILER "${CMAKE_BINARY_DIR}/clang")
+      set(DRAKE_CXX_COMPILER "${CMAKE_BINARY_DIR}/clang++")
+      file(CREATE_LINK "${CMAKE_C_COMPILER}" "${DRAKE_C_COMPILER}" SYMBOLIC)
+      file(CREATE_LINK "${CMAKE_CXX_COMPILER}" "${DRAKE_CXX_COMPILER}" SYMBOLIC)
+    elseif(CMAKE_C_COMPILER_ID STREQUAL GNU)
+      set(DRAKE_C_COMPILER "${CMAKE_BINARY_DIR}/gcc")
+      set(DRAKE_CXX_COMPILER "${CMAKE_BINARY_DIR}/g++")
+      file(CREATE_LINK "${CMAKE_C_COMPILER}" "${DRAKE_C_COMPILER}" SYMBOLIC)
+      file(CREATE_LINK "${CMAKE_CXX_COMPILER}" "${DRAKE_CXX_COMPILER}" SYMBOLIC)
+    endif()
+  endif()
+
   string(APPEND BAZEL_REPO_ENV
-    " --repo_env=CC=${CMAKE_C_COMPILER}"
-    " --repo_env=CXX=${CMAKE_CXX_COMPILER}"
+    " --repo_env=CC=${DRAKE_C_COMPILER}"
+    " --repo_env=CXX=${DRAKE_CXX_COMPILER}"
   )
-  # Pass a Bazel flag which enforces the underlying compiler so that
-  # compiler- and version-specific logic (e.g., warnings) is enforced in the
-  # build, since rules_cc's support for compiler identification is not as
-  # robust as using CMake's CMAKE_<LANG>_COMPILER_ID.
-  if(CMAKE_C_COMPILER_ID STREQUAL Clang)
-    string(APPEND BAZEL_REPO_ENV
-      " --@drake//tools/cc_toolchain:compiler=clang"
-    )
-  elseif(CMAKE_C_COMPILER_ID STREQUAL GNU)
-    string(APPEND BAZEL_REPO_ENV
-      " --@drake//tools/cc_toolchain:compiler=gcc"
-    )
-  endif()
 endif()
 
 get_filename_component(PROJECT_BINARY_DIR_REALPATH

tools/cc_toolchain/BUILD.bazel

@@ -26,28 +26,16 @@ string_flag(
     ],
 )
 
-# In our developer builds and CMake installs, we use the following two flags
-# as optional hints for configuring compiler warnings. It's not strictly
-# necessary to set these, but doing so will tweak the compiler flags and
-# reduce warning spam during the build.
+# In our developer builds and CMake installs, we use the following flag as
+# an optional hint for configuring compiler warnings. It's not strictly
+# necessary to set, but doing so will tweak the compiler flags and reduce
+# warning spam during the build.
 #
-# These are intended for internal use only, not for users to configure.
+# This is intended for internal use only, not for users to configure.
 #
-# TODO(jwnimmer-tri) Ideally, rules_cc would provide,
-# * for 'compiler', a check for the real underlying compiler by invoking it
-#   (akin to CMake) instead of just parsing the filename; and
-# * for 'compiler_major', some toolchain attribute that we could query.
-# Neither of these seem to exist yet.
-string_flag(
-    name = "compiler",
-    build_setting_default = "",
-    values = [
-        "",
-        "gcc",
-        "clang",
-    ],
-)
-
+# TODO(jwnimmer-tri) Ideally, rules_cc would provide some toolchain attribute
+# that we could query for the version of the compiler, but that doesn't seem to
+# exist yet.
 int_flag(
     name = "compiler_major",
     build_setting_default = 0,
@@ -73,37 +61,11 @@ config_setting(
     flag_values = {":error_severity": "error"},
 )
 
-config_setting(
-    name = "compiler_clang",
-    flag_values = {":compiler": "clang"},
-)
-
-config_setting(
-    name = "compiler_gcc",
-    flag_values = {":compiler": "gcc"},
-)
-
-selects.config_setting_group(
-    name = "clang",
-    match_any = [
-        "@rules_cc//cc/compiler:clang",
-        ":compiler_clang",
-    ],
-)
-
-selects.config_setting_group(
-    name = "gcc",
-    match_any = [
-        "@rules_cc//cc/compiler:gcc",
-        ":compiler_gcc",
-    ],
-)
-
 selects.config_setting_group(
     name = "apple_clang_with_warnings",
     match_all = [
         ":apple",
-        ":clang",
+        "@rules_cc//cc/compiler:clang",
         ":error_severity_warning",
     ],
 )
@@ -112,23 +74,23 @@ selects.config_setting_group(
     name = "apple_clang_with_errors",
     match_all = [
         ":apple",
-        ":clang",
+        "@rules_cc//cc/compiler:clang",
         ":error_severity_error",
     ],
 )
 
 selects.config_setting_group(
     name = "gcc_with_warnings",
     match_all = [
-        ":gcc",
+        "@rules_cc//cc/compiler:gcc",
         ":error_severity_warning",
     ],
 )
 
 selects.config_setting_group(
     name = "gcc_with_errors",
     match_all = [
-        ":gcc",
+        "@rules_cc//cc/compiler:gcc",
         ":error_severity_error",
     ],
 )
@@ -137,7 +99,7 @@ selects.config_setting_group(
     name = "linux_clang_with_warnings",
     match_all = [
         ":linux",
-        ":clang",
+        "@rules_cc//cc/compiler:clang",
         ":error_severity_warning",
     ],
 )
@@ -146,7 +108,7 @@ selects.config_setting_group(
     name = "linux_clang_with_errors",
     match_all = [
         ":linux",
-        ":clang",
+        "@rules_cc//cc/compiler:clang",
         ":error_severity_error",
     ],
 )
@@ -191,7 +153,7 @@ config_setting(
 selects.config_setting_group(
     name = "gcc_13_with_warnings",
     match_all = [
-        ":gcc",
+        "@rules_cc//cc/compiler:gcc",
         ":compiler_major_13",
         ":error_severity_warning",
     ],
@@ -200,7 +162,7 @@ selects.config_setting_group(
 selects.config_setting_group(
     name = "gcc_13_with_errors",
     match_all = [
-        ":gcc",
+        "@rules_cc//cc/compiler:gcc",
         ":compiler_major_13",
         ":error_severity_error",
     ],

tools/skylark/drake_cc.bzl

@@ -141,8 +141,8 @@ def _platform_copts(rule_copts, rule_gcc_copts, rule_clang_copts, cc_test = 0):
     test_gcc_copts = (CC_TEST_FLAGS + GCC_CC_TEST_FLAGS) if cc_test else []
     test_clang_copts = CC_TEST_FLAGS if cc_test else []
     return BASE_COPTS + rule_copts + select({
-        "//tools/cc_toolchain:gcc": rule_gcc_copts + test_gcc_copts,
-        "//tools/cc_toolchain:clang": rule_clang_copts + test_clang_copts,
+        "@rules_cc//cc/compiler:gcc": rule_gcc_copts + test_gcc_copts,
+        "@rules_cc//cc/compiler:clang": rule_clang_copts + test_clang_copts,
         "//conditions:default": [],
     })
 

tools/skylark/pybind.bzl

@@ -5,7 +5,7 @@ load("//tools/skylark:drake_py.bzl", "drake_py_library", "drake_py_test")
 load("//tools/skylark:py.bzl", "py_library")
 
 EXTRA_PYBIND_COPTS = select({
-    "@drake//tools/cc_toolchain:clang": [
+    "@rules_cc//cc/compiler:clang": [
         # GCC and Clang don't always agree / succeed when inferring storage
         # duration (#9600). Workaround it for now.
         "-Wno-unused-lambda-capture",

tools/wheel/image/build-drake.sh

@@ -50,5 +50,7 @@ cmake ../drake-src \
     -DDRAKE_VERSION_OVERRIDE="${DRAKE_VERSION}" \
     -DDRAKE_GIT_SHA_OVERRIDE="${DRAKE_GIT_SHA}" \
     -DCMAKE_INSTALL_PREFIX=/tmp/drake-wheel-build/drake-dist \
-    -DPython_EXECUTABLE=/usr/local/bin/python
+    -DPython_EXECUTABLE=/usr/local/bin/python \
+    -DCMAKE_C_COMPILER=/usr/bin/gcc \
+    -DCMAKE_CXX_COMPILER=/usr/bin/g++
 make install