[libc] Add compile tests for each public header (llvm#122527)

This adds a test that consists of compiling `#include <...>`,
pretty much alone, for each public header file in each different
language mode (`-std=...` compiler switch) with -Werror and many
warnings enabled.

There are several headers that have bugs when used alone, and
many more headers that have bugs in certain language modes.  So
for now, compiling the new tests is gated on the cmake switch
-DLLVM_LIBC_BUILD_HEADER_TESTS=ON.  When all the bugs are fixed,
the switch will be removed so future regressions don't land.

libc/cmake/modules/LLVMLibCHeaderRules.cmake

@@ -66,6 +66,7 @@ function(add_header target_name)
   set_target_properties(
     ${fq_target_name}
     PROPERTIES
+      HEADER_NAME ${dest_leaf_filename}
       HEADER_FILE_PATH ${dest_file}
       DEPS "${fq_deps_list}"
   )
@@ -164,6 +165,7 @@ function(add_gen_header target_name)
   set_target_properties(
     ${fq_target_name}
     PROPERTIES
+      HEADER_NAME ${ADD_GEN_HDR_GEN_HDR}
       HEADER_FILE_PATH ${out_file}
       DECLS_FILE_PATH "${decl_out_file}"
       DEPS "${fq_deps_list}"

libc/test/include/CMakeLists.txt

@@ -422,7 +422,7 @@ add_libc_test(
     -Werror
   DEPENDS
     libc.include.llvm-libc-macros.math_function_macros
-)  
+)
 
 add_libc_test(
   isfinite_c_test
@@ -483,3 +483,70 @@ add_libc_test(
   DEPENDS
     libc.include.llvm-libc-macros.math_function_macros
 )
+
+# Test `#include <...>` of each header in each available language mode.
+# This is gated on -DLLVM_LIBC_BUILD_HEADER_TESTS=ON until all the bugs
+# in headers are fixed so the tests all compile.
+set(TEST_STDC_VERSIONS 89;99;11;17;23)
+set(TEST_STDCXX_VERSIONS 03;11;14;17;20;23;26)
+
+function(add_header_test target_name source_file deps std_mode)
+  if(LLVM_LIBC_BUILD_HEADER_TESTS)
+    add_libc_test(
+      ${target_name}
+      C_TEST
+      HERMETIC_TEST_ONLY
+      SUITE
+	libc_include_tests
+      SRCS
+	${source_file}
+      COMPILE_OPTIONS
+	-Werror
+	-Wsystem-headers
+	-Wall
+	-Wextra
+	-std=${std_mode}
+      DEPENDS
+	${deps}
+    )
+  endif()
+endfunction()
+
+foreach(target ${TARGET_PUBLIC_HEADERS})
+  string(REPLACE "libc.include." "" header ${target})
+  get_target_property(HEADER_NAME ${target} HEADER_NAME)
+
+  set(test_stdc_file "${CMAKE_CURRENT_BINARY_DIR}/${header}_test.c")
+  configure_file(header-test-template.c ${test_stdc_file} @ONLY)
+  foreach(stdc_version ${TEST_STDC_VERSIONS})
+    add_header_test(
+      "${header}_c${stdc_version}_test"
+      ${test_stdc_file}
+      ${target}
+      "c${stdc_version}"
+    )
+    add_header_test(
+      "${header}_gnu${stdc_version}_test"
+      ${test_stdc_file}
+      ${target}
+      "gnu${stdc_version}"
+    )
+  endforeach()
+
+  set(test_stdcxx_file "${CMAKE_CURRENT_BINARY_DIR}/${header}_test.cpp")
+  configure_file(header-test-template.c ${test_stdcxx_file} @ONLY)
+  foreach(stdcxx_version ${TEST_STDCXX_VERSIONS})
+    add_header_test(
+      "${header}_cpp${stdcxx_version}_test"
+      ${test_stdcxx_file}
+      ${target}
+      "c++${stdcxx_version}"
+    )
+    add_header_test(
+      "${header}_gnucpp${stdcxx_version}_test"
+      ${test_stdcxx_file}
+      ${target}
+      "gnu++${stdcxx_version}"
+    )
+  endforeach()
+endforeach()

libc/test/include/header-test-template.c

@@ -0,0 +1,14 @@
+/*===-- Test for <@HEADER_NAME@> ----------------------------------------===
+ *
+ * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+ * See https://llvm.org/LICENSE.txt for license information.
+ * SPDXList-License-Identifier: Apache-2.0 WITH LLVM-exception
+ */
+
+#include <@HEADER_NAME@>
+
+int main(int argc, char **argv) {
+  (void)argc;
+  (void)argv;
+  return 0;
+}