Features: Record C features for MSVC

The MSVC C compiler has no notion of C language standards or flags.
Tell CMake to assume that all language standards are available.
Record available C language features depending on the version of
the compiler.

Fixes: #17858

Help/release/dev/features-msvc-c.rst

@@ -0,0 +1,5 @@
+features-msvc-c
+---------------
+
+* The :manual:`Compile Features <cmake-compile-features(7)>` functionality
+  is now aware of the availability of C features in MSVC since VS 2010.

Modules/Compiler/MSVC-C-FeatureTests.cmake

@@ -0,0 +1,8 @@
+set(_cmake_oldestSupported "_MSC_VER >= 1600")
+
+# Not yet supported:
+#set(_cmake_feature_test_c_static_assert "")
+#set(_cmake_feature_test_c_restrict "")
+
+set(_cmake_feature_test_c_variadic_macros "${_cmake_oldestSupported}")
+set(_cmake_feature_test_c_function_prototypes "${_cmake_oldestSupported}")

Modules/Compiler/MSVC-C.cmake

@@ -0,0 +1,25 @@
+# MSVC has no specific options to set C language standards, but set them as
+# empty strings anyways so the feature test infrastructure can at least check
+# to see if they are defined.
+set(CMAKE_C90_STANDARD_COMPILE_OPTION "")
+set(CMAKE_C90_EXTENSION_COMPILE_OPTION "")
+set(CMAKE_C99_STANDARD_COMPILE_OPTION "")
+set(CMAKE_C99_EXTENSION_COMPILE_OPTION "")
+set(CMAKE_C11_STANDARD_COMPILE_OPTION "")
+set(CMAKE_C11_EXTENSION_COMPILE_OPTION "")
+
+# There is no meaningful default for this
+set(CMAKE_C_STANDARD_DEFAULT "")
+
+# There are no C compiler modes so we only need to test features once.
+# Override the default macro for this special case.  Pretend that
+# all language standards are available so that at least compilation
+# can be attempted.
+macro(cmake_record_c_compile_features)
+  list(APPEND CMAKE_C_COMPILE_FEATURES
+    c_std_90
+    c_std_99
+    c_std_11
+    )
+  _record_compiler_features(C "" CMAKE_C_COMPILE_FEATURES)
+endmacro()

Tests/CMakeCommands/target_compile_features/CMakeLists.txt

@@ -1,5 +1,4 @@
-cmake_minimum_required(VERSION 3.0)
-cmake_policy(SET CMP0057 NEW)
+cmake_minimum_required(VERSION 3.3)
 project(target_compile_features)
 
 set(CMAKE_VERBOSE_MAKEFILE ON)
@@ -19,7 +18,8 @@ if (c_restrict IN_LIST CMAKE_C_COMPILE_FEATURES)
   target_link_libraries(c_restrict_user_specific c_lib_restrict_specific)
 endif()
 
-if (c_std_99 IN_LIST CMAKE_C_COMPILE_FEATURES)
+if (c_std_99 IN_LIST CMAKE_C_COMPILE_FEATURES AND
+    NOT "${CMAKE_C_COMPILER_ID}" STREQUAL "MSVC")
   add_executable(c_target_compile_features_meta main.c)
   target_compile_features(c_target_compile_features_meta
     PRIVATE c_std_99

Tests/CompileFeatures/CMakeLists.txt

@@ -254,6 +254,11 @@ if (C_expected_features)
         EXPECT_C_STATIC_ASSERT=0
         )
     endif()
+  elseif(CMAKE_C_COMPILER_ID STREQUAL "MSVC")
+    list(APPEND expected_defs
+      EXPECT_C_RESTRICT=0
+      EXPECT_C_STATIC_ASSERT=0
+      )
   elseif (CMAKE_C_COMPILER_ID STREQUAL "SunPro")
     if (NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 5.13)
       list(APPEND expected_defs