c-family: Implement __has_feature and __has_extension [PR60512]

This patch implements clang's __has_feature and __has_extension in GCC.
Currently the patch aims to implement all documented features (and some
undocumented ones) following the documentation at
https://clang.llvm.org/docs/LanguageExtensions.html with the exception
of the legacy features for C++ type traits.  These are omitted, since as
the clang documentation notes, __has_builtin is the correct "modern" way
to query for these (which GCC already implements).

gcc/c-family/ChangeLog:

	PR c++/60512
	* c-common.cc (struct hf_feature_info): New.
	(c_common_register_feature): New.
	(init_has_feature): New.
	(has_feature_p): New.
	* c-common.h (c_common_has_feature): New.
	(c_family_register_lang_features): New.
	(c_common_register_feature): New.
	(has_feature_p): New.
	* c-lex.cc (init_c_lex): Plumb through has_feature callback.
	(c_common_has_builtin): Generalize and move common part ...
	(c_common_lex_availability_macro): ... here.
	(c_common_has_feature): New.
	* c-ppoutput.cc (init_pp_output): Plumb through has_feature.

gcc/c/ChangeLog:

	PR c++/60512
	* c-lang.cc (c_family_register_lang_features): New.
	* c-objc-common.cc (struct c_feature_info): New.
	(c_register_features): New.
	* c-objc-common.h (c_register_features): New.

gcc/cp/ChangeLog:

	PR c++/60512
	* cp-lang.cc (c_family_register_lang_features): New.
	* cp-objcp-common.cc (struct cp_feature_selector): New.
	(cp_feature_selector::has_feature): New.
	(struct cp_feature_info): New.
	(cp_register_features): New.
	* cp-objcp-common.h (cp_register_features): New.

gcc/ChangeLog:

	PR c++/60512
	* doc/cpp.texi: Document __has_{feature,extension}.

gcc/objc/ChangeLog:

	PR c++/60512
	* objc-act.cc (struct objc_feature_info): New.
	(objc_nonfragile_abi_p): New.
	(objc_common_register_features): New.
	* objc-act.h (objc_common_register_features): New.
	* objc-lang.cc (c_family_register_lang_features): New.

gcc/objcp/ChangeLog:

	PR c++/60512
	* objcp-lang.cc (c_family_register_lang_features): New.

libcpp/ChangeLog:

	PR c++/60512
	* include/cpplib.h (struct cpp_callbacks): Add has_feature.
	(enum cpp_builtin_type): Add BT_HAS_{FEATURE,EXTENSION}.
	* init.cc: Add __has_{feature,extension}.
	* macro.cc (_cpp_builtin_macro_text): Handle
	BT_HAS_{FEATURE,EXTENSION}.

gcc/testsuite/ChangeLog:

	PR c++/60512
	* c-c++-common/has-feature-common.c: New test.
	* c-c++-common/has-feature-pedantic.c: New test.
	* g++.dg/ext/has-feature.C: New test.
	* gcc.dg/asan/has-feature-asan.c: New test.
	* gcc.dg/has-feature.c: New test.
	* gcc.dg/ubsan/has-feature-ubsan.c: New test.
	* obj-c++.dg/has-feature.mm: New test.
	* objc.dg/has-feature.m: New test.

Co-Authored-By: Iain Sandoe <iain@sandoe.co.uk>

3 files changed, 45 insertions, 0 deletions

diff --git a/gcc/c/c-lang.cc b/gcc/c/c-lang.cc
index ddfd3e8..ff6633e 100644
--- a/gcc/c/c-lang.cc
+++ b/gcc/c/c-lang.cc
@@ -63,6 +63,15 @@ c_get_sarif_source_language (const char *)
   return "c";
 }
 
+/* Implement c-family hook to register language-specific features for
+   __has_{feature,extension}.  */
+
+void
+c_family_register_lang_features ()
+{
+  c_register_features ();
+}
+
 #if CHECKING_P
 
 namespace selftest {
diff --git a/gcc/c/c-objc-common.cc b/gcc/c/c-objc-common.cc
index c8f49aa..53eda7f 100644
--- a/gcc/c/c-objc-common.cc
+++ b/gcc/c/c-objc-common.cc
@@ -34,6 +34,39 @@ along with GCC; see the file COPYING3.  If not see
 static bool c_tree_printer (pretty_printer *, text_info *, const char *,
 			    int, bool, bool, bool, bool *, const char **);
 
+/* Info for C language features which can be queried through
+   __has_{feature,extension}.  */
+
+struct c_feature_info
+{
+  const char *ident;
+  const int *enable_flag;
+};
+
+static const c_feature_info c_feature_table[] =
+{
+  { "c_alignas", &flag_isoc11 },
+  { "c_alignof", &flag_isoc11 },
+  { "c_atomic", &flag_isoc11 },
+  { "c_generic_selections", &flag_isoc11 },
+  { "c_static_assert", &flag_isoc11 },
+  { "c_thread_local", &flag_isoc11 },
+  { "cxx_binary_literals", &flag_isoc23 }
+};
+
+/* Register features specific to the C language.  */
+
+void
+c_register_features ()
+{
+  for (unsigned i = 0; i < ARRAY_SIZE (c_feature_table); i++)
+    {
+      const c_feature_info *info = c_feature_table + i;
+      const bool feat_p = !info->enable_flag || *info->enable_flag;
+      c_common_register_feature (info->ident, feat_p);
+    }
+}
+
 bool
 c_missing_noreturn_ok_p (tree decl)
 {
diff --git a/gcc/c/c-objc-common.h b/gcc/c/c-objc-common.h
index ede451c..63aff70 100644
--- a/gcc/c/c-objc-common.h
+++ b/gcc/c/c-objc-common.h
@@ -21,6 +21,9 @@ along with GCC; see the file COPYING3.  If not see
 #ifndef GCC_C_OBJC_COMMON
 #define GCC_C_OBJC_COMMON
 
+/* Implemented in c-objc-common.cc.  */
+extern void c_register_features ();
+
 /* Lang hooks that are shared between C and ObjC are defined here.  Hooks
    specific to C or ObjC go in c-lang.cc and objc/objc-lang.cc, respectively.  */