Add support for the __has_c_attribute builtin preprocessor macro.

This behaves similar to the __has_cpp_attribute builtin macro in that it allows users to detect whether an attribute is supported with the [[]] spelling syntax, which can be enabled in C with -fdouble-square-bracket-attributes.

llvm-svn: 320088

clang/docs/LanguageExtensions.rst

@@ -139,6 +139,35 @@ and following ``__`` (double underscore) to avoid interference from a macro with
 the same name.  For instance, ``gnu::__const__`` can be used instead of
 ``gnu::const``.
 
+``__has_c_attribute``
+---------------------
+
+This function-like macro takes a single argument that is the name of an
+attribute exposed with the double square-bracket syntax in C mode. The argument
+can either be a single identifier or a scoped identifier. If the attribute is
+supported, a nonzero value is returned. If the attribute is not supported by the
+current compilation target, this macro evaluates to 0. It can be used like this:
+
+.. code-block:: c
+
+  #ifndef __has_c_attribute         // Optional of course.
+    #define __has_c_attribute(x) 0  // Compatibility with non-clang compilers.
+  #endif
+
+  ...
+  #if __has_c_attribute(fallthrough)
+    #define FALLTHROUGH [[fallthrough]]
+  #else
+    #define FALLTHROUGH
+  #endif
+  ...
+
+The attribute identifier (but not scope) can also be specified with a preceding
+and following ``__`` (double underscore) to avoid interference from a macro with
+the same name.  For instance, ``gnu::__const__`` can be used instead of
+``gnu::const``.
+
+
 ``__has_attribute``
 -------------------
 

clang/docs/ReleaseNotes.rst

@@ -139,6 +139,11 @@ Clang now supports the ...
 Attribute Changes in Clang
 --------------------------
 
+- Added the ``__has_c_attribute()`` builtin preprocessor macro which allows
+  users to dynamically detect whether a double square-bracket attribute is
+  supported in C mode. This attribute syntax can be enabled with the
+  ``-fdouble-square-bracket-attributes`` flag.
+  
 - The presence of __attribute__((availability(...))) on a declaration no longer
   implies default visibility for that declaration on macOS.
 

clang/include/clang/Lex/Preprocessor.h

@@ -173,6 +173,7 @@ class Preprocessor {
   IdentifierInfo *Ident__building_module;          // __building_module
   IdentifierInfo *Ident__MODULE__;                 // __MODULE__
   IdentifierInfo *Ident__has_cpp_attribute;        // __has_cpp_attribute
+  IdentifierInfo *Ident__has_c_attribute;          // __has_c_attribute
   IdentifierInfo *Ident__has_declspec;             // __has_declspec_attribute
 
   SourceLocation DATELoc, TIMELoc;

clang/test/Preprocessor/has_c_attribute.c

@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -fdouble-square-bracket-attributes -std=c11 -E %s -o - | FileCheck %s
+
+// CHECK: has_fallthrough
+#if __has_c_attribute(fallthrough)
+  int has_fallthrough();
+#endif
+
+// CHECK: does_not_have_selectany
+#if !__has_c_attribute(selectany)
+  int does_not_have_selectany();
+#endif
+