Simplify some mojo macros by assuming C++11 or C11 are used.

No intended behavior change.

Bug: none
Change-Id: I6a3f8b5c56a760920a7d21fc88b348b3168f8846
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1537997
Auto-Submit: Nico Weber <thakis@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#643872}

mojo/public/c/system/macros.h

@@ -7,49 +7,37 @@
 
 #include <stddef.h>
 
+#if !defined(__cplusplus)
+#include <assert.h>    // Defines static_assert() in C11.
+#include <stdalign.h>  // Defines alignof() in C11.
+#endif
+
 // Assert things at compile time. (|msg| should be a valid identifier name.)
 // Use like:
 //   MOJO_STATIC_ASSERT(sizeof(struct Foo) == 12, "Foo has invalid size");
-#if defined(__cplusplus)
 #define MOJO_STATIC_ASSERT(expr, msg) static_assert(expr, msg)
-#elif defined(__clang__)
-// TODO(thakis): Use #include <assert.h> and static_assert() in C11 mode
-// (__STDC_VERSION__>= 201112L) once https://reviews.llvm.org/D17444 made its
-// way into Chromium.
-#define MOJO_STATIC_ASSERT(expr, msg) _Static_assert(expr, msg)
-#else
-#define MOJO_STATIC_ASSERT(expr, msg)
-#endif
 
 // Defines a pointer-sized struct field of the given type. This ensures that the
 // field has an 8-byte footprint on both 32-bit and 64-bit systems, using an
-// anonymous bitfield of either 32 or 0 bits, depending on pointer size. Weird
-// formatting here courtesy of clang-format.
+// anonymous bitfield of either 32 or 0 bits, depending on pointer size.
+// clang-format off
 #define MOJO_POINTER_FIELD(type, name) \
   type name;                           \
-  uint32_t:                            \
-  (sizeof(void*) == 4 ? 32 : 0)
+  uint32_t : (sizeof(void*) == 4 ? 32 : 0)
+// clang-format on
 
 // Like the C++11 |alignof| operator.
-#if __cplusplus >= 201103L
 #define MOJO_ALIGNOF(type) alignof(type)
-#elif defined(__GNUC__)
-#define MOJO_ALIGNOF(type) __alignof__(type)
-#elif defined(_MSC_VER)
-// The use of |sizeof| is to work around a bug in MSVC 2010 (see
-// http://goo.gl/isH0C; supposedly fixed since then).
-#define MOJO_ALIGNOF(type) (sizeof(type) - sizeof(type) + __alignof(type))
-#else
-#error "Please define MOJO_ALIGNOF() for your compiler."
-#endif
 
 // Specify the alignment of a |struct|, etc.
 // Use like:
 //   struct MOJO_ALIGNAS(8) Foo { ... };
 // Unlike the C++11 |alignas()|, |alignment| must be an integer. It may not be a
 // type, nor can it be an expression like |MOJO_ALIGNOF(type)| (due to the
 // non-C++11 MSVS version).
-#if __cplusplus >= 201103L
+// This can't use alignas() in C11 mode because unlike the C++11 version the
+// C11 version can't be used on struct declarations.
+#if defined(__cplusplus)
 #define MOJO_ALIGNAS(alignment) alignas(alignment)
 #elif defined(__GNUC__)
 #define MOJO_ALIGNAS(alignment) __attribute__((aligned(alignment)))