Enhance Py_ARRAY_LENGTH(): fail at build time if the argument is not an array

Move other various macros to pymcacro.h

Thanks Rusty Russell for having written these amazing C macros!

Include/Python.h

@@ -48,6 +48,7 @@
 #include <assert.h>
 
 #include "pyport.h"
+#include "pymacro.h"
 
 #include "pyatomic.h"
 
@@ -126,24 +127,6 @@
 #include "pystrcmp.h"
 #include "dtoa.h"
 #include "fileutils.h"
-
-/* Argument must be a char or an int in [-128, 127] or [0, 255]. */
-#define Py_CHARMASK(c) ((unsigned char)((c) & 0xff))
-
 #include "pyfpe.h"
 
-/* Define macros for inline documentation. */
-#define PyDoc_VAR(name) static char name[]
-#define PyDoc_STRVAR(name,str) PyDoc_VAR(name) = PyDoc_STR(str)
-#ifdef WITH_DOC_STRINGS
-#define PyDoc_STR(str) str
-#else
-#define PyDoc_STR(str) ""
-#endif
-
-#define Py_ARRAY_LENGTH(array) (sizeof(array) / sizeof((array)[0]))
-
-#define Py_MIN(x, y) (((x) > (y)) ? (y) : (x))
-#define Py_MAX(x, y) (((x) > (y)) ? (x) : (y))
-
 #endif /* !Py_PYTHON_H */

Include/pymacro.h

@@ -0,0 +1,57 @@
+#ifndef Py_PYMACRO_H
+#define Py_PYMACRO_H
+
+#define Py_MIN(x, y) (((x) > (y)) ? (y) : (x))
+#define Py_MAX(x, y) (((x) > (y)) ? (x) : (y))
+
+/* Argument must be a char or an int in [-128, 127] or [0, 255]. */
+#define Py_CHARMASK(c) ((unsigned char)((c) & 0xff))
+
+
+/* Assert a build-time dependency, as an expression.
+
+   Your compile will fail if the condition isn't true, or can't be evaluated
+   by the compiler. This can be used in an expression: its value is 0.
+
+   Example:
+
+   #define foo_to_char(foo)  \
+       ((char *)(foo)        \
+        + Py_BUILD_ASSERT(offsetof(struct foo, string) == 0))
+
+   Written by Rusty Russell, public domain. */
+#define Py_BUILD_ASSERT(cond) \
+    (sizeof(char [1 - 2*!(cond)]) - 1)
+
+#if defined(__GNUC__)
+/* Two gcc extensions.
+   &a[0] degrades to a pointer: a different type from an array */
+#define _Py_ARRAY_LENGTH_CHECK(array) \
+    Py_BUILD_ASSERT(!__builtin_types_compatible_p(typeof(array), \
+                                                  typeof(&(array)[0])))
+#else
+#define _Py_ARRAY_LENGTH_CHECK(array) 0
+#endif
+
+
+/* Get the number of elements in a visible array
+
+   This does not work on pointers, or arrays declared as [], or function
+   parameters. With correct compiler support, such usage will cause a build
+   error (see Py_BUILD_ASSERT).
+
+   Written by Rusty Russell, public domain. */
+#define Py_ARRAY_LENGTH(array) \
+    (sizeof(array) / sizeof((array)[0]) + _Py_ARRAY_LENGTH_CHECK(array))
+
+
+/* Define macros for inline documentation. */
+#define PyDoc_VAR(name) static char name[]
+#define PyDoc_STRVAR(name,str) PyDoc_VAR(name) = PyDoc_STR(str)
+#ifdef WITH_DOC_STRINGS
+#define PyDoc_STR(str) str
+#else
+#define PyDoc_STR(str) ""
+#endif
+
+#endif /* Py_PYMACRO_H */

Makefile.pre.in

@@ -718,6 +718,7 @@ PYTHON_HEADERS= \
 		Include/pyfpe.h \
 		Include/pymath.h \
 		Include/pygetopt.h \
+		Include/pymacro.h \
 		Include/pymem.h \
 		Include/pyport.h \
 		Include/pystate.h \

Misc/ACKS

@@ -825,6 +825,7 @@ Rauli Ruohonen
 Jeff Rush
 Sam Rushing
 Mark Russell
+Rusty Russell
 Nick Russo
 Patrick Sabin
 Sébastien Sablé

PCbuild/pythoncore.vcproj

@@ -886,6 +886,10 @@
 				RelativePath="..\Include\pytime.h"
 				>
 			</File>
+			<File
+				RelativePath="..\Include\pymacro.h"
+				>
+			</File>
 			<File
 				RelativePath="..\Include\pymem.h"
 				>