abi check: prefer nim constant to enable, document

compiler/ccgtypes.nim

@@ -703,7 +703,7 @@ proc getTypeDescAux(m: BModule, typ: PType, check: var IntSet): Rope =
                     else: getTupleDesc(m, t, result, check)
       if not isImportedType(t):
         add(m.s[cfsTypes], recdesc)
-      elif tfIncompleteStruct notin t.flags:
+      elif tfIncompleteStruct notin t.flags and isDefined("checkabi"):
         addf(m.s[cfsTypeInfo], "NIM_CHECK_SIZE($1, $2);$n", [result, rope(getSize(t))])
   of tySet:
     result = getTypeName(t.lastSon) & "Set"

doc/nimc.rst

@@ -258,6 +258,10 @@ Define               Effect
 ``ssl``              Enables OpenSSL support for the sockets module.
 ``memProfiler``      Enables memory profiling for the native GC.
 ``uClibc``           Use uClibc instead of libc. (Relevant for Unix-like OSes)
+``checkAbi``         When using types from C headers, add checks that compare
+                     what's in the Nim file with what's in the C header
+                     (requires a C compiler with _Static_assert support, like
+                     any C11 compiler)
 ==================   =========================================================
 
 

lib/nimbase.h

@@ -460,10 +460,6 @@ typedef int Nim_and_C_compiler_disagree_on_target_architecture[sizeof(NI) == siz
 #  include <sys/types.h>
 #endif
 
-/* Compile with -t:-DNIM_CHECK_ABI to enable */
-#ifdef NIM_CHECK_ABI
-#  define NIM_CHECK_SIZE(typ, sz) \
+/* Compile with -d:checkAbi and a sufficiently C11:ish compiler to enable */
+#define NIM_CHECK_SIZE(typ, sz) \
   _Static_assert(sizeof(typ) == sz, "Nim & C disagree on type size")
-#else
-#  define NIM_CHECK_SIZE(typ, sz)
-#endif