Swap order of extern/static and pg_nodiscard

When pg_nodiscard was first added, the C standard draft had it as a
function specifier, and so the code comment about placement was
written with that in mind.  The final C23 standard has it as an
attribute and the placement rules are a bit different for that.
Specifically, it needs to be before extern or static.  (Or at least
both current clang and gcc require that.)  So just swap these.  (To be
clear: The current implementation with gcc attributes doesn't care.
This change is just for maximum forward compatibility for non-gcc
compilers.)  This also keeps the order consistent with the previously
introduced pg_noreturn.  Also update the code comment to reflect the
mentioned developments since its introduction.

Reviewed-by: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/flat/pxr5b3z7jmkpenssra5zroxi7qzzp6eswuggokw64axmdixpnk@zbwxuq7gbbcw

src/include/c.h

@@ -135,9 +135,9 @@
 
 /*
  * pg_nodiscard means the compiler should warn if the result of a function
- * call is ignored.  The name "nodiscard" is chosen in alignment with
- * (possibly future) C and C++ standards.  For maximum compatibility, use it
- * as a function declaration specifier, so it goes before the return type.
+ * call is ignored.  The name "nodiscard" is chosen in alignment with the C23
+ * standard attribute with the same name.  For maximum forward compatibility,
+ * place it before the declaration.
  */
 #ifdef __GNUC__
 #define pg_nodiscard __attribute__((warn_unused_result))

src/include/common/base64.h

@@ -11,8 +11,8 @@
 #define BASE64_H
 
 /* base 64 */
-extern pg_nodiscard int pg_b64_encode(const char *src, int len, char *dst, int dstlen);
-extern pg_nodiscard int pg_b64_decode(const char *src, int len, char *dst, int dstlen);
+pg_nodiscard extern int pg_b64_encode(const char *src, int len, char *dst, int dstlen);
+pg_nodiscard extern int pg_b64_decode(const char *src, int len, char *dst, int dstlen);
 extern int	pg_b64_enc_len(int srclen);
 extern int	pg_b64_dec_len(int srclen);
 

src/include/nodes/pg_list.h

@@ -608,23 +608,23 @@ extern List *list_make5_impl(NodeTag t, ListCell datum1, ListCell datum2,
 							 ListCell datum3, ListCell datum4,
 							 ListCell datum5);
 
-extern pg_nodiscard List *lappend(List *list, void *datum);
-extern pg_nodiscard List *lappend_int(List *list, int datum);
-extern pg_nodiscard List *lappend_oid(List *list, Oid datum);
-extern pg_nodiscard List *lappend_xid(List *list, TransactionId datum);
+pg_nodiscard extern List *lappend(List *list, void *datum);
+pg_nodiscard extern List *lappend_int(List *list, int datum);
+pg_nodiscard extern List *lappend_oid(List *list, Oid datum);
+pg_nodiscard extern List *lappend_xid(List *list, TransactionId datum);
 
-extern pg_nodiscard List *list_insert_nth(List *list, int pos, void *datum);
-extern pg_nodiscard List *list_insert_nth_int(List *list, int pos, int datum);
-extern pg_nodiscard List *list_insert_nth_oid(List *list, int pos, Oid datum);
+pg_nodiscard extern List *list_insert_nth(List *list, int pos, void *datum);
+pg_nodiscard extern List *list_insert_nth_int(List *list, int pos, int datum);
+pg_nodiscard extern List *list_insert_nth_oid(List *list, int pos, Oid datum);
 
-extern pg_nodiscard List *lcons(void *datum, List *list);
-extern pg_nodiscard List *lcons_int(int datum, List *list);
-extern pg_nodiscard List *lcons_oid(Oid datum, List *list);
+pg_nodiscard extern List *lcons(void *datum, List *list);
+pg_nodiscard extern List *lcons_int(int datum, List *list);
+pg_nodiscard extern List *lcons_oid(Oid datum, List *list);
 
-extern pg_nodiscard List *list_concat(List *list1, const List *list2);
-extern pg_nodiscard List *list_concat_copy(const List *list1, const List *list2);
+pg_nodiscard extern List *list_concat(List *list1, const List *list2);
+pg_nodiscard extern List *list_concat_copy(const List *list1, const List *list2);
 
-extern pg_nodiscard List *list_truncate(List *list, int new_size);
+pg_nodiscard extern List *list_truncate(List *list, int new_size);
 
 extern bool list_member(const List *list, const void *datum);
 extern bool list_member_ptr(const List *list, const void *datum);
@@ -632,15 +632,15 @@ extern bool list_member_int(const List *list, int datum);
 extern bool list_member_oid(const List *list, Oid datum);
 extern bool list_member_xid(const List *list, TransactionId datum);
 
-extern pg_nodiscard List *list_delete(List *list, void *datum);
-extern pg_nodiscard List *list_delete_ptr(List *list, void *datum);
-extern pg_nodiscard List *list_delete_int(List *list, int datum);
-extern pg_nodiscard List *list_delete_oid(List *list, Oid datum);
-extern pg_nodiscard List *list_delete_first(List *list);
-extern pg_nodiscard List *list_delete_last(List *list);
-extern pg_nodiscard List *list_delete_first_n(List *list, int n);
-extern pg_nodiscard List *list_delete_nth_cell(List *list, int n);
-extern pg_nodiscard List *list_delete_cell(List *list, ListCell *cell);
+pg_nodiscard extern List *list_delete(List *list, void *datum);
+pg_nodiscard extern List *list_delete_ptr(List *list, void *datum);
+pg_nodiscard extern List *list_delete_int(List *list, int datum);
+pg_nodiscard extern List *list_delete_oid(List *list, Oid datum);
+pg_nodiscard extern List *list_delete_first(List *list);
+pg_nodiscard extern List *list_delete_last(List *list);
+pg_nodiscard extern List *list_delete_first_n(List *list, int n);
+pg_nodiscard extern List *list_delete_nth_cell(List *list, int n);
+pg_nodiscard extern List *list_delete_cell(List *list, ListCell *cell);
 
 extern List *list_union(const List *list1, const List *list2);
 extern List *list_union_ptr(const List *list1, const List *list2);
@@ -657,25 +657,25 @@ extern List *list_difference_ptr(const List *list1, const List *list2);
 extern List *list_difference_int(const List *list1, const List *list2);
 extern List *list_difference_oid(const List *list1, const List *list2);
 
-extern pg_nodiscard List *list_append_unique(List *list, void *datum);
-extern pg_nodiscard List *list_append_unique_ptr(List *list, void *datum);
-extern pg_nodiscard List *list_append_unique_int(List *list, int datum);
-extern pg_nodiscard List *list_append_unique_oid(List *list, Oid datum);
+pg_nodiscard extern List *list_append_unique(List *list, void *datum);
+pg_nodiscard extern List *list_append_unique_ptr(List *list, void *datum);
+pg_nodiscard extern List *list_append_unique_int(List *list, int datum);
+pg_nodiscard extern List *list_append_unique_oid(List *list, Oid datum);
 
-extern pg_nodiscard List *list_concat_unique(List *list1, const List *list2);
-extern pg_nodiscard List *list_concat_unique_ptr(List *list1, const List *list2);
-extern pg_nodiscard List *list_concat_unique_int(List *list1, const List *list2);
-extern pg_nodiscard List *list_concat_unique_oid(List *list1, const List *list2);
+pg_nodiscard extern List *list_concat_unique(List *list1, const List *list2);
+pg_nodiscard extern List *list_concat_unique_ptr(List *list1, const List *list2);
+pg_nodiscard extern List *list_concat_unique_int(List *list1, const List *list2);
+pg_nodiscard extern List *list_concat_unique_oid(List *list1, const List *list2);
 
 extern void list_deduplicate_oid(List *list);
 
 extern void list_free(List *list);
 extern void list_free_deep(List *list);
 
-extern pg_nodiscard List *list_copy(const List *oldlist);
-extern pg_nodiscard List *list_copy_head(const List *oldlist, int len);
-extern pg_nodiscard List *list_copy_tail(const List *oldlist, int nskip);
-extern pg_nodiscard List *list_copy_deep(const List *oldlist);
+pg_nodiscard extern List *list_copy(const List *oldlist);
+pg_nodiscard extern List *list_copy_head(const List *oldlist, int len);
+pg_nodiscard extern List *list_copy_tail(const List *oldlist, int nskip);
+pg_nodiscard extern List *list_copy_deep(const List *oldlist);
 
 typedef int (*list_sort_comparator) (const ListCell *a, const ListCell *b);
 extern void list_sort(List *list, list_sort_comparator cmp);

src/include/storage/buffile.h

@@ -38,7 +38,7 @@ typedef struct BufFile BufFile;
 
 extern BufFile *BufFileCreateTemp(bool interXact);
 extern void BufFileClose(BufFile *file);
-extern pg_nodiscard size_t BufFileRead(BufFile *file, void *ptr, size_t size);
+pg_nodiscard extern size_t BufFileRead(BufFile *file, void *ptr, size_t size);
 extern void BufFileReadExact(BufFile *file, void *ptr, size_t size);
 extern size_t BufFileReadMaybeEOF(BufFile *file, void *ptr, size_t size, bool eofOK);
 extern void BufFileWrite(BufFile *file, const void *ptr, size_t size);

src/include/utils/guc.h

@@ -444,7 +444,7 @@ extern ArrayType *GUCArrayDelete(ArrayType *array, const char *name);
 extern ArrayType *GUCArrayReset(ArrayType *array);
 
 extern void *guc_malloc(int elevel, size_t size);
-extern pg_nodiscard void *guc_realloc(int elevel, void *old, size_t size);
+pg_nodiscard extern void *guc_realloc(int elevel, void *old, size_t size);
 extern char *guc_strdup(int elevel, const char *src);
 extern void guc_free(void *ptr);
 

src/include/utils/palloc.h

@@ -79,10 +79,10 @@ extern void *palloc(Size size);
 extern void *palloc0(Size size);
 extern void *palloc_extended(Size size, int flags);
 extern void *palloc_aligned(Size size, Size alignto, int flags);
-extern pg_nodiscard void *repalloc(void *pointer, Size size);
-extern pg_nodiscard void *repalloc_extended(void *pointer,
+pg_nodiscard extern void *repalloc(void *pointer, Size size);
+pg_nodiscard extern void *repalloc_extended(void *pointer,
 											Size size, int flags);
-extern pg_nodiscard void *repalloc0(void *pointer, Size oldsize, Size size);
+pg_nodiscard extern void *repalloc0(void *pointer, Size oldsize, Size size);
 extern void pfree(void *pointer);
 
 /*
@@ -110,7 +110,7 @@ extern void pfree(void *pointer);
 
 /* Higher-limit allocators. */
 extern void *MemoryContextAllocHuge(MemoryContext context, Size size);
-extern pg_nodiscard void *repalloc_huge(void *pointer, Size size);
+pg_nodiscard extern void *repalloc_huge(void *pointer, Size size);
 
 /*
  * Although this header file is nominally backend-only, certain frontend