IWYU widely useful pragmas

Add various widely useful "IWYU pragma" annotations, such as

- Common header files such as c.h, postgres.h should be "always_keep".

- System headers included in c.h, postgres.h etc. should be considered
  "export".

- Some portability headers such as getopt_long.h should be
  "always_keep", so they are not considered superfluous on some
  platforms.

- Certain system headers included from portability headers should be
  considered "export" because the purpose of the portability header is
  to wrap them.

- Superfluous includes marked as "for backward compatibility" get a
  formal IWYU annotation.

- Generated header included in utils/syscache.h is marked exported.
  This is a very commonly used include and this avoids lots of
  complaints.

Discussion: https://www.postgresql.org/message-id/flat/9395d484-eff4-47c2-b276-8e228526c8ae@eisentraut.org

1 files changed, 5 insertions, 0 deletions

diff --git a/src/include/c.h b/src/include/c.h
index ab6a49aeca4..a14c6315162 100644
--- a/src/include/c.h
+++ b/src/include/c.h
@@ -16,6 +16,7 @@
  *
  *-------------------------------------------------------------------------
  */
+/* IWYU pragma: always_keep */
 /*
  *----------------------------------------------------------------
  *	 TABLE OF CONTENTS
@@ -46,6 +47,8 @@
 #ifndef C_H
 #define C_H
 
+/* IWYU pragma: begin_exports */
+
 /*
  * These headers must be included before any system headers, because on some
  * platforms they affect the behavior of the system headers (for example, by
@@ -1327,4 +1330,6 @@ typedef intptr_t sigjmp_buf[5];
 /* /port compatibility functions */
 #include "port.h"
 
+/* IWYU pragma: end_exports */
+
 #endif							/* C_H */