add #include guard hack

According to MSVC manual (*1), cl.exe can skip including a header file
when that:

- contains #pragma once, or
- starts with #ifndef, or
- starts with #if ! defined.

GCC has a similar trick (*2), but it acts more stricter (e. g. there
must be _no tokens_ outside of #ifndef...#endif).

Sun C lacked #pragma once for a looong time.  Oracle Developer Studio
12.5 finally implemented it, but we cannot assume such recent version.

This changeset modifies header files so that each of them include
strictly one #ifndef...#endif.  I believe this is the most portable way
to trigger compiler optimizations. [Bug #16770]

*1: https://docs.microsoft.com/en-us/cpp/preprocessor/once
*2: https://gcc.gnu.org/onlinedocs/cppinternals/Guard-Macros.html

1 files changed, 7 insertions, 9 deletions

diff --git a/include/ruby/3/static_assert.h b/include/ruby/3/static_assert.h
index 78ce4def4b..b9c842f3b6 100644
--- a/include/ruby/3/static_assert.h
+++ b/include/ruby/3/static_assert.h
@@ -1,4 +1,6 @@
-/**                                                     \noop-*-C++-*-vi:ft=cpp
+#ifndef RUBY3_STATIC_ASSERT_H                        /*-*-C++-*-vi:se ft=cpp:*/
+#define RUBY3_STATIC_ASSERT_H
+/**
  * @file
  * @author     Ruby developers <ruby-core@ruby-lang.org>
  * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
@@ -22,10 +24,7 @@
 #include "ruby/3/has/extension.h"
 
 /** @cond INTERNAL_MACRO */
-#if defined(RUBY3_STATIC_ASSERT0)
-# /* Take that. */
-
-#elif defined(__cplusplus) && defined(__cpp_static_assert)
+#if defined(__cplusplus) && defined(__cpp_static_assert)
 # /* https://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations */
 # define RUBY3_STATIC_ASSERT0 static_assert
 
@@ -62,10 +61,7 @@
  * @param  expr  Expression to assert.
  * @note   `name` shall not be a string literal.
  */
-#if defined(RUBY3_STATIC_ASSERT)
-# /* Take that. */
-
-#elif defined(__DOXYGEN__)
+#if defined(__DOXYGEN__)
 # define RUBY3_STATIC_ASSERT static_assert
 
 #elif defined(RUBY3_STATIC_ASSERT0)
@@ -76,3 +72,5 @@
 # define RUBY3_STATIC_ASSERT(name, expr) \
     typedef int static_assert_ ## name ## _check[1 - 2 * !(expr)]
 #endif
+
+#endif /* RUBY3_STATIC_ASSERT_H */