mv include/ruby/{impl,internal}

Devs do not love "impl".

include/ruby/internal/attr/noexcept.h

@@ -0,0 +1,91 @@
+#ifndef RBIMPL_ATTR_NOEXCEPT_H                       /*-*-C++-*-vi:se ft=cpp:*/
+#define RBIMPL_ATTR_NOEXCEPT_H
+/**
+ * @file
+ * @author     Ruby developers <ruby-core@ruby-lang.org>
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ * @warning    Symbols   prefixed  with   either  `RBIMPL`   or  `rbimpl`   are
+ *             implementation details.   Don't take  them as canon.  They could
+ *             rapidly appear then vanish.  The name (path) of this header file
+ *             is also an  implementation detail.  Do not expect  it to persist
+ *             at the place it is now.  Developers are free to move it anywhere
+ *             anytime at will.
+ * @note       To  ruby-core:  remember  that   this  header  can  be  possibly
+ *             recursively included  from extension  libraries written  in C++.
+ *             Do not  expect for  instance `__VA_ARGS__` is  always available.
+ *             We assume C99  for ruby itself but we don't  assume languages of
+ *             extension libraries. They could be written in C++98.
+ * @brief      Defines #RBIMPL_ATTR_NOEXCEPT.
+ *
+ * This isn't actually an attribute in C++ but who cares...
+ *
+ * Mainly due  to aesthetic reasons,  this one is  rarely used in  the project.
+ * But can  be handy on  occasions, especially when a  function's noexcept-ness
+ * depends on its calling functions.
+ *
+ * ### Q&A ###
+ *
+ * - Q: Can a function that raises Ruby exceptions be attributed `noexcept`?
+ *
+ * - A: Yes.   `noexcept` is  about  C++ exceptions,  not  Ruby's.  They  don't
+ *      interface each other.  You can  safely attribute a function that raises
+ *      Ruby exceptions as `noexcept`.
+ *
+ * - Q: How, then, can I assert that  a function I wrote doesn't raise any Ruby
+ *      exceptions?
+ *
+ * - A: `__attribute__((__leaf__))` is for that purpose.  A function attributed
+ *      as leaf can still throw C++  exceptions, but not Ruby's.  Note however,
+ *      that it's extremely difficult -- if  not impossible -- to assert that a
+ *      function  doesn't  raise any  Ruby  exceptions  at  all.  Use  of  that
+ *      attribute is not  recommended; mere mortals can't properly  use that by
+ *      hand.
+ *
+ * - Q: Does it make sense to attribute an inline function `noexcept`?
+ *
+ * - A: I thought so before.  But no, I don't think they are useful any longer.
+ *
+ *     - When an  inline function attributed `noexcept`  actually doesn't throw
+ *       any  exceptions at  all:  these days  I don't  see  any difference  in
+ *       generated assembly  by adding/removing this attribute.   C++ compilers
+ *       get smarter and  smarter.  Today they can infer if  it actually throws
+ *       or not without any annotations by humans (correct me if I'm wrong).
+ *
+ *     - When an inline function attributed `noexcepr` actually _does_ throw an
+ *       exception:  they  have to  call  `std::terminate`  then (C++  standard
+ *       mandates  so).  This  means exception  handling routines  are actually
+ *       enforced, not  omitted.  This doesn't impact  runtime performance (The
+ *       Itanium C++ ABI has zero-cost  exception handling), but does impact on
+ *       generated binary size.  This is bad.
+ */
+#include "ruby/impl/compiler_since.h"
+#include "ruby/impl/has/feature.h"
+
+/** Wraps (or simulates) C++11 `noexcept` */
+#if ! defined(__cplusplus)
+# /* Doesn't make sense. */
+# define RBIMPL_ATTR_NOEXCEPT(_) /* void */
+
+#elif RBIMPL_HAS_FEATURE(cxx_noexcept)
+# define RBIMPL_ATTR_NOEXCEPT(_) noexcept(noexcept(_))
+
+#elif defined(__GXX_EXPERIMENTAL_CXX0X__) && __GXX_EXPERIMENTAL_CXX0X__
+# define RBIMPL_ATTR_NOEXCEPT(_) noexcept(noexcept(_))
+
+#elif defined(__INTEL_CXX11_MODE__)
+# define RBIMPL_ATTR_NOEXCEPT(_) noexcept(noexcept(_))
+
+#elif RBIMPL_COMPILER_SINCE(MSVC, 19, 0, 0)
+# define RBIMPL_ATTR_NOEXCEPT(_) noexcept(noexcept(_))
+
+#elif __cplusplus >= 201103L
+# define RBIMPL_ATTR_NOEXCEPT(_) noexcept(noexcept(_))
+
+#else
+# define RBIMPL_ATTR_NOEXCEPT(_) /* void */
+#endif
+
+#endif /* RBIMPL_ATTR_NOEXCEPT_H */