[libc++][vector] include formatter only in C++23

[atetubou]

This is to reduce included header file size in pre C++23 build.

e.g. 2% of total included file coming from formatter_bool.h in chrome build.
https://commondatastorage.googleapis.com/chromium-browser-clang/chrome_includes_2024-10-17_004801.html#view=edges&filter=&sort=asize&reverse=&includer=%5Ethird_party%2Flibc%5C%2B%5C%2B%2Fsrc%2Finclude%2Fvector%24&included=&limit=1000

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Takuto Ikuta (atetubou)

Changes

This is to reduce included header file size in pre C++23 build.

e.g. 2% of total included file coming from formatter_bool.h in chrome build.
https://commondatastorage.googleapis.com/chromium-browser-clang/chrome_includes_2024-10-17_004801.html#view=edges&filter=&sort=asize&reverse=&includer=%5Ethird_party%2Flibc%5C%2B%5C%2B%2Fsrc%2Finclude%2Fvector%24&included=&limit=1000

---

Full diff: https://github.com/llvm/llvm-project/pull/112675.diff

1 Files Affected:

• (modified) libcxx/include/vector (+5-2)

diff --git a/libcxx/include/vector b/libcxx/include/vector
index dc31f31838264c..93755afdde711c 100644
--- a/libcxx/include/vector
+++ b/libcxx/include/vector
@@ -324,8 +324,6 @@ template<class T, class charT> requires is-vector-bool-reference<T> // Since C++
 #include <__config>
 #include <__debug_utils/sanitizers.h>
 #include <__format/enable_insertable.h>
-#include <__format/formatter.h>
-#include <__format/formatter_bool.h>
 #include <__functional/hash.h>
 #include <__functional/unary_function.h>
 #include <__fwd/vector.h>
@@ -391,6 +389,11 @@ template<class T, class charT> requires is-vector-bool-reference<T> // Since C++
 #include <compare>
 #include <initializer_list>
 
+#if _LIBCPP_STD_VER >= 23
+#  include <__format/formatter.h>
+#  include <__format/formatter_bool.h>
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif

[frederick-vs-ja]

Perhaps we should correspondingly remove related entries in cxx11.csv ~ cxx20.csv in libcxx/test/libcxx/transitive_includes.

[zmodem]

There was some discussion of this in https://reviews.llvm.org/D149543 with @mordante

I'm not familiar with the standard's requirements, but if they're only needed only in C++23, making the includes conditional sounds like a good option.

[atetubou]

I updated libcxx/test/libcxx/transitive_includes for some C++ versions.

From my understanding, formatter headers are only used from C++23's code for vector<bool> and we don't need to include then in earlier version.