[gcc r14-7072] libstdc++: Simplify some chrono formatters

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:8ae50799cd25a63fa028eb6aad4f93be1d8eeb39

commit r14-7072-g8ae50799cd25a63fa028eb6aad4f93be1d8eeb39
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Dec 14 15:29:13 2023 +0000

    libstdc++: Simplify some chrono formatters
    
    I don't remember exactly why I made these bits of code reserve space in
    a COW string and append to it, rather than just use the string returned
    from std::format (which will undergo copy elision). The _Str_sink type
    used by std::format means the string only performs a single allocation
    for the formatted output, and the returned string's reference count will
    be one, so won't reallocate when indexing into it. We can remove these
    non-optimizations.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/chrono_io.h (__formatter_chrono::_M_F): Simplify
            handling of string returned from std::format.
            (__formatter_chrono::_M_R_T): Likewise.

Diff:
---
 libstdc++-v3/include/bits/chrono_io.h | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/libstdc++-v3/include/bits/chrono_io.h b/libstdc++-v3/include/bits/chrono_io.h
index ec2ae9d53cc..7b5876b24e6 100644
--- a/libstdc++-v3/include/bits/chrono_io.h
+++ b/libstdc++-v3/include/bits/chrono_io.h
@@ -898,11 +898,8 @@ namespace __format
 	     _FormatContext&) const
 	{
 	  auto __ymd = _S_date(__t);
-	  basic_string<_CharT> __s;
-#if ! _GLIBCXX_USE_CXX11_ABI
-	  __s.reserve(11);
-#endif
-	  __s += std::format(_GLIBCXX_WIDEN("{:04d}-  -  "), (int)__ymd.year());
+	  auto __s = std::format(_GLIBCXX_WIDEN("{:04d}-  -  "),
+				 (int)__ymd.year());
 	  auto __sv = _S_two_digits((unsigned)__ymd.month());
 	  __s[__s.size() - 5] = __sv[0];
 	  __s[__s.size() - 4] = __sv[1];
@@ -1093,11 +1090,8 @@ namespace __format
 	  // %T Equivalent to %H:%M:%S
 	  auto __hms = _S_hms(__t);
 
-	  basic_string<_CharT> __s;
-#if ! _GLIBCXX_USE_CXX11_ABI
-	  __s.reserve(11);
-#endif
-	  __s = std::format(_GLIBCXX_WIDEN("{:02d}:00"), __hms.hours().count());
+	  auto __s = std::format(_GLIBCXX_WIDEN("{:02d}:00"),
+				 __hms.hours().count());
 	  auto __sv = _S_two_digits(__hms.minutes().count());
 	  __s[__s.size() - 2] = __sv[0];
 	  __s[__s.size() - 1] = __sv[1];