libstdc++: Define std::string::resize_and_overwrite for C++11 and COW string
There are several places in the library where we can improve performance
using resize_and_overwrite so it's inconvenient only being able to use
it in C++23 mode, and only for cxx11 strings. This adds it for COW
strings, and also adds __resize_and_overwrite as an extension for C++11
mode.
The new __resize_and_overwrite is available for C++11 and later, so
within the library we can use that consistently even in C++23. In order
to avoid making a copy (which might not be possible for non-copyable,
non-movable types) the callable is passed to resize_and_overwrite as an
lvalue reference. Unlike wrapping it in std::ref(op) this ensures that
invoking it as std::move(op)(n, p) will use the correct value category.
It also avoids any overhead that would be added by wrapping it in a
lambda like [&op](auto p, auto n) { return std::move(op)(p, n); }.
Adjust std::format to use the new __resize_and_overwrite, which we can
assume exists because we only use std::basic_string<char> and
std::basic_string<wchar_t>, so no program-defined specializations.
The uses in <experimental/internet> cannot be replaced, because those
are type-dependent on an Allocator template parameter, which could mean
they use program-defined specializations of std::basic_string that don't
have the __resize_and_overwrite extension.
libstdc++-v3/ChangeLog:
* include/bits/basic_string.h (__resize_and_overwrite): New
function.
* include/bits/basic_string.tcc (__resize_and_overwrite): New
function.
(resize_and_overwrite): Simplify by using reserve instead of
growing the string manually. Adjust for C++11 compatibility.
* include/bits/cow_string.h (resize_and_overwrite): New
function.
(__resize_and_overwrite): New function.
* include/bits/version.def (__cpp_lib_string_resize_and_overwrite):
Do not depend on cxx11abi.
* include/bits/version.h: Regenerate.
* include/std/format (__formatter_fp::_S_resize_and_overwrite):
Remove.
(__formatter_fp::format, __formatter_fp::_M_localize): Use
__resize_and_overwrite instead of _S_resize_and_overwrite.
* testsuite/21_strings/basic_string/capacity/char/resize_and_overwrite.cc:
Adjust for C++11 compatibility when included by ...
* testsuite/21_strings/basic_string/capacity/char/resize_and_overwrite_ext.cc:
New test.