[Bug libstdc++/112925] Optimize std::string construction from std::istreambuf_iterator

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112925

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #0)
> +                 auto __r = std::__copy_n_a(__beg, capacity() - size(),
> +                                            data() + size(), false);

This uses an optimized overload of __copy_n_a that takes istreambuf_iterator,
which just uses streambuf::sgetn to extract directly to the string's char
buffer.

For the basic_string(from_range_t, R&&) constructor we don't have an optimized
ranges::copy_n for streambuf iterators (see Bug 111053).  But the string
from_range ctor can write its own loop using streambuf::sgetn.

We can also consider using streambuf::showmanyc() to find how many characters
the streambuf has available. For filebuf that can tell us the total number of
bytes so we can size the string correctly in one allocation.