[Bug libstdc++/111052] New: std::format_to(std::back_inserter(str), "") should write directly to the string

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111052

            Bug ID: 111052
           Summary: std::format_to(std::back_inserter(str), "") should
                    write directly to the string
           Product: gcc
           Version: 13.1.1
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

fmt::format_to has a nice optimization where it recognizes a
back_insert_iterator<string> and extracts the contained string so that it can
format directly to the string, instead of writing one character at a time
through the output iterator.

We could get the string, move it into a _Seq_sink<string> to format into it,
then move assign (or swap) the populated string back to the original one that
the iterator referred to.