Instead of using a temporary `string` in `__vformat_to_wrapped` use a new
generic iterator. This aids to reduce the number of template instantions
and avoids using a `string` to buffer the entire formatted output.
This changes the type of `format_context` and `wformat_context`, this can
still be done since the code isn't ABI stable yet.
Several approaches have been evaluated:
- Using a __output_buffer base class with:
- a put function to store the buffer in its internal buffer
- a virtual flush function to copy the internal buffer to the output
- Using a `function` to forward the output operation to the output buffer,
much like the next method.
- Using a type erased function point to store the data in the buffer.
The last version resulted in the best performance. For some cases there's
still a loss of speed over the original method. This loss many becomes
apparent when large strings are copied to a pointer like iterator, before
the compiler optimized this using `memcpy`.
Reviewed By: ldionne, vitaut, #libc
Differential Revision: https://reviews.llvm.org/D110495
555214cbcc