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Advantages of the patch.
* finer control on how the buffer is allocated: only overallocate if the replacement string (while handling an encoding error) is longer than 1 byte/character. The "replace" error handler should never use overallocation for example. Overallocation (when misused, when it was not needed) has a cost at the end of the encoder, because the buffer must be resized (shrink)
* use a buffer allocated on the stack for short strings. I'm not really convinced of this optimization. The data is still copied when the result is converted to a bytes objects (PyBytes_FromStringAndSize). It may be interesting if the encoder has to handle one or more errors: no need to resize the buffer until we reach the size of the small buffer (ex: 512 bytes).
* handle correctly integer overflow: most encoders do not catch integer overflow errors and may fail to handle (very) long strings (ex: encoded string longer than PY_SSIZE_T_MAX).
I'm not convinced that the patch would permit to design faster code. According to the assembler, it is the opposite (when "*writer.str++" is used in a loop). I don't know if it's possible to design a more efficient _PyBytesWriter API (to help GCC to generate more efficient machine code), nor if the overhead is important in a "normal case" (bench_encoders.py tests border cases, text with many many errors).