internal int DrainLeftoverDataForGetCharCount(ReadOnlySpan<byte> bytes, out int bytesConsumed)
{
// Quick check: we _should not_ have leftover fallback data from a previous invocation,
// as we'd end up consuming any such data and would corrupt whatever Convert call happens
// to be in progress. Unlike EncoderNLS, this is simply a Debug.Assert. No exception is thrown.
Debug.Assert(_fallbackBuffer is null || _fallbackBuffer.Remaining == 0, "Should have no data remaining in the fallback buffer.");
Debug.Assert(HasLeftoverData, "Caller shouldn't invoke this routine unless there's leftover data in the decoder.");
// Copy the existing leftover data plus as many bytes as possible of the new incoming data
// into a temporary concated buffer, then get its char count by decoding it.
Span<byte> combinedBuffer = stackalloc byte[4];
combinedBuffer = combinedBuffer.Slice(0, ConcatInto(GetLeftoverData(), bytes, combinedBuffer));
int charCount = 0;
Debug.Assert(_encoding != null);
switch (_encoding.DecodeFirstRune(combinedBuffer, out Rune value, out int combinedBufferBytesConsumed))
{
