Comment by purplesyringa

Instead of

    let surrogate_mask = surrogate_bit << 2 | surrogate_bit << 1 | surrogate_bit;
    len &= !surrogate_mask;

consider

    len &= surrogate_bit.wrapping_sub(1);

This should still work better. Alternatively, invert the conditions and do

    len &= non_surrogate_bit.wrapping_neg();