That's very useful feedback. I suspect if the solution is irreducible it's OK, which in this case is close to true (this is for bit-unpacking integers):

    // bytes are added to the buffer until a value of bitwidth is available
    for _ in 0..<bitWidth {
        let byte = try UInt8(parsing: &input)
        buffer |= UInt64(byte) << bitsInBuffer
        bitsInBuffer += 8
      // Values of bitwidth are right-shifted off the buffer.
      while bitsInBuffer >= bitWidth && outPos < numValues {
          let value = Int(buffer & mask)
          out[outPos + outOffset] = value
          outPos += 1
          buffer >>= bitWidth
          bitsInBuffer -= bitWidth
      }
    }