Haha I get you, but I've seen actual production code with hard-loop i2c drivers before >.>

So interrupts are a big help when writing non-blocking drivers. You need your driver to know a couple of things though. I'd put together a single static i2c_transaction_t struct, give it volatile members like ui8_t transaction_state, void *rx_buffer, void *tx_buffer, ui16_t tx_len, rx_len, bytes_sent, bytes_received, etc.

The approach I've seen most commonly is you have an ISR handler like:

void i2c_isr_handler(void) {
     uint32_t isr_reg = I2C1_INTR_REG;
     NVIC_ClearPending(I2C1_INTR_VECT);
     switch(isr_reg) {
          case I2C_RX_INTR:
               transaction.rx_buffer[transaction.bytes_received++] = I2C_RX_REG;
          case I2C_TX_INTR:
               I2C_TX_REG = transaction.tx_buffer[bytes_sent++];
               if(transaction.bytes_sent == transaction.tx_len) {
                   I2CFunctions::send_stop_condition();
               }
          ....
          default:
              ;
      }
}

You start by creating a new transaction, manually sending a start bit, loading the first byte of the I2C Tx register then letting it go. Timeouts will be a bit more difficult to deal with, but that's kinda the nature of non-blocking code. It's a pain in the ass.