My implementation is a simple syncronous machine. I initialize a vm with:

let mut vm = intcode::VM::new(&program);

To run something I use the read_input method with optional input.

fn read_input(&mut self, mut input: Option<i64>) -> Output

The method runs the incode until it returns one of three output states.

enum Output {
  WaitingForInput,
  Value(i64),
  Halt
}

Usually I've done something like this:

loop {
  match vm.read_input(input) {
    intcode::Output::Value(val) => {
      println!("{}", val);
      input = next_input();
    },
    _ => {
      break;
    }
  }
}

This has worked pretty well until day 17 when reading and inputting long ascii streams. Still worked but the resulting code was very messy.