Shared via Rust Playground

playground.rs

struct ParseState {
    stack: Vec<u64>,
    num: Option<u64>
}

fn flush_state(state: &mut ParseState) {
    match state.num {
        Some(x) => state.stack.push(x),
        None => {}
    }
    state.num = None;
}

fn process_char(state: &mut ParseState, c: char) {
    if c.is_digit(10) {
        match state.num {
            Some(x) => state.num = Some(x * 10 + c.to_digit(10).unwrap() as u64),
            None => state.num = Some(c.to_digit(10).unwrap() as u64)
        }
    } else {
        flush_state(state);
        match c {
            '+' => {
                let val = state.stack.pop().unwrap() + state.stack.pop().unwrap();
                state.stack.push(val);
            },
            '-' => {
                let val = state.stack.pop().unwrap() - state.stack.pop().unwrap();
                state.stack.push(val);
            },
            '.' => println!("{}", state.stack.pop().unwrap()),
            ' ' => {}
            _ => panic!("Invalid input")
        }
    }
}

fn main() {
    let mut state = ParseState {
        stack: vec![],
        num: None
    };
    let input = "1 2 3 + + .";
    for c in input.chars() {
        process_char(&mut state, c);
    }
}