Rust decimal to roman numeral kata

Just wrote this decimal to roman numeral converter, inspired Kevlin Henney’s talk “Get Kata”.

I have been reading the rust doc’s little by little and this is one of my first attempts at some code.

I’d like to get feedback on style, general rust idioms and how I could have implemented this algorithm in more succinct manner.

code also available here

use std::env;
use std::io;
use std::io::BufWriter;
use std::io::Write;

static BASE10_NUMERALS: (&str; 7) = ("I", "X", "C", "M", "X̄", "C̄", "M̄");

static CENTRE_NUMERALS: (&str; 6) = ("V", "L", "D", "V̄", "L̄", "D̄");

struct Bases {
    base: usize
}


impl Iterator for Bases {
    type Item = usize;

    fn next(&mut self) -> Option<usize> {
        self.base = self.base - 1;
        Some(self.base)
    }
}

fn iter_bases(largest_base: usize) -> Bases {
    Bases { base: largest_base }
}


fn encode((decimal_number, base, ): (char, usize, )) -> String {
    let digit = decimal_number.to_digit(10).unwrap();
    let max_base = CENTRE_NUMERALS.len();

    if base >= max_base {
        BASE10_NUMERALS(BASE10_NUMERALS.len() - 1)
            //This pow function is the main limiter for decimal size
            .repeat((10_u32.pow((base - max_base) as u32) * digit) as usize)
    } else {
        if digit == 9 {
            format!("{}{}"
                    , if base == 3 { "Ī"} else { BASE10_NUMERALS(base) }
                    , BASE10_NUMERALS(base + 1)
            )
        } else if digit >= 5 {
            format!("{}{}", CENTRE_NUMERALS(
                base), BASE10_NUMERALS(base)
                        .repeat((digit - 5) as usize))
        } else if digit == 4 {
            format!("{}{}", BASE10_NUMERALS(base), CENTRE_NUMERALS(base))
        } else {
            // Less than 4
            BASE10_NUMERALS(base).repeat(digit as usize)
        }
    }
}


fn main() {
    let args: Vec<String> = env::args().collect();
    let mut writer = BufWriter::new(io::stdout());
    let input = &args(1);
    for roman_numeral in input
        .chars()
        .zip(iter_bases(input.len()))
        .map(encode) {
        writer.write(roman_numeral.as_bytes())
            .expect("Unable to write to stdout");
    }
}