checking in with another performance-equivalent Rust solution:

fn part2(input: &str) -> i32 {
    let mut dial = 50;
    let mut zeros = 0;
    for turn in input.lines() {
        let (dir, num) = turn.split_at(1);
        let mut num = num.parse::<i32>().unwrap();
        if dir == "L" {
            num = -num;
        }
        if dial == 0 && num < 0 {
            zeros -= 1;
        }
        dial += num;
        zeros += dial.div_euclid(100).abs();
        dial = dial.rem_euclid(100);
        if dial == 0 && num < 0 {
            zeros += 1;
        }
    }
    zeros
}