For sorting sequential fields you can chain comparisons with .then():

    struct Date {
        year: u32,
        month: u32,
        day: u32,
    }
    
    let mut vec: Vec<Date> = Vec::new();
    
    vec.sort_by(|a,b| {
       a.year.cmp(&b.year)
        .then(a.month.cmp(&b.month))
        .then(a.day.cmp(&b.day))
    });

Alternatively you can derive PartialEq, PartialOrd, Eq and Ord for your struct, which will produce a lexicographic ordering based on the top-to-bottom declaration order of the struct's members:

    #[derive(PartialEq, PartialOrd, Eq, Ord)]
    struct Date {
        year: u32,
        month: u32,
        day: u32,
    }