I’m just starting in the Rust (come from python and C#) world and I know this question has been asked many times, but I’m struggling to understand the answers as they are very generic (Foo, Bar, Baz). I’m not even sure if what I need is achievable in Rust.

I need a class to inherit a method from its parent and grand-parent as well and force it to implement functions defined in its parent.

Case:

As you can see in the Code section bellow, Days30BondCounter and Days30ECounter structs return the day_count with the same expresion. All the difference is in how d1 and d2 is calculated.

My Idea is to create a Days30 abstract class that inherits from DayCounter but that requires implementing get_d1 and get_d2 functions. So that Days30 looks something like

trait Days30 : DayCounter {
    fn get_d1(&self, start_date: NaiveDate, end_date: NaiveDate) -> i32;
    fn get_d2(&self, start_date: NaiveDate, end_date: NaiveDate) -> i32;

    fn day_count(&self, start_date: NaiveDate, end_date: NaiveDate) -> i64 {
        let d1 = self.get_d1(start_date, end_date);
        let d2 = self.get_d2(start_date, end_date);
        (360 * (end_date.year() - start_date.year()) + 30 * ((end_date.month() - start_date.month()) as i32) + d2 - d1) as i64
    }
}

struct Days30BondCounter;
impl Days30 for Days30BondCounter { // this fails
    fn get_d1(&self, start_date: NaiveDate, end_date: NaiveDate) -> i32 {
        1 // d1 logic
    }
    fn get_d2(&self, start_date: NaiveDate, end_date: NaiveDate) -> i32 {
        2 // d2 logic
    }
}

Code:

use chrono::{Datelike, NaiveDate, TimeDelta};
use std::cmp::min;

trait DayCounter { // Base Class
    fn day_count(&self, start_date: NaiveDate, end_date: NaiveDate) -> i64;

    fn day_count_vector(&self, start_date: NaiveDate, end_dates: &Vec<NaiveDate>) -> Vec<i64> {
        let mut result: Vec<i64> = Vec::with_capacity(end_dates.len()); // Replace with VLA
        for end_date in end_dates {
            result.push(self.day_count(start_date, *end_date));
        }
        result
    }

}

struct ActualCounter; // Simple child
impl DayCounter for ActualCounter {
    fn day_count(&self, start_date: NaiveDate, end_date: NaiveDate) -> i64 {
        let duration: TimeDelta = end_date.signed_duration_since(start_date);
        duration.num_days()    
    }
}

struct Days30BondCounter; // This is currently a Child. I'd like it to inherit from Days30 which inherits from DayCounter
impl DayCounter for Days30BondCounter {
    fn day_count(&self, start_date: NaiveDate, end_date: NaiveDate) -> i64 {
        let mut d1: i32 = start_date.day() as i32;
        d1 = min(d1, 30);
        let mut d2: i32 = end_date.day() as i32;
        if d1 > 29 {
            d2 = min(d2, 30);
        }

        (360 * (end_date.year() - start_date.year()) + 30 * ((end_date.month() - start_date.month()) as i32) + d2 - d1) as i64
    }
}

struct Days30ECounter; // This is currently a Child. I'd like it to inherit from Days30 which inherits from DayCounter
impl DayCounter for Days30ECounter {
    fn day_count(&self, start_date: NaiveDate, end_date: NaiveDate) -> i64 {
        let d1: i32 = min(start_date.day(), 30) as i32;
        let d2: i32 = min(end_date.day(), 30) as i32;
        
        (360 * (end_date.year() - start_date.year()) + 30 * ((end_date.month() - start_date.month()) as i32) + d2 - d1) as i64
    }
}