https://cmcenroe.me/2016/08/22/rust-extending-type.html

rust-extending-type.rs

use std::ops::{Range, RangeFrom, RangeTo};

pub trait RangeExt<T> {
    fn before(&self) -> RangeTo<T>;
    fn after(&self) -> RangeFrom<T>;
}

impl<T: Copy> RangeExt<T> for Range<T> {
    fn before(&self) -> RangeTo<T> {
        ..self.start
    }

    fn after(&self) -> RangeFrom<T> {
        self.end..
    }
}

#[cfg(test)]
mod tests {
    use super::RangeExt;

    #[test]
    fn range_ext() {
        assert_eq!(..3, (3..5).before());
        assert_eq!(5.., (3..5).after());
    }
}