返回一个大迭代器

3.rs

use bigdecimal::num_traits::ops::overflowing::OverflowingAdd;
use rand::{thread_rng, RngCore};
use std::iter::{Map, Sum};
use std::ops::Add;

pub struct OverflowingAddWrapper<T: OverflowingAdd>(pub T);

impl<T: OverflowingAdd> From<T> for OverflowingAddWrapper<T> {
    fn from(value: T) -> Self {
        Self(value)
    }
}

impl<T: OverflowingAdd> Add for OverflowingAddWrapper<T> {
    type Output = OverflowingAddWrapper<T>;

    fn add(self, rhs: Self) -> Self::Output {
        self.0.overflowing_add(&rhs.0).0.into()
    }
}

impl<T: OverflowingAdd + Default> Sum for OverflowingAddWrapper<T> {
    fn sum<I: Iterator<Item = Self>>(iter: I) -> Self {
        let mut sum = OverflowingAddWrapper(T::default());
        for x in iter {
            sum = sum.add(x)
        }
        sum
    }
}

fn baz(n: usize) -> Map<impl Iterator<Item = usize>, impl FnMut(usize) -> u32> {
    let mut rng = [(); 100000].map(|_| thread_rng());
    (0..n).map(move |_| {
        let rng = &mut rng;
        rng.iter_mut()
            .map(|x| OverflowingAddWrapper(x.next_u32()))
            .sum::<OverflowingAddWrapper<u32>>()
            .0
    })
}

fn main() {
    let iter = baz(10);
    println!("{}", size_of_val(&iter));
}