#![allow(unused_variables)]
#![feature(vec_push_within_capacity)]
fn main() {
    use std::collections::TryReserveError;
    fn from_iter_fallible<T>(
        iter: impl Iterator<Item = T>,
    ) -> Result<Vec<T>, TryReserveError> {
        let mut vec = Vec::new();
        for value in iter {
            if let Err(value) = vec.push_within_capacity(value) {
                vec.try_reserve(1)?;
                let _ = vec.push_within_capacity(value);
            }
        }
        Ok(vec)
    }
    assert_eq!(from_iter_fallible(0..100), Ok(Vec::from_iter(0..100)));
}