Collect result + extend()

[sorhawell]

I know of
.extend() to avoid reallocation during .collect()

let mut new_vec = Vec::with_capacity(size);
new_vec.extend(your_iterator);

Is there a function to set the size hint of an iterator?

There's no function to set the size hint of an iterator. The easiest way to ensure that it is preallocated is to do this instead of using collect: let mut new_vec = Vec::with_capacity(size); new_vec.extend(your_iterator);

I know of combining results of iterator items durring collect

let new: Result<Vec<T>, E> = v.into_iter().collect()

How do I convert a Vec<Result<T, E>> to Result<Vec<T>, E>?

Is there a way to use both concepts at once?

[alice]

fn collect_hint<T, E>(size: usize, iter: impl IntoIterator<Item = Result<T,E>>) -> Result<Vec<T>, E> {
    let mut new_vec = Vec::with_capacity(size);
    for item in your_iterator {
        new_vec.push(item?);
    }
    Ok(new_vec)
}

[sorhawell]

excellent so simple in hindsight

tiny edit: iter was one placed named your_iterator

fn collect_hint<T, E>(
    size: usize,
    iter: impl IntoIterator<Item = Result<T, E>>,
) -> Result<Vec<T>, E> {
    let mut new_vec = Vec::with_capacity(size);
    for item in iter {
        new_vec.push(item?);
    }
    Ok(new_vec)
}