Possibility of a VacantEntry to return an OccupiedEntry on insertion

I am working on some code that looks like this:

fn my_operation() -> Result<()> {
    let mut map = BTreeMap::new();
    let object = serde_json::from_str(/*some str*/)?;
    for (key, value) in object {
        let &mut entry = map.entry(key);
        process_value(value, &mut entry);

fn process_value(value: Value, entry: &mut Entry<String, Vec<usize>>) {
    match value {
        Value::String(s) => {
           // conditionally insert into the map, nothing may be added, so no entry is created in this case
           if s.len() > 10 {
       Value::Array(a) => a.into_iter().for_each(|item| process_value(item, entry),
       _ => todo!()

The issue is that insert on VacantEntry consumes self so I am not able to reuse the entry once I have inserted in it once.

I have considered passing the key and the map to the process_value function. This would means performing a lookup every time, and cloning the key every time I need to insert. I also tought about passing a Either<Entry<String, Vec<usize>>, &mut Vec<usize>> but that's rather ugly.

I am wondering if it would be possible for the Entry::insert_* method to mutate in place the Entry and turn a VavantEntry into a OccupiedEntry, and thus only mutably borrowing self, or for VacantEntry to expose a method that returns an OccupiedEntry upon insert?

Looks like there’s an unstable/nightly-only method Entry::insert that does this. Some more discussion in the corresponding issue #65225.

1 Like

Thanks @steffahn ! Indeed I didn't see that it was implemented on HashMap, thanks for the link to the tracking issue

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