Feature request: insertion that bails if reallocation would occur

[cole-miller]

For a custom data structure I'm implementing that uses hashbrown::HashMap internally, I'd like to access something like the following interface:

// returns the value of `map.insert(key, val)` (wrapped in `Ok`) if this call would not cause `map` to reallocate
// if reallocation would occur, returns `Err((key, val))` without performing the insertion
fn try_insert_no_grow<K: Eq + Hash, V, S: BuildHasher>(
    map: &mut HashMap<K, V, S>,
    key: K,
    val: V,
) -> Result<Option<V>, (K, V)> {
    unimplemented!()
}

Preemptively comparing map.len() to map.capacity() will not achieve this precisely due to the "lower bound" nature of HashMap::capacity mentioned in the docs. I want this power so that I can determine precisely when reallocation occurs and "rebalance" the containing data structure at the same time.

I asked about how to achieve this on URLO here and didn't get any leads on how to do it using the current interface (including RawTable), but if there is a way of course I'd love to hear about it.

[cole-miller]

closed by 4c3ba70