Function apply 

pub fn apply<C, E>(target: &mut C, delta: EntryDelta<E::Key, E::Value>)
where
    C: IntoIterator<Item = E> + Extend<E> + TryIndex<E::Key, Output = E::Value>,
    E: MapEntry,

Applies a keyed membership diff to target in place.

Each removal is a single lookup rather than a scan, so this costs the same as diff does. Membership is preserved but position is not — additions land wherever the collection decides to put them. Use ordered where that matters.

A removal naming a key that target does not have is ignored, and an addition overwrites whatever the key held, which together make applying the same delta twice harmless.

use delta_struct::entry::{apply, diff};
use std::collections::BTreeMap;

let labels = |entries: Vec<(&'static str, &'static str)>| {
    entries.into_iter().collect::<BTreeMap<&str, &str>>()
};

let delta = diff(
    labels(vec![("tier", "web"), ("zone", "a")]),
    labels(vec![("tier", "edge")]),
);
let mut target = labels(vec![("tier", "web"), ("zone", "a")]);
apply(&mut target, delta);
assert_eq!(target, labels(vec![("tier", "edge")]));