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sometimes we want to change external values during an upsert (pseudo-code):

let mut is_one = false;
map.upsert(key, || { is_one = true; 1 }, |mut n| { n += 1; if n == 1 { is_one = true; } });

this code doesn't compile because is_one is mutably borrowed more than once.

It's possible to workaround this awkwardly:

let mut is_one1 = false;
let mut is_one2 = false;
map.upsert(key, || { is_one1 = true; 1 }, |mut n| { n += 1; if n == 1 { is_one2 = true; }});
// look at both is_one1 and is_one2

if there's a way to return a WriteGuard<K, V> with upsert that this can be solved more elegantly:

let is_one = {
    let n = map.upsert(key, || 1, |mut n| n+= 1);
    n == 1;
}