`Itertools::dedup_by[_key]` vs `Vec`

[Philippe-Cholet]

First, Itertools::dedup_by[_with_count] uses FnMut(&Self::Item, &Self::Item) -> bool while Vec::dedup_by uses FnMut(&mut T, &mut T) -> bool. It differs in mutability: & vs &mut. Not sure why yet.

Second, Vec::dedup_by_key exists but Itertools::dedup_by_key[_with_count] do not:

trait Itertools {
    fn dedup_by_key<F, K>(self, key: F) -> DedupByKey<Self, F>
    where
        Self: Sized,
        F: FnMut(&/*mut ??*/ Self::Item) -> K,
        K: PartialEq;

    fn dedup_by_key_with_count<F, K>(self, key: F) -> DedupByKeyWithCount<Self, F>
    where
        Self: Sized,
        F: FnMut(&/*mut ??*/ Self::Item) -> K,
        K: PartialEq;
}