Equivalent trait to pass custom "borrowed" structs as key

[PoOnesNerfect]

Have you thought about introducing Equivalent trait, which you can implement for struct, so that you can create a custom borrowed struct for key?

For example, currently, if you want have a struct as a key:

pub struct Key {
    field1: String,
    field2: String,
}

To get a value from a cache, you have to own the key then reference it. So, if you don't own the key fields, you must clone to get the cache.

However, by using Equivalent trait, you can create a custom borrowed struct:

pub struct RefKey<'a> {
    field1: &'a str,
    field2: &'a str,
}

impl<'a> Equivalent<Key> for RefKey<'a> {
    fn equivalent(&self, key: &Key) -> bool {
        self.field1 == &key.field1 && self.field2 == &key.field2
    }
}

...
my_cache.get(&ref_key).await;

This way, you don't have to own all the values in the key in order to fetch the value.

I have a fork that I've been using for this exact purpose: thuo-io#1

If you think this could be a good addition to the library, I can open a PR; or it's totally fine to not want to have this change.

p.s. the trait itself is a copy from another popular hashmap library, I forget where since it's been so long that i've copied it.