Use Hashbrown's raw entry API to reduce hashes and clone in priority

[charliermarsh]

Summary

I'm open to not merging this -- I was kind of just interested in what the API looked like. But the idea is: we can avoid hashing values twice and unnecessarily cloning within the priority map by using the raw entry API.

[zanieb]

I don't find this hard to read

[konstin]

What about using the EntryRef API?

        let len = self.virtual_package_tiebreaker.len();
        self.virtual_package_tiebreaker
            .entry_ref(package)
            .or_insert_with(|| {
                PubGrubTiebreaker::from(u32::try_from(len).expect("Less than 2**32 packages"))
            });

[charliermarsh]

That doesn’t work unless PubGrubPackage implements From<&PubGrubPackage> for PubGrubPackage.

[konstin]

A bit annoying that they use From instead of ToOwned, but treating it as clone works

The relationships I use between the key K and borrowed key Q is K: Borrow<Q> + From<&Q>. The reason for not using Q: ToOwned<K> is to support key types like Rc<str> (which is used in a couple of the doctests), which would not be possible with to_owned().

(from rust-lang/hashbrown#301)

impl From<&PubGrubPackage> for PubGrubPackage {
    fn from(package: &PubGrubPackage) -> Self {
        package.clone()
    }
}

[charliermarsh]

Ok, we can do that instead.