Implement std::borrow::Borrow for Atom

[frewsxcv]

I want to be able to use connect() on Atoms, which requires
Borrow be implemented

[SimonSapin]

I think this would potentially break hashmaps, but I don’t know how much of a problem that is in practice.

HashMap<K, V> has methods like:

fn get<Q: ?Sized>(&self, k: &Q) -> Option<&V> where K: Borrow<Q>, Q: Hash + Eq

which, as far as I understand, assume that the Hash impls of K and Q are compatible. But hashing of Atom is not based on the derefed str, it’s derived and thus based only on the internal u64. This makes hashing fast, in the same way that comparison is fast. I don’t know if Servo ever hashes atoms, though.

[SimonSapin]

Also, SliceConcatExt (which defines connect) should probably rely on AsRef rather than Borrow: rust-lang/rust#26780

[frewsxcv]

I think this would potentially break hashmaps

Very good point. I didn't even think about this as a side effect.

Also, considering the behavior might be changing soon (and the rename from connect to join), I'll close this and use this pattern for the time being.

[SimonSapin]

It’s not as pretty, but something like this would avoid allocating the intermediate vector of that pattern:

let mut iter =  atoms.iter();
if let Some(first) = iter.next() {
    iter.fold(String::new(first), |s, atom| { s.push_str(" "); s.push_str(atom) })
} else {
    String::new()
}