HashMap (and therefore DOM nodes) grew by 8 bytes

[SimonSapin]

HashMap grew by 8 bytes in rust-lang/rust#38368. script::dom::event_target::EventTarget contains:

    handlers: DOMRefCell<HashMap<Atom, EventListeners, BuildHasherDefault<FnvHasher>>>,

and DOM nodes contain EventTarget, so they all grew by the same amount, and unit tests in tests/unit/script/size_of.rs fail when updating the compiler: #15565 (comment)

@jdm @Ms2ger @nox What do you think? Should we take the hit and increase the expect size in unit tests, or try to get this space back somehow?

[SimonSapin]

Is the number of handlers for a given event target expected to be small? Could we get away with a Vec and linear scan instead of a HashMap?

I’m also looking at BTreeMap which is smaller (24 bytes), but in the current implementation even an empty map allocates an internal LeafNode with space for 11 key/value pairs, which in this case is 352 bytes. (For comparison, HashMap grew from 40 to 48 bytes but an empty map does not allocate.)

[SimonSapin]

Correction: 40/48 is with the default RandomState which takes 16 bytes. BuildHasherDefault is zero bytes, so the HashMap in handlers grew from 24 to 32 bytes.

[nox]

I don't care either way. One day we will optimise such things extensively, but today is not the day.

[SimonSapin]

Ok. Let’s increase expected sizes in unit tests next time we rustup.