Support Rc in html! attributes

[tv42]

Yew 0.17.4 silently cloned Strings passed in e.g. html!{<p foo=&self.a_String}. String::clone copies the contents of the string, which is wasteful.

v0.18.0 removes the silent clone, and while the easy fix would be to just move the clone up the stack to my Component::view, copying string contents on every view seems wasteful. That was the motivation for the change in the first place!

At first, it seems using Cow would help, but after some discussion I believe the current use of Cow<'static, str> still causes String content copies when the string has a lifetime less than 'static, e.g. strings originating from messages to Component::update.

It seems using Rc<str> or such would lead to string content copying happening only in Component::update (and typically, only for one variant of the Component::Message enum), so that would be a nice improvement.

It would be great if yew's vdom understood Rc<str> (and perhaps Arc also) and allowed using reference counts to enable cheap clones, instead of expensive string content copies.

Concretely:

struct Model {
    foo: Rc<str>
}
...
enum Msg {
    UpdateFoo { s: String },
...
    fn update(&mut self, msg: Self::Message) -> ShouldRender {
        match msg {
            Msg::UpdateFoo { s } => {
                // make String into a str, but then pass a reference to Rc::from
                self.foo = Rc::from(&*s);
            }
...
    fn view(&self) -> Html {
        html! {
            <p foo=&self.foo>
            </p>

It should also be possible to push the Rc (or just str) up front all the way to the message source, to avoid even the copy in `Component::update``, but that shouldn't require anything from yew.

[ranile]

A good solution would be have a custom type for this:

enum AttrValue {
    Borrowed(&'static str),
    Owned(String),
    ReferenceCounted(Rc<str>),
}

The VDom can then use this value.