xilem_core: Add a WithoutElements ViewSequence

[Philipp-M]

This is similar as the fork view, but it allows using Views/ViewSequences without elements within a ViewSequence context that contains elements, see #461 (comment) for the original motivation.

Taking xilem_web as example, this should be possible (as well as something similar as in the linked comment):

fn run_once_fragment() -> impl DomFragment<()> {
    without_elements(run_once(|| {}))
}

The WidgetViewSequence is "fixed" as well, by using the "right" widget type (DynWidget is only used in AnyView, whereas I think Box<dyn Widget> is more general and correct).

[DJMcNab]

I haven't had chance to review this yet. One thing I'm wondering is whether we can make an alternative for ViewMarker for these kinds of sequences?

[Philipp-M]

Good point.

You mean to avoid the extra wrapper, and directly use the no element views in sequences?

At least something like this:

impl<V: View<State, Action, Context, Message, Element = NoElement> + NoElementViewMarker, ...>
    ViewSequence<State, Action, Context, Element, Message> for V {}

is not possible, because of ambiguity again... (We would need something like negative trait bounds?)

The other possible idea I tried, is directly implementing the ViewSequence for the concrete NoElement views, but this has another issue because of the super-trait bound View: ViewMarker which leads to ambiguity again.

I'm not sure if it's worth it, but we could remove the super trait requirement View: ViewMarker and put it to the concrete Views (like WidgetView), not sure about the exact implications, but it's probably not optimal, that the compiler doesn't scream when implementing View without also implementing ViewMarker...

Or do you mean something else?

[DJMcNab]

I mean, I was especially saying that we should do that investigation.