Require View: ViewMarker to avoid the leaky abstraction `fn view() …

…-> impl View + ViewMarker`

Motivation: to avoid requiring `fn view() -> impl View + ViewMarker` and instead just using `fn view() -> impl View`

I don't see a reason for types that implement `View`, but not `ViewMarker`. This requirement has the neat side-effect of avoiding weird error-messages otherwise, in case `impl ViewMarker for T` has been forgotten for `impl View for T`.

The whole purpose AFAIK of `ViewMarker` is a workaround of the orphan rule, that something like this:
```rust
impl<T, A, V: View<T, A> + ViewMarker> ViewSequence<T, A> for V
```
is possible.

So I tried adding this as super trait requirement and it seems to be possible.
I think this makes the leaky abstraction/workaround much more feasible as it's only required now for actual `View` implementations.

crates/xilem_core/src/view/mod.rs

@@ -9,16 +9,18 @@ mod memoize;
 /// Arguments are
 ///
 ///  - `$viewtrait` - The name of the view trait we want to generate.
+///  - `$viewmarker` - The name of the viewmarker trait as a workaround for Rust's orphan rules,
+///     to allow views to be also viewsequences.
 ///  - `$bound` - A bound on all element types that will be used.
 ///  - `$cx` - The name of text context type that will be passed to the `build`/`rebuild`
 ///    methods, and be responsible for managing element creation & deletion.
 ///  - `$changeflags` - The type that reports down/up the tree. Can be used to avoid
 ///    doing work when we can prove nothing needs doing.
-///  - `$ss` - (optional) parent traits to this trait (e.g. `:Send`). Also applied to
-///    the state type requirements
+///  - `$super_bounds` - (optional) parent traits to this trait (e.g. `+ Send`).
+///  - `$state_bounds` - (optional) trait bounds for the associated type `State` (e.g. `: Send`).
 #[macro_export]
 macro_rules! generate_view_trait {
-    ($viewtrait:ident, $bound:ident, $cx:ty, $changeflags:ty; $($ss:tt)*) => {
+    ($viewtrait:ident, $viewmarker:ident, $bound:ident, $cx:ty, $changeflags:ty; ($($super_bounds:tt)*), ($($state_bounds:tt)*)) => {
         /// A view object representing a node in the UI.
         ///
         /// This is a central trait for representing UI. An app will generate a tree of
@@ -36,9 +38,9 @@ macro_rules! generate_view_trait {
         /// and also a type for actions which are passed up the tree in message
         /// propagation. During message handling, mutable access to the app state is
         /// given to view nodes, which in turn can expose it to callbacks.
-        pub trait $viewtrait<T, A = ()> $( $ss )* {
+        pub trait $viewtrait<T, A = ()>: $viewmarker $( $super_bounds )* {
             /// Associated state for the view.
-            type State $( $ss )*;
+            type State $( $state_bounds )*;
 
             /// The associated element for the view.
             type Element: $bound;

crates/xilem_web/src/interfaces.rs

@@ -1,4 +1,4 @@
-use crate::{Pointer, PointerMsg, View, ViewMarker};
+use crate::{Pointer, PointerMsg, View};
 use std::borrow::Cow;
 
 use gloo::events::EventListenerOptions;
@@ -29,7 +29,7 @@ macro_rules! event_handler_mixin {
     };
 }
 
-pub trait Element<T, A = ()>: View<T, A> + ViewMarker + sealed::Sealed
+pub trait Element<T, A = ()>: View<T, A> + sealed::Sealed
 where
     Self: Sized,
 {

crates/xilem_web/src/view.rs

@@ -84,7 +84,7 @@ impl Pod {
     }
 }
 
-xilem_core::generate_view_trait! {View, DomNode, Cx, ChangeFlags;}
+xilem_core::generate_view_trait! {View, ViewMarker, DomNode, Cx, ChangeFlags;(),()}
 xilem_core::generate_viewsequence_trait! {ViewSequence, View, ViewMarker, DomNode, Cx, ChangeFlags, Pod;}
 xilem_core::generate_anyview_trait! {AnyView, View, ViewMarker, Cx, ChangeFlags, AnyNode, BoxedView;}
 xilem_core::generate_memoize_view! {Memoize, MemoizeState, View, ViewMarker, Cx, ChangeFlags, static_view, memoize;}

src/view/view.rs

@@ -23,7 +23,7 @@ use xilem_core::{Id, IdPath};
 
 use crate::widget::{AnyWidget, ChangeFlags, Pod, Widget};
 
-xilem_core::generate_view_trait! {View, Widget, Cx, ChangeFlags; : Send}
+xilem_core::generate_view_trait! {View, ViewMarker, Widget, Cx, ChangeFlags; (+ Send), (: Send)}
 xilem_core::generate_viewsequence_trait! {ViewSequence, View, ViewMarker, Widget, Cx, ChangeFlags, Pod; : Send}
 xilem_core::generate_anyview_trait! {AnyView, View, ViewMarker, Cx, ChangeFlags, AnyWidget, BoxedView; + Send}
 xilem_core::generate_memoize_view! {Memoize, MemoizeState, View, ViewMarker, Cx, ChangeFlags, s, memoize; + Send}