Work around trait_upcasting being unstable

Credits for Masonry/Xilem for introducing me to this trick.

.envrc

@@ -1,2 +1 @@
 export RUST_BACKTRACE=1
-export RUSTC_BOOTSTRAP=1

.github/workflows/rust.yml

@@ -6,7 +6,6 @@ on:
 
 env:
   CARGO_TERM_COLOR: always
-  RUSTC_BOOTSTRAP: 1
 
 jobs:
   build:

ravel-web/src/any.rs

@@ -7,6 +7,19 @@ use crate::{
     BuildCx, Builder, RebuildCx, State, View, Web,
 };
 
+/// Trait for upcasting to [`Any`], implemented automatically.
+///
+/// This is a workaround until `trait_upcasting` is stabilized.
+pub trait AsAny: Any {
+    fn as_mut_dyn_any(&mut self) -> &mut dyn Any;
+}
+
+impl<T: Any> AsAny for T {
+    fn as_mut_dyn_any(&mut self) -> &mut dyn Any {
+        self
+    }
+}
+
 /// A wrapper around a [`View`], erasing its [`State`] type.
 pub struct AnyView<V: View, Output> {
     inner: V,
@@ -35,7 +48,7 @@ where
     }
 
     fn rebuild(self, cx: RebuildCx, state: &mut Self::State) {
-        match (state.state.deref_mut() as &mut dyn Any)
+        match (state.state.as_mut_dyn_any().deref_mut() as &mut dyn Any)
             .downcast_mut::<V::State>()
         {
             Some(state) => self.inner.rebuild(cx, state),

ravel-web/src/lib.rs

@@ -1,7 +1,6 @@
 //! A web/DOM/HTML backend for [`ravel`].
 
-#![feature(trait_upcasting)]
-use std::{any::Any, sync::Arc};
+use std::sync::Arc;
 
 use atomic_waker::AtomicWaker;
 use dom::Position;
@@ -43,7 +42,7 @@ pub struct RebuildCx<'cx> {
 }
 
 /// Trait for the state of a [`Web`] component.
-pub trait State<Output>: Any {
+pub trait State<Output>: AsAny {
     /// Processes a "frame".
     ///
     /// This method can respond to externally triggered events by changing the