winit-core/as_any: fix Box<AsAny> casting

The casting was doing an incorrect check on the `ref` instead of
actually trying to downcast a ref as `cast_ref` does. So use `cast_ref`
to check whether we can safely `cast` to owned type.

winit-core/src/as_any.rs

@@ -55,8 +55,7 @@ macro_rules! impl_dyn_casting {
             ///
             /// Returns `Err` with `self` if the object was not from that backend.
             pub fn cast<T: $trait>(self: Box<Self>) -> Result<Box<T>, Box<Self>> {
-                let reference: &dyn std::any::Any = self.__as_any();
-                if reference.is::<T>() {
+                if self.cast_ref::<T>().is_some() {
                     let this: Box<dyn std::any::Any> = self.__into_any();
                     // Unwrap is okay, we just checked the type of `self` is `T`.
                     Ok(this.downcast::<T>().unwrap())
@@ -69,3 +68,25 @@ macro_rules! impl_dyn_casting {
 }
 
 pub use impl_dyn_casting;
+
+#[cfg(test)]
+mod tests {
+    use super::AsAny;
+
+    struct Foo;
+    trait FooTrait: AsAny {}
+    impl FooTrait for Foo {}
+    impl_dyn_casting!(FooTrait);
+
+    #[test]
+    fn dyn_casting() {
+        let foo_owned: Box<dyn FooTrait> = Box::new(Foo);
+        assert!(foo_owned.cast::<Foo>().is_ok());
+
+        let mut foo = Foo;
+        let foo_ref: &mut dyn FooTrait = &mut foo;
+        assert!((foo_ref).cast_ref::<Foo>().is_some());
+        assert!((&&&&foo_ref).cast_ref::<Foo>().is_some());
+        assert!(foo_ref.cast_mut::<Foo>().is_some());
+    }
+}