Move more foundational types to script_bindings (#35280)

* script: Move DOMClass to script_bindings.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* script: Move DOMJSClass and get_dom_class to script_bindings.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* script: Move Castable/DerivedFrom/IDLInterface to script_bindings.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

---------

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

components/script_bindings/inheritance.rs

@@ -0,0 +1,53 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+//! The `Castable` trait.
+
+use std::mem;
+
+use crate::conversions::{get_dom_class, DerivedFrom, IDLInterface};
+use crate::reflector::DomObject;
+use crate::script_runtime::runtime_is_alive;
+
+/// A trait to hold the cast functions of IDL interfaces that either derive
+/// or are derived from other interfaces.
+pub trait Castable: IDLInterface + DomObject + Sized {
+    /// Check whether a DOM object implements one of its deriving interfaces.
+    fn is<T>(&self) -> bool
+    where
+        T: DerivedFrom<Self>,
+    {
+        // This is a weird place for this check to live, but it should catch any
+        // attempts to interact with DOM objects from Drop implementations that run
+        // as a result of the runtime shutting down and finalizing all remaining objects.
+        debug_assert!(
+            runtime_is_alive(),
+            "Attempting to interact with DOM objects after JS runtime has shut down."
+        );
+
+        let class = unsafe { get_dom_class(self.reflector().get_jsobject().get()).unwrap() };
+        T::derives(class)
+    }
+
+    /// Cast a DOM object upwards to one of the interfaces it derives from.
+    fn upcast<T>(&self) -> &T
+    where
+        T: Castable,
+        Self: DerivedFrom<T>,
+    {
+        unsafe { mem::transmute::<&Self, &T>(self) }
+    }
+
+    /// Cast a DOM object downwards to one of the interfaces it might implement.
+    fn downcast<T>(&self) -> Option<&T>
+    where
+        T: DerivedFrom<Self>,
+    {
+        if self.is::<T>() {
+            Some(unsafe { mem::transmute::<&Self, &T>(self) })
+        } else {
+            None
+        }
+    }
+}