Auto merge of #8041 - nox:castable, r=jdm

Introduce trait Castable

Removes all those messy FooCast structures in InheritTypes.rs.

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components/script/dom/bindings/js.rs

@@ -24,6 +24,7 @@
 //!
 
 use core::nonzero::NonZero;
+use dom::bindings::conversions::{Castable, DerivedFrom};
 use dom::bindings::trace::JSTraceable;
 use dom::bindings::trace::trace_reflector;
 use dom::bindings::utils::{Reflectable, Reflector};
@@ -34,6 +35,7 @@ use layout_interface::TrustedNodeAddress;
 use script_task::STACK_ROOTS;
 use std::cell::UnsafeCell;
 use std::default::Default;
+use std::mem;
 use std::ops::Deref;
 use std::ptr;
 use util::mem::HeapSizeOf;
@@ -112,6 +114,24 @@ pub struct LayoutJS<T> {
     ptr: NonZero<*const T>
 }
 
+impl<T: Castable> LayoutJS<T> {
+    /// Cast a DOM object root upwards to one of the interfaces it derives from.
+    pub fn upcast<U>(&self) -> LayoutJS<U> where U: Castable, T: DerivedFrom<U> {
+        unsafe { mem::transmute_copy(self) }
+    }
+
+    /// Cast a DOM object downwards to one of the interfaces it might implement.
+    pub fn downcast<U>(&self) -> Option<LayoutJS<U>> where U: DerivedFrom<T> {
+        unsafe {
+            if (*self.unsafe_get()).is::<U>() {
+                Some(mem::transmute_copy(self))
+            } else {
+                None
+            }
+        }
+    }
+}
+
 impl<T: Reflectable> LayoutJS<T> {
     /// Get the reflector.
     pub unsafe fn get_jsobject(&self) -> *mut JSObject {
@@ -440,6 +460,22 @@ pub struct Root<T: Reflectable> {
     root_list: *const RootCollection,
 }
 
+impl<T: Castable> Root<T> {
+    /// Cast a DOM object root upwards to one of the interfaces it derives from.
+    pub fn upcast<U>(root: Root<T>) -> Root<U> where U: Castable, T: DerivedFrom<U> {
+        unsafe { mem::transmute(root) }
+    }
+
+    /// Cast a DOM object root downwards to one of the interfaces it might implement.
+    pub fn downcast<U>(root: Root<T>) -> Option<Root<U>> where U: DerivedFrom<T> {
+        if root.is::<U>() {
+            Some(unsafe { mem::transmute(root) })
+        } else {
+            None
+        }
+    }
+}
+
 impl<T: Reflectable> Root<T> {
     /// Create a new stack-bounded root for the provided JS-owned value.
     /// It cannot not outlive its associated `RootCollection`, and it gives