Make OpaqueLayoutAndStyleData just a bit less opaque

It now stores a NonNull<dyn Any>.

components/script_layout_interface/lib.rs

@@ -7,6 +7,7 @@
 //! to depend on script.
 
 #![deny(unsafe_code)]
+#![feature(box_into_raw_non_null)]
 
 #[macro_use]
 extern crate html5ever;
@@ -24,6 +25,7 @@ use libc::c_void;
 use net_traits::image_cache::PendingImageId;
 use script_traits::UntrustedNodeAddress;
 use servo_url::{ImmutableOrigin, ServoUrl};
+use std::any::Any;
 use std::ptr::NonNull;
 use std::sync::atomic::AtomicIsize;
 use style::data::ElementData;
@@ -54,7 +56,34 @@ pub struct OpaqueStyleAndLayoutData {
     // NB: We really store a `StyleAndLayoutData` here, so be careful!
     #[ignore_malloc_size_of = "TODO(#6910) Box value that should be counted but \
                                the type lives in layout"]
-    pub ptr: NonNull<StyleData>,
+    ptr: NonNull<dyn Any + Send + Sync>,
+}
+
+impl OpaqueStyleAndLayoutData {
+    #[inline]
+    pub fn new<T>(value: T) -> Self
+    where
+        T: Any + Send + Sync,
+    {
+        Self {
+            ptr: Box::into_raw_non_null(Box::new(value) as Box<dyn Any + Send + Sync>),
+        }
+    }
+
+    #[inline]
+    pub fn as_ptr(&self) -> *mut (dyn Any + Send + Sync) {
+        self.ptr.as_ptr()
+    }
+
+    /// Extremely cursed.
+    #[allow(unsafe_code)]
+    #[inline]
+    pub unsafe fn downcast_ref<'extended, T>(&self) -> Option<&'extended T>
+    where
+        T: Any + Send + Sync,
+    {
+        (*self.ptr.as_ptr()).downcast_ref()
+    }
 }
 
 #[allow(unsafe_code)]