Make Opaque a newtype

And add Sync to its bounds

Fixes: xiph#2641

src/api/test.rs

@@ -1176,7 +1176,7 @@ fn send_frame_kf<T: Pixel>(ctx: &mut Context<T>, keyframe: bool) {
   let frame_type_override =
     if keyframe { FrameTypeOverride::Key } else { FrameTypeOverride::No };
 
-  let opaque = Some(Box::new(keyframe) as Box<dyn std::any::Any + Send>);
+  let opaque = Some(Opaque::new(keyframe));
 
   let fp = FrameParameters { frame_type_override, opaque };
 

src/api/util.rs

@@ -20,7 +20,27 @@ use std::sync::Arc;
 use thiserror::*;
 
 /// Opaque type to be passed from Frame to Packet
-pub type Opaque = Box<dyn Any + Send>;
+#[derive(Debug)]
+pub struct Opaque(Box<dyn Any + Send + Sync>);
+
+impl Opaque {
+  /// Wrap a type in the opaque struct
+  pub fn new<T: Any + Send + Sync>(t: T) -> Self {
+    Opaque(Box::new(t) as Box<dyn Any + Send + Sync>)
+  }
+
+  /// Attempt to downcast the opaque to a concrete type.
+  pub fn downcast<T: Any + Send + Sync>(self) -> Result<Box<T>, Opaque> {
+    if self.0.is::<T>() {
+      unsafe {
+        let raw: *mut (dyn Any + Send + Sync) = Box::into_raw(self.0);
+        Ok(Box::from_raw(raw as *mut T))
+      }
+    } else {
+      Err(self)
+    }
+  }
+}
 
 // TODO: use the num crate?
 /// A rational number.

src/capi.rs

@@ -67,6 +67,7 @@ struct FrameOpaque {
 }
 
 unsafe impl Send for FrameOpaque {}
+unsafe impl Sync for FrameOpaque {}
 
 impl Default for FrameOpaque {
   fn default() -> Self {
@@ -1062,7 +1063,7 @@ pub unsafe extern fn rav1e_send_frame(
   let maybe_opaque = if frame.is_null() {
     None
   } else {
-    (*frame).opaque.take().map(|o| Box::new(o) as rav1e::Opaque)
+    (*frame).opaque.take().map(|o| rav1e::Opaque::new(o))
   };
 
   let ret = (*ctx)