Currently add_route is defined as:

pub fn add_route(&self, receiver: OpaqueIpcReceiver, callback: RouterHandler);
pub type RouterHandler = Box<FnMut(OpaqueIpcMessage) + Send>;

It is up to the user to remember and properly turn the OpaqueIpcMessage into the correct type. This leads to wrong types being used as in servo/servo#23818

Instead I recommend adding a typed version as:

pub fn add_route<T>(&self, receiver: IpcReceiver<T>, mut callback: RouterHandler<T>)

The user no longer has to learn about OpaqueIpcReceiver and it is compile-time checked, so the type can never be used wrong! Internally, the implementation converts the OpaqueIpcMessage into T so it can never be misused:

|msg: OpaqueIpcMessage| callback(msg.to::<T>().unwrap());

Eventually the existing add_route could be deprecated in favor of the typed one.