Returning traits with dyn - rust

Question

I've been trying to get this code to compile
pub trait Packet: PacketSerializer + ProtocolToID + Default {
fn serialize_with_id(&self, ver: &ProtocolVersion) -> Box<ByteBuf>;
fn deserialize_gen(buf: &mut ByteBuf) -> Box<Self>;
}
impl<T: PacketSerializer + ProtocolToID + PacketHandler + Default> Packet for T
{
fn serialize_with_id(&self, ver: &ProtocolVersion) -> Box<ByteBuf> {
let mut buf = Box::new(ByteBuf::new());
buf.write_var_int(self.resolve_id(ver));
self.serialize(&mut buf, &ver);
buf
}
fn deserialize_gen(buf: &mut ByteBuf) -> Box<T> {
let mut p: T = Default::default();
p.deserialize(buf);
Box::new(p)
}
}
pub fn invoke_packet_handler(id: i32, buf: &mut ByteBuf) -> Box<dyn Packet> {
Box::new(clientbound::SetCompression { threshold: 256 })
}
specifically invoke_packet_handler is supposed to return a Box<dyn Packet>
According to the docs, this should work https://doc.rust-lang.org/rust-by-example/trait/dyn.html if you "statically" define the trait so the compiler can see it.
I get the following error at runtime
error[E0038]: the trait `Packet` cannot be made into an object
--> src/serialize/packet.rs:43:61
|
43 | pub fn invoke_packet_handler(id: i32, buf: &mut ByteBuf) -> Box<dyn Packet> {
| ^^^^^^^^^^^^^^^ `Packet` cannot be made into an object
I'm assuming this is because of the implementation for Packet? It's generic on every type that implements PacketSerializer, ProtocolToID, PacketHandler and Default
However,
pub fn invoke_packet_handler(id: i32, buf: &mut ByteBuf) -> Box<dyn PacketHandler> {
Box::new(clientbound::SetCompression { threshold: 256 })
}
does compile if I specify a single trait