Generalize Reply impl for Box<dyn Reply>

[mikeyhew]

We had a case where we were boxing up replies, and needed them to impl Send + Sync to use in other code. Box<dyn Reply + Send + Sync> unfortunately doesn't implement Reply, because the impl is too specific. This generalizes the impl of Reply to include any Box<T> where T: Reply + ?Sized.

[seanmonstar]

Thanks!

I suppose technically this could be breaking if someone has impl Reply for Box<MyThingy>, right? Or am I remembering wrong and that hits the orphan rule? Anywho, I can't imagine people actually doing so...

[mikeyhew]

Looks like it is, the example below compiles without the blanket impl. Probably because Box has the #[fundamental] attribute.

struct MyThingy;

impl Reply for Box<MyThingy> {
    fn into_response(self) -> warp::reply::Response {
        todo!()
    }
}

Arc, on the other hand, isn't #[fundamental], and this impl fails to compile:

impl Reply for std::sync::Arc<MyThingy> {
    fn into_response(self) -> warp::reply::Response {
        todo!()
    }
}

with the error message:

error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
 --> server/src/api/products.rs:6:1
  |
6 | impl Reply for std::sync::Arc<MyThingy> {
  | ^^^^^^^^^^^^^^^------------------------
  | |              |
  | |              `std::sync::Arc` is not defined in the current crate
  | impl doesn't use only types from inside the current crate
  |
  = note: define and implement a trait or new type instead