send error #1

[LucioFranco]

No description provided.

[LucioFranco]

This change causes line 53 to error with:

tower-playground on  lucio/send-error-1 [$?] is 📦 v0.1.0 via 🦀 v1.70.0-nightly
❯ cargo test
   Compiling tower v0.1.0 (/Users/luciofra/code/tower-playground)
error[E0277]: `impl Future<Output = Result<<T as Service<Req>>::Res, <T as Service<Req>>::Error>>` cannot be sent between threads safely
  --> src/box_service.rs:53:9
   |
53 |         Box::pin(self.call(req))
   |         ^^^^^^^^^^^^^^^^^^^^^^^^ `impl Future<Output = Result<<T as Service<Req>>::Res, <T as Service<Req>>::Error>>` cannot be sent between threads safely
   |
   = help: the trait `Send` is not implemented for `impl Future<Output = Result<<T as Service<Req>>::Res, <T as Service<Req>>::Error>>`
   = note: required for the cast from `impl Future<Output = Result<<T as Service<Req>>::Res, <T as Service<Req>>::Error>>` to the object type `dyn Future<Output = Result<<T as Service<Req>>::Res, <T as Service<Req>>::Error>> + Send`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `tower` (lib) due to previous error
warning: build failed, waiting for other jobs to finish...
error: could not compile `tower` (lib test) due to previous error

Related rust-lang/rust#103854