fix(transport): remove needless BoxFuture (#644)

Should save some allocation. Didn't actually benchmark it.

tonic/src/transport/server/mod.rs

@@ -23,10 +23,7 @@ pub(crate) use tokio_rustls::server::TlsStream;
 use crate::transport::Error;
 
 use self::recover_error::RecoverError;
-use super::{
- service::{GrpcTimeout, Or, Routes, ServerIo},
- BoxFuture,
-};
+use super::service::{GrpcTimeout, Or, Routes, ServerIo};
 use crate::{body::BoxBody, request::ConnectionInfo};
 use futures_core::Stream;
 use futures_util::{
@@ -643,7 +640,7 @@ where
 {
  type Response = BoxService;
  type Error = crate::Error;
- type Future = BoxFuture<Self::Response, Self::Error>;
+ type Future = future::Ready<Result<Self::Response, Self::Error>>;
 
  fn poll_ready(&mut self, _cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
  Ok(()).into()
@@ -660,21 +657,19 @@ where
  let timeout = self.timeout;
  let trace_interceptor = self.trace_interceptor.clone();
 
- Box::pin(async move {
- let svc = ServiceBuilder::new()
- .layer_fn(RecoverError::new)
- .option_layer(concurrency_limit.map(ConcurrencyLimitLayer::new))
- .layer_fn(|s| GrpcTimeout::new(s, timeout))
- .service(svc);
+ let svc = ServiceBuilder::new()
+ .layer_fn(RecoverError::new)
+ .option_layer(concurrency_limit.map(ConcurrencyLimitLayer::new))
+ .layer_fn(|s| GrpcTimeout::new(s, timeout))
+ .service(svc);
 
-  let svc = BoxService::new(Svc {
-  inner: svc,
-  trace_interceptor,
-  conn_info,
-  });
+ let svc = BoxService::new(Svc {
+ inner: svc,
+ trace_interceptor,
+ conn_info,
+ });
 
- Ok(svc)
- })
+ future::ready(Ok(svc))
  }
 }