feat: make pub Channel fns that allow creating it from a custom con…

…nector (#2015)

* feat: make pub `Channel` fns that allow creating it from a custom connector

The connector is required to be a `Service` that accepts a `http::Uri` and
returns a connection implementing hyper's IO traits, which is a very reasonable
requirement that can be fulfilled by advanced users that need to customize any
mechanics that don't fit into the paved path for HTTP and HTTPS with tonic.

One such requirement is customizing `rustls::ClientConfig`, which has come up
for a number of users. They have been pointed in the direction of a custom
connector, but the APIs on `Endpoint` are not always suitable:

`Enedpoint::connect_(lazy_)with_connector` always wraps the provided connector
with `transport::channel::service::Connector`. This wrapper validates if the
scheme is HTTPS, the TLS feature is enabled, and TLS was not configured on the
endpoint, and raises an error `HttpsUriWithoutTlsSupport` in this situation.
This is a good safety feature in general, but not if the wrapped connector is
taking care of TLS.

As a side bonus, `tonic::transport::channel::service::io::BoxedIo` can be
avoided.

* chore: remove redundant bound on connections

tonic/src/transport/channel/mod.rs

@@ -16,7 +16,6 @@ use http::{
     uri::{InvalidUri, Uri},
     Request, Response,
 };
-use hyper_util::client::legacy::connect::Connection as HyperConnection;
 use std::{
     fmt,
     future::Future,
@@ -144,12 +143,15 @@ impl Channel {
         (Self::balance(list, DEFAULT_BUFFER_SIZE, executor), tx)
     }
 
-    pub(crate) fn new<C>(connector: C, endpoint: Endpoint) -> Self
+    /// Create a new [`Channel`] using a custom connector to the provided [Endpoint].
+    ///
+    /// This is a lower level API, prefer to use [`Endpoint::connect_lazy`] if you are not using a custom connector.
+    pub fn new<C>(connector: C, endpoint: Endpoint) -> Self
     where
         C: Service<Uri> + Send + 'static,
         C::Error: Into<crate::BoxError> + Send,
         C::Future: Send,
-        C::Response: rt::Read + rt::Write + HyperConnection + Unpin + Send + 'static,
+        C::Response: rt::Read + rt::Write + Unpin + Send + 'static,
     {
         let buffer_size = endpoint.buffer_size.unwrap_or(DEFAULT_BUFFER_SIZE);
         let executor = endpoint.executor.clone();
@@ -162,12 +164,15 @@ impl Channel {
         Channel { svc }
     }
 
-    pub(crate) async fn connect<C>(connector: C, endpoint: Endpoint) -> Result<Self, super::Error>
+    /// Connect to the provided [`Endpoint`] using the provided connector, and return a new [`Channel`].
+    ///
+    /// This is a lower level API, prefer to use [`Endpoint::connect`] if you are not using a custom connector.
+    pub async fn connect<C>(connector: C, endpoint: Endpoint) -> Result<Self, super::Error>
     where
         C: Service<Uri> + Send + 'static,
         C::Error: Into<crate::BoxError> + Send,
         C::Future: Unpin + Send,
-        C::Response: rt::Read + rt::Write + HyperConnection + Unpin + Send + 'static,
+        C::Response: rt::Read + rt::Write + Unpin + Send + 'static,
     {
         let buffer_size = endpoint.buffer_size.unwrap_or(DEFAULT_BUFFER_SIZE);
         let executor = endpoint.executor.clone();