Retain newlines when reordering imports

cargo-fmt did this:

diff --git a/async-await-echo/src/main.rs b/async-await-echo/src/main.rs
index 1531b22..a654e92 100644
--- a/async-await-echo/src/main.rs
+++ b/async-await-echo/src/main.rs
@@ -1,16 +1,4 @@
 use {
-    hyper::{
-        // Miscellaneous types from Hyper for working with HTTP.
-        Body, Client, Request, Response, Server, Uri,
-
-        // This function turns a closure which returns a future into an
-        // implementation of the the Hyper `Service` trait, which is an
-        // asynchronous function from a generic `Request` to a `Response`.
-        service::service_fn,
-
-        // A function which runs a future to completion using the Hyper runtime.
-        rt::run,
-    },
     futures::{
         // Extension trait for futures 0.1 futures, adding the `.compat()` method
         // which allows us to use `.await` on 0.1 futures.
@@ -20,6 +8,22 @@ use {
         // `TryFutureExt` adds methods to futures that return `Result` types.
         future::{FutureExt, TryFutureExt},
     },
+    hyper::{
+        // A function which runs a future to completion using the Hyper runtime.
+        rt::run,
+        // This function turns a closure which returns a future into an
+        // implementation of the the Hyper `Service` trait, which is an
+        // asynchronous function from a generic `Request` to a `Response`.
+        service::service_fn,
+
+        // Miscellaneous types from Hyper for working with HTTP.
+        Body,
+        Client,
+        Request,
+        Response,
+        Server,
+        Uri,
+    },
     std::net::SocketAddr,
 };
 

that rt::run, line could use an extra new line, me thinks.

rustfmt 1.4.8-nightly (6b0a4471 2019-09-20)

code is from: https://rust-lang.github.io/async-book/01_getting_started/05_http_server_example.html

Thanks.

PS: someone could probably title this better to describe this case?

Reproduce the formatting change using the async-book HTTP server example linked in the issue and the shown import block. Trace the import-reordering behavior that moves rt::run; done means the reorder preserves the intended blank lines and comments rather than collapsing their grouping.