style(lib): use rust 2018 edition idioms (#1910)

src/common/exec.rs

@@ -57,13 +57,13 @@ impl Exec {
                     struct TokioSpawnError;
 
                     impl fmt::Debug for TokioSpawnError {
-                        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+                        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                             fmt::Debug::fmt("tokio::spawn failed (is a tokio runtime running this future?)", f)
                         }
                     }
 
                     impl fmt::Display for TokioSpawnError {
-                        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+                        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                             fmt::Display::fmt("tokio::spawn failed (is a tokio runtime running this future?)", f)
                         }
                     }
@@ -99,7 +99,7 @@ impl Exec {
 }
 
 impl fmt::Debug for Exec {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("Exec")
             .finish()
     }

src/common/io/rewind.rs

@@ -129,8 +129,7 @@ mod tests {
     use super::Rewind;
     /*
     use super::*;
-    extern crate tokio_mockstream;
-    use self::tokio_mockstream::MockStream;
+    use tokio_mockstream::MockStream;
     use std::io::Cursor;
 
     // Test a partial rewind

src/common/never.rs

@@ -9,7 +9,7 @@ use std::fmt;
 pub enum Never {}
 
 impl fmt::Display for Never {
-    fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
         match *self {}
     }
 }

src/common/task.rs

@@ -4,7 +4,7 @@ use super::Never;
 /// A function to help "yield" a future, such that it is re-scheduled immediately.
 ///
 /// Useful for spin counts, so a future doesn't hog too much time.
-pub(crate) fn yield_now(cx: &mut Context) -> Poll<Never> {
+pub(crate) fn yield_now(cx: &mut Context<'_>) -> Poll<Never> {
     cx.waker().wake_by_ref();
     Poll::Pending
 }