Add suggestion for missing .await keyword

src/test/ui/async-await/dont-suggest-missing-await.rs

@@ -0,0 +1,21 @@
+// edition:2018
+
+// This test ensures we don't make the suggestion in bodies that aren't `async`.
+
+#![feature(async_await)]
+
+fn take_u32(x: u32) {}
+
+async fn make_u32() -> u32 {
+    22
+}
+
+async fn dont_suggest_await_in_closure() {
+    || {
+        let x = make_u32();
+        take_u32(x)
+        //~^ ERROR mismatched types [E0308]
+    };
+}
+
+fn main() {}

src/test/ui/async-await/dont-suggest-missing-await.stderr

@@ -0,0 +1,12 @@
+error[E0308]: mismatched types
+  --> $DIR/dont-suggest-missing-await.rs:16:18
+   |
+LL |         take_u32(x)
+   |                  ^ expected u32, found opaque type
+   |
+   = note: expected type `u32`
+              found type `impl std::future::Future`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/async-await/suggest-missing-await.fixed

@@ -0,0 +1,32 @@
+// edition:2018
+// run-rustfix
+
+#![feature(async_await)]
+
+fn take_u32(_x: u32) {}
+
+async fn make_u32() -> u32 {
+    22
+}
+
+#[allow(unused)]
+async fn suggest_await_in_async_fn() {
+    let x = make_u32();
+    take_u32(x.await)
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider using `.await` here
+    //~| SUGGESTION x.await
+}
+
+#[allow(unused)]
+async fn suggest_await_in_async_closure() {
+    async || {
+        let x = make_u32();
+        take_u32(x.await)
+        //~^ ERROR mismatched types [E0308]
+        //~| HELP consider using `.await` here
+        //~| SUGGESTION x.await
+    };
+}
+
+fn main() {}

src/test/ui/async-await/suggest-missing-await.rs

@@ -0,0 +1,32 @@
+// edition:2018
+// run-rustfix
+
+#![feature(async_await)]
+
+fn take_u32(_x: u32) {}
+
+async fn make_u32() -> u32 {
+    22
+}
+
+#[allow(unused)]
+async fn suggest_await_in_async_fn() {
+    let x = make_u32();
+    take_u32(x)
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider using `.await` here
+    //~| SUGGESTION x.await
+}
+
+#[allow(unused)]
+async fn suggest_await_in_async_closure() {
+    async || {
+        let x = make_u32();
+        take_u32(x)
+        //~^ ERROR mismatched types [E0308]
+        //~| HELP consider using `.await` here
+        //~| SUGGESTION x.await
+    };
+}
+
+fn main() {}

src/test/ui/async-await/suggest-missing-await.stderr

@@ -0,0 +1,27 @@
+error[E0308]: mismatched types
+  --> $DIR/suggest-missing-await.rs:15:14
+   |
+LL |     take_u32(x)
+   |              ^
+   |              |
+   |              expected u32, found opaque type
+   |              help: consider using `.await` here: `x.await`
+   |
+   = note: expected type `u32`
+              found type `impl std::future::Future`
+
+error[E0308]: mismatched types
+  --> $DIR/suggest-missing-await.rs:25:18
+   |
+LL |         take_u32(x)
+   |                  ^
+   |                  |
+   |                  expected u32, found opaque type
+   |                  help: consider using `.await` here: `x.await`
+   |
+   = note: expected type `u32`
+              found type `impl std::future::Future`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.