When expecting BoxFuture and using async {}, suggest Box::pin

src/test/ui/suggestions/expected-boxed-future-isnt-pinned.fixed

@@ -0,0 +1,16 @@
+// edition:2018
+// run-rustfix
+#![allow(dead_code)]
+use std::future::Future;
+use std::pin::Pin;
+
+type BoxFuture<'a, T> = Pin<Box<dyn Future<Output = T> + Send + 'a>>;
+//   ^^^^^^^^^ This would come from the `futures` crate in real code.
+
+fn foo() -> BoxFuture<'static, i32> {
+    Box::pin(async { //~ ERROR mismatched types
+        42
+    })
+}
+
+fn main() {}

src/test/ui/suggestions/expected-boxed-future-isnt-pinned.rs

@@ -0,0 +1,16 @@
+// edition:2018
+// run-rustfix
+#![allow(dead_code)]
+use std::future::Future;
+use std::pin::Pin;
+
+type BoxFuture<'a, T> = Pin<Box<dyn Future<Output = T> + Send + 'a>>;
+//   ^^^^^^^^^ This would come from the `futures` crate in real code.
+
+fn foo() -> BoxFuture<'static, i32> {
+    async { //~ ERROR mismatched types
+        42
+    }
+}
+
+fn main() {}

src/test/ui/suggestions/expected-boxed-future-isnt-pinned.stderr

@@ -0,0 +1,27 @@
+error[E0308]: mismatched types
+  --> $DIR/expected-boxed-future-isnt-pinned.rs:11:5
+   |
+LL |   fn foo() -> BoxFuture<'static, i32> {
+   |               ----------------------- expected `std::pin::Pin<std::boxed::Box<(dyn std::future::Future<Output = i32> + std::marker::Send + 'static)>>` because of return type
+LL | /     async {
+LL | |         42
+LL | |     }
+   | |_____^ expected struct `std::pin::Pin`, found opaque type
+   | 
+  ::: $SRC_DIR/libstd/future.rs:LL:COL
+   |
+LL |   pub fn from_generator<T: Generator<Yield = ()>>(x: T) -> impl Future<Output = T::Return> {
+   |                                                            ------------------------------- the found opaque type
+   |
+   = note:   expected struct `std::pin::Pin<std::boxed::Box<(dyn std::future::Future<Output = i32> + std::marker::Send + 'static)>>`
+           found opaque type `impl std::future::Future`
+help: you need to pin and box this expression
+   |
+LL |     Box::pin(async {
+LL |         42
+LL |     })
+   |
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.