Add a test case to make sure we don't reborrow twice

Author: eholk

Diff for: tests/ui/async-await/pin-reborrow-arg.rs

@@ -15,7 +15,7 @@ impl Foo {
 fn foo(_: Pin<&mut Foo>) {
 }
 
-fn bar(mut x: Pin<&mut Foo>) {
+fn bar(x: Pin<&mut Foo>) {
     foo(x);
     foo(x); // for this to work we need to automatically reborrow,
             // as if the user had written `foo(x.as_mut())`.

Diff for: tests/ui/async-await/pin-reborrow-once.rs

@@ -0,0 +1,13 @@
+#![feature(pin_ergonomics)]
+#![allow(dead_code, incomplete_features)]
+
+// Make sure with pin reborrowing that we can only get one mutable reborrow of a pinned reference.
+
+use std::pin::{pin, Pin};
+
+fn twice(_: Pin<&mut i32>, _: Pin<&mut i32>) {}
+
+fn main() {
+    let x = pin!(42);
+    twice(x, x); //~ ERROR cannot borrow
+}

Diff for: tests/ui/async-await/pin-reborrow-once.stderr

@@ -0,0 +1,12 @@
+error[E0499]: cannot borrow `*x.__pointer` as mutable more than once at a time
+  --> $DIR/pin-reborrow-once.rs:12:14
+   |
+LL |     twice(x, x);
+   |     ----- -  ^ second mutable borrow occurs here
+   |     |     |
+   |     |     first mutable borrow occurs here
+   |     first borrow later used by call
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0499`.