Add test for suggestion of array_into_iter in for loop.

src/test/ui/iterators/into-iter-on-arrays-2018.rs

@@ -25,6 +25,9 @@ fn main() {
 
     // But you can always use the trait method explicitly as an array.
     let _: IntoIter<i32, 10> = IntoIterator::into_iter(array);
+
+    for _ in [1, 2, 3].into_iter() {}
+    //~^ WARNING this method call resolves to `<&[T; N] as IntoIterator>::into_iter`
 }
 
 /// User type that dereferences to an array.

src/test/ui/iterators/into-iter-on-arrays-2018.stderr

@@ -33,5 +33,20 @@ help: or use `IntoIterator::into_iter(..)` instead of `.into_iter()` to explicit
 LL |     let _: Iter<'_, i32> = IntoIterator::into_iter(Box::new(array));
    |                            ^^^^^^^^^^^^^^^^^^^^^^^^               ^
 
-warning: 2 warnings emitted
+warning: this method call resolves to `<&[T; N] as IntoIterator>::into_iter` (due to backwards compatibility), but will resolve to <[T; N] as IntoIterator>::into_iter in Rust 2021.
+  --> $DIR/into-iter-on-arrays-2018.rs:27:24
+   |
+LL |     for _ in [1, 2, 3].into_iter() {}
+   |                        ^^^^^^^^^
+   |
+help: use `.iter()` instead of `.into_iter()` to avoid ambiguity
+   |
+LL |     for _ in [1, 2, 3].iter() {}
+   |                        ^^^^
+help: or remove `.into_iter()` to iterate by value
+   |
+LL |     for _ in [1, 2, 3] {}
+   |                      --
+
+warning: 3 warnings emitted