Add test for 2021 ambiguous [T; N].into_iter().

src/test/ui/rust-2021/array-into-iter-ambiguous.fixed

@@ -0,0 +1,27 @@
+// See https://github.com/rust-lang/rust/issues/88475
+// run-rustfix
+// edition:2018
+// check-pass
+#![warn(array_into_iter)]
+#![allow(unused)]
+
+struct FooIter;
+
+trait MyIntoIter {
+    fn into_iter(self) -> FooIter;
+}
+
+impl<T, const N: usize> MyIntoIter for [T; N] {
+    fn into_iter(self) -> FooIter {
+        FooIter
+    }
+}
+
+struct Point;
+
+pub fn main() {
+    let points: [Point; 1] = [Point];
+    let y = MyIntoIter::into_iter(points);
+    //~^ WARNING trait method `into_iter` will become ambiguous in Rust 2021
+    //~| WARNING this changes meaning in Rust 2021
+}

src/test/ui/rust-2021/array-into-iter-ambiguous.rs

@@ -0,0 +1,27 @@
+// See https://github.com/rust-lang/rust/issues/88475
+// run-rustfix
+// edition:2018
+// check-pass
+#![warn(array_into_iter)]
+#![allow(unused)]
+
+struct FooIter;
+
+trait MyIntoIter {
+    fn into_iter(self) -> FooIter;
+}
+
+impl<T, const N: usize> MyIntoIter for [T; N] {
+    fn into_iter(self) -> FooIter {
+        FooIter
+    }
+}
+
+struct Point;
+
+pub fn main() {
+    let points: [Point; 1] = [Point];
+    let y = points.into_iter();
+    //~^ WARNING trait method `into_iter` will become ambiguous in Rust 2021
+    //~| WARNING this changes meaning in Rust 2021
+}

src/test/ui/rust-2021/array-into-iter-ambiguous.stderr

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+warning: trait method `into_iter` will become ambiguous in Rust 2021
+  --> $DIR/array-into-iter-ambiguous.rs:24:13
+   |
+LL |     let y = points.into_iter();
+   |             ^^^^^^^^^^^^^^^^^^ help: disambiguate the associated function: `MyIntoIter::into_iter(points)`
+   |
+note: the lint level is defined here
+  --> $DIR/array-into-iter-ambiguous.rs:5:9
+   |
+LL | #![warn(array_into_iter)]
+   |         ^^^^^^^^^^^^^^^
+   = warning: this changes meaning in Rust 2021
+   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/IntoIterator-for-arrays.html>
+
+warning: 1 warning emitted
+