Add UI test for array.into_iter() lint

src/test/ui/iterators/into-iter-on-arrays-lint.fixed

@@ -0,0 +1,33 @@
+// run-pass
+// run-rustfix
+
+fn main() {
+    let small = [1, 2];
+    let big = [0u8; 33];
+
+    // Expressions that should trigger the lint
+    small.iter();
+    //~^ WARNING this method call currently resolves to `<&[T; N] as IntoIterator>::into_iter`
+    //~| WARNING this was previously accepted by the compiler but is being phased out
+    [1, 2].iter();
+    //~^ WARNING this method call currently resolves to `<&[T; N] as IntoIterator>::into_iter`
+    //~| WARNING this was previously accepted by the compiler but is being phased out
+    big.iter();
+    //~^ WARNING this method call currently resolves to `<&[T] as IntoIterator>::into_iter`
+    //~| WARNING this was previously accepted by the compiler but is being phased out
+    [0u8; 33].iter();
+    //~^ WARNING this method call currently resolves to `<&[T] as IntoIterator>::into_iter`
+    //~| WARNING this was previously accepted by the compiler but is being phased out
+
+
+    // Expressions that should not
+    (&[1, 2]).into_iter();
+    (&small).into_iter();
+    (&[0u8; 33]).into_iter();
+    (&big).into_iter();
+
+    for _ in &[1, 2] {}
+    (&small as &[_]).into_iter();
+    small[..].into_iter();
+    std::iter::IntoIterator::into_iter(&[1, 2]);
+}

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

@@ -0,0 +1,33 @@
+// run-pass
+// run-rustfix
+
+fn main() {
+    let small = [1, 2];
+    let big = [0u8; 33];
+
+    // Expressions that should trigger the lint
+    small.into_iter();
+    //~^ WARNING this method call currently resolves to `<&[T; N] as IntoIterator>::into_iter`
+    //~| WARNING this was previously accepted by the compiler but is being phased out
+    [1, 2].into_iter();
+    //~^ WARNING this method call currently resolves to `<&[T; N] as IntoIterator>::into_iter`
+    //~| WARNING this was previously accepted by the compiler but is being phased out
+    big.into_iter();
+    //~^ WARNING this method call currently resolves to `<&[T] as IntoIterator>::into_iter`
+    //~| WARNING this was previously accepted by the compiler but is being phased out
+    [0u8; 33].into_iter();
+    //~^ WARNING this method call currently resolves to `<&[T] as IntoIterator>::into_iter`
+    //~| WARNING this was previously accepted by the compiler but is being phased out
+
+
+    // Expressions that should not
+    (&[1, 2]).into_iter();
+    (&small).into_iter();
+    (&[0u8; 33]).into_iter();
+    (&big).into_iter();
+
+    for _ in &[1, 2] {}
+    (&small as &[_]).into_iter();
+    small[..].into_iter();
+    std::iter::IntoIterator::into_iter(&[1, 2]);
+}

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

@@ -0,0 +1,37 @@
+warning: this method call currently resolves to `<&[T; N] as IntoIterator>::into_iter` (due to autoref coercions), but that might change in the future when `IntoIterator` impls for arrays are added.
+  --> $DIR/into-iter-on-arrays-lint.rs:9:11
+   |
+LL |     small.into_iter();
+   |           ^^^^^^^^^ help: use `.iter()` instead of `.into_iter()` to avoid ambiguity: `iter`
+   |
+   = note: `#[warn(array_into_iter)]` on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #66145 <https://github.com/rust-lang/rust/issues/66145>
+
+warning: this method call currently resolves to `<&[T; N] as IntoIterator>::into_iter` (due to autoref coercions), but that might change in the future when `IntoIterator` impls for arrays are added.
+  --> $DIR/into-iter-on-arrays-lint.rs:12:12
+   |
+LL |     [1, 2].into_iter();
+   |            ^^^^^^^^^ help: use `.iter()` instead of `.into_iter()` to avoid ambiguity: `iter`
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #66145 <https://github.com/rust-lang/rust/issues/66145>
+
+warning: this method call currently resolves to `<&[T] as IntoIterator>::into_iter` (due to autoref coercions), but that might change in the future when `IntoIterator` impls for arrays are added.
+  --> $DIR/into-iter-on-arrays-lint.rs:15:9
+   |
+LL |     big.into_iter();
+   |         ^^^^^^^^^ help: use `.iter()` instead of `.into_iter()` to avoid ambiguity: `iter`
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #66145 <https://github.com/rust-lang/rust/issues/66145>
+
+warning: this method call currently resolves to `<&[T] as IntoIterator>::into_iter` (due to autoref coercions), but that might change in the future when `IntoIterator` impls for arrays are added.
+  --> $DIR/into-iter-on-arrays-lint.rs:18:15
+   |
+LL |     [0u8; 33].into_iter();
+   |               ^^^^^^^^^ help: use `.iter()` instead of `.into_iter()` to avoid ambiguity: `iter`
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #66145 <https://github.com/rust-lang/rust/issues/66145>
+