Skip into_iter() for arrays before 2021

library/core/src/array/mod.rs

@@ -155,6 +155,10 @@ impl<T: fmt::Debug, const N: usize> fmt::Debug for [T; N] {
     }
 }
 
+// Note: the `#[rustc_skip_array_during_method_dispatch]` on `trait IntoIterator`
+// hides this implementation from explicit `.into_iter()` calls on editions < 2021,
+// so those calls will still resolve to the slice implementation, by reference.
+#[cfg(not(bootstrap))]
 #[stable(feature = "array_into_iter_impl", since = "1.53.0")]
 impl<T, const N: usize> IntoIterator for [T; N] {
     type Item = T;

library/core/src/iter/traits/collect.rs

@@ -198,6 +198,7 @@ pub trait FromIterator<A>: Sized {
 /// }
 /// ```
 #[rustc_diagnostic_item = "IntoIterator"]
+#[cfg_attr(not(bootstrap), rustc_skip_array_during_method_dispatch)]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait IntoIterator {
     /// The type of the elements being iterated over.

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

@@ -0,0 +1,17 @@
+// check-pass
+
+use std::array::IntoIter;
+use std::slice::Iter;
+
+fn main() {
+    let array = [0; 10];
+
+    // Before 2021, the method dispatched to `IntoIterator for &[T]`,
+    // which we continue to support for compatibility.
+    let _: Iter<'_, i32> = array.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
+
+    // But you can always use the trait method explicitly as an array.
+    let _: IntoIter<i32, 10> = IntoIterator::into_iter(array);
+}

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

@@ -0,0 +1,23 @@
+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-2018.rs:11:34
+   |
+LL |     let _: Iter<'_, i32> = array.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: 1 warning emitted
+
+Future incompatibility report: Future breakage date: None, diagnostic:
+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-2018.rs:11:34
+   |
+LL |     let _: Iter<'_, i32> = array.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>
+

src/test/ui/iterators/into-iter-on-arrays-2021.rs

@@ -0,0 +1,15 @@
+// check-pass
+// edition:2021
+// compile-flags: -Zunstable-options
+
+use std::array::IntoIter;
+
+fn main() {
+    let array = [0; 10];
+
+    // In 2021, the method dispatches to `IntoIterator for [T; N]`.
+    let _: IntoIter<i32, 10> = array.into_iter();
+
+    // And you can always use the trait method explicitly as an array.
+    let _: IntoIter<i32, 10> = IntoIterator::into_iter(array);
+}