Point out that behavior might be switched on 2015 and 2018 too one day

library/std/src/primitive_docs.rs

@@ -553,8 +553,10 @@ mod prim_pointer {}
 /// # Editions
 ///
 /// Prior to Rust 1.53, arrays did not implement `IntoIterator` by value, so the method call
-/// `array.into_iter()` auto-referenced into a slice iterator. That behavior is preserved in the
-/// 2015 and 2018 editions of Rust for compatability, ignoring `IntoIterator` by value.
+/// `array.into_iter()` auto-referenced into a slice iterator. Right now, the old behavior
+/// is preserved in the 2015 and 2018 editions of Rust for compatibility, ignoring
+/// `IntoIterator` by value. In the future, the behavior on the 2015 and 2018 edition
+/// might be made consistent to the behavior of later editions.
 ///
 #[cfg_attr(bootstrap, doc = "```rust,edition2018,ignore")]
 #[cfg_attr(not(bootstrap), doc = "```rust,edition2018")]
@@ -601,6 +603,49 @@ mod prim_pointer {}
 /// }
 /// ```
 ///
+/// Future language versions might start treating the `array.into_iter()`
+/// syntax on editions 2015 and 2018 the same as on edition 2021. So code using
+/// those older editions should still be written with this change in mind, to
+/// prevent breakage in the future. The safest way to accomplish this is to
+/// avoid the `into_iter` syntax on those editions. If an edition update is not
+/// viable/desired, there are multiple alternatives:
+/// * use `iter`, equivalent to the old behavior, creating references
+/// * use [`array::IntoIter`], equivalent to the post-2021 behavior (Rust 1.51+)
+/// * replace `for ... in array.into_iter() {` with `for ... in array {`,
+///   equivalent to the post-2021 behavior (Rust 1.53+)
+///
+#[cfg_attr(bootstrap, doc = "```rust,edition2018,ignore")]
+#[cfg_attr(not(bootstrap), doc = "```rust,edition2018")]
+/// use std::array::IntoIter;
+///
+/// let array: [i32; 3] = [0; 3];
+///
+/// // This iterates by reference:
+/// for item in array.iter() {
+///     let x: &i32 = item;
+///     println!("{}", x);
+/// }
+///
+/// // This iterates by value:
+/// for item in IntoIter::new(array) {
+///     let x: i32 = item;
+///     println!("{}", x);
+/// }
+///
+/// // This iterates by value:
+/// for item in array {
+///     let x: i32 = item;
+///     println!("{}", x);
+/// }
+///
+/// // IntoIter can also start a chain.
+/// // This iterates by value:
+/// for item in IntoIter::new(array).enumerate() {
+///     let (i, x): (usize, i32) = item;
+///     println!("array[{}] = {}", i, x);
+/// }
+/// ```
+///
 /// [slice]: prim@slice
 /// [`Debug`]: fmt::Debug
 /// [`Hash`]: hash::Hash