Add a paragraph with possible alternatives on older editions

rust-lang · bors · Apr 30, 2021 · Apr 22, 2021 · Apr 26, 2021 · Apr 27, 2021
commit 12642d99a6a7f5302efb9079ece54cf18c337571
diff --git a/library/std/src/primitive_docs.rs b/library/std/src/primitive_docs.rs
@@ -603,6 603,48 @@ 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 )
 ///
 /// ```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