Add slide on enumerate

src/SUMMARY.md

@@ -193,6 +193,7 @@ SPDX-License-Identifier: CC-BY-4.0
   - [Motivation](iterators/motivation.md)
   - [`Iterator` Trait](iterators/iterator.md)
   - [`Iterator` Helper Methods](iterators/helpers.md)
+  - [`enumerate`](iterators/enumerate.md)
   - [`collect`](iterators/collect.md)
   - [`IntoIterator`](iterators/intoiterator.md)
   - [Exercise: Iterator Method Chaining](iterators/exercise.md)

src/iterators/enumerate.md

@@ -0,0 +1,43 @@
+# `enumerate`
+
+The [`enumerate`] helper gives us a way to get the index of each element for any
+iterator.
+
+```rust,editable
+# // Copyright 2023 Google LLC
+# // SPDX-License-Identifier: Apache-2.0
+#
+fn main() {
+    let array = ['a', 'b', 'c', 'd', 'e'];
+
+    // By default iterating over an array just gives its elements.
+    for letter in array {
+        println!("The element is {letter:?}, but what's its index?");
+    }
+
+    // `enumerate` gives both the element and its index.
+    for (i, letter) in array.iter().enumerate() {
+        println!("{letter:?} has index {i}");
+    }
+}
+```
+
+<details>
+
+- A nuance of Rust's iterators and `for` loops is that an iterator by default
+  does not give you the index of each element. This is fine when all you need
+  are e.g. the values in an array, but it's common to also want the index of
+  each element as you iterate.
+
+- `enumerate` creates an iterator that produces a tuple where the first element
+  is the index and the second element is the original iterator's item.
+
+- Point out that we can use a pattern for the loop variable in a `for` loop.
+  This allows us to destructure the tuple produced by `enumerate`.
+
+- Note that we had to do `.iter()` first; `enumerate` is an iterator method, not
+  a method on the array type itself.
+
+</details>
+
+[`enumerate`]: https://doc.rust-lang.org/stable/std/iter/trait.Iterator.html#method.enumerate