Reframe FromIterator slide to focus on collect (#2516)

src/SUMMARY.md

@@ -171,7 +171,7 @@
 - [Iterators](iterators.md)
   - [`Iterator`](iterators/iterator.md)
   - [`IntoIterator`](iterators/intoiterator.md)
-  - [`FromIterator`](iterators/fromiterator.md)
+  - [`collect`](iterators/collect.md)
   - [Exercise: Iterator Method Chaining](iterators/exercise.md)
     - [Solution](iterators/solution.md)
 - [Modules](modules.md)

src/iterators/collect.md

@@ -0,0 +1,48 @@
+---
+minutes: 5
+---
+
+# `collect`
+
+The [`collect`][3] method lets you build a collection from an [`Iterator`][2].
+
+```rust,editable
+fn main() {
+    let primes = vec![2, 3, 5, 7];
+    let prime_squares = primes.into_iter().map(|p| p * p).collect::<Vec<_>>();
+    println!("prime_squares: {prime_squares:?}");
+}
+```
+
+<details>
+
+- Any iterator can be collected in to a `Vec`, `VecDeque`, or `HashSet`.
+  Iterators that produce key-value pairs (i.e. a two-element tuple) can also be
+  collected into `HashMap` and `BTreeMap`.
+
+Show the students the definition for `collect` in the standard library docs.
+There are two ways to specify the generic type `B` for this method:
+
+- With the "turbofish": `some_iterator.collect::<COLLECTION_TYPE>()`, as shown.
+  The `_` shorthand used here lets Rust infer the type of the `Vec` elements.
+- With type inference: `let prime_squares: Vec<_> = some_iterator.collect()`.
+  Rewrite the example to use this form.
+
+## More to Explore
+
+- If students are curious about how this works, you can bring up the
+  [`FromIterator`][1] trait, which defines how each type of collection gets
+  built from an iterator.
+- In addition to the basic implementations of `FromIterator` for `Vec`,
+  `HashMap`, etc., there are also more specialized implementations which let you
+  do cool things like convert an `Iterator<Item = Result<V, E>>` into a
+  `Result<Vec<V>, E>`.
+- The reason type annotations are often needed with `collect` is because it's
+  generic over its return type. This makes it harder for the compiler to infer
+  the correct type in a lot of cases.
+
+</details>
+
+[1]: https://doc.rust-lang.org/std/iter/trait.FromIterator.html
+[2]: https://doc.rust-lang.org/std/iter/trait.Iterator.html
+[3]: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.collect