Fix rust-lang#1060: add page on Impl Trait

Author: adamchalmers

src/SUMMARY.md

@@ -137,6 +137,7 @@
     - [Operator Overloading](trait/ops.md)
     - [Drop](trait/drop.md)
     - [Iterators](trait/iter.md)
+    - [impl Trait](trait/impl_trait.md)
     - [Clone](trait/clone.md)
 
 - [macro_rules!](macros.md)

src/trait/impl_trait.md

@@ -0,0 +1,46 @@
+# impl Trait
+
+If your function returns a type that implements `MyTrait`, you can write its return type as `-> impl MyTrait`. This can help simplify your type signatures quite a lot!
+
+```rust,editable
+use std::iter;
+use std::vec::IntoIter;
+
+// This function combines two Vec<i32> and returns an iterator over it.
+// Look how complicated its return type is!
+fn combine_vecs_explicit_return_type<'a>(
+    v: Vec<i32>,
+    u: Vec<i32>,
+) -> iter::Cycle<iter::Chain<IntoIter<i32>, IntoIter<i32>>> {
+    v.into_iter().chain(u.into_iter()).cycle()
+}
+
+// This is the exact same function, but its return type uses `impl Trait`.
+// Look how much simpler it is!
+fn combine_vecs<'a>(
+    v: Vec<i32>,
+    u: Vec<i32>,
+) -> impl Iterator<Item=i32> {
+    v.into_iter().chain(u.into_iter()).cycle()
+}
+```
+
+More importantly, some Rust types can't be written out. For example, every closure has its own unnamed concrete type. Before `impl Trait` syntax, you couldn't write a function like this:
+
+```rust,editable
+fn return_closure() -> impl Fn(i32) {
+    let closure = |x: i32| { println!("the value of x is {}", x) };
+    closure
+}
+```
+
+Here's a more realistic example: what if you want to return an iterator that uses `map` or `filter` closures? Because the closure's types don't have names, you can't write out an explicit return type. But with `impl Trait` you can do this easily:
+
+```rust,editable
+fn double_positives<'a>(numbers: &'a Vec<i32>) -> impl Iterator<Item = i32> + 'a {
+    numbers
+        .iter()
+        .filter(|x| x > &&0)
+        .map(|x| x * 2)
+}
+```