Add contents for Rust 1.35

Fixes #204

Author: steveklabnik

src/SUMMARY.md

@@ -96,4 +96,5 @@
     - [`literal` macro matcher](rust-next/literal-macro-matcher.md)
     - [`?` operator in macros](rust-next/qustion-mark-operator-in-macros.md)
     - [const fn](rust-next/const-fn.md)
-    - [Pinning](rust-next/pin.md)
+    - [Pinning](rust-next/pin.md)
+    - [No more FnBox](rust-next/no-more-fnbox.md)

src/rust-next/no-more-fnbox.md

@@ -0,0 +1,38 @@
+# No more FnBox
+
+![Minimum Rust version: 1.35](https://img.shields.io/badge/Minimum%20Rust%20Version-1.35-brightgreen.svg)
+
+The book used to have this code in Chapter 20, section 2:
+
+```rust
+trait FnBox {
+    fn call_box(self: Box<Self>);
+}
+
+impl<F: FnOnce()> FnBox for F {
+    fn call_box(self: Box<F>) {
+        (*self)()
+    }
+}
+
+type Job = Box<dyn FnBox + Send + 'static>;
+```
+
+Here, we define a new trait called `FnBox`, and then implement it for all
+`FnOnce` closures. All the implementation does is call the closure. These
+sorts of hacks were needed because a `Box<dyn FnOnce>` didn't implement
+`FnOnce`. This was true for all three posibilities:
+
+* `Box<dyn Fn>` and `Fn`
+* `Box<dyn FnMut>` and `FnMut`
+* `Box<dyn FnOnce>` and `FnOnce`
+
+However, as of Rust 1.35, these traits are implemented for these types,
+and so the `FnBox` trick is no longer required. In the latest version of
+the book, the `Job` type looks like this:
+
+```rust
+type Job = Box<dyn FnOnce() + Send + 'static>;
+```
+
+No need for all that other code.