write summary

Author: steveklabnik

src/ch19-00-advanced-features.md

@@ -1,4 +1,18 @@
-# Advanced Type System Features
+# Advanced Features
+
+We've come a long way! By now, we've learned 99% of the things you'll need to
+know when writing Rust. We'll wrap the book up by doing one more project, but
+before we get to that, let's talk about a few things that you may run into that
+last 1% of the time. Feel free to skip this chapter and come back to it once
+you run into these things in the wild; the tools we'll learn to use here are
+useful in very specific situations. We don't want to leave them out, but you
+won't find yourself reaching for them often.
+
+Here's a quick summary:
+
+* Unsafe Rust: for when you need to tell Rust "just trust me, promise!"
+* Advanced Lifetimes: Additional lifetime syntax for complex situations.
+* Advanced Traits: Associated Types, coherence, and disambiguation.
 
 ## Unsafe Rust