Traits: Improve index layout, add chalk blurb

Author: tmandry

src/traits/index.md

@@ -1,17 +1,26 @@
 # Trait solving (new-style)
 
-🚧 This chapter describes "new-style" trait solving. This is still in the
-[process of being implemented][wg]; this chapter serves as a kind of
-in-progress design document. If you would prefer to read about how the
-current trait solver works, check out
-[this other chapter](./resolution.html). (By the way, if you
-would like to help in hacking on the new solver, you will find
-instructions for getting involved in the
-[Traits Working Group tracking issue][wg].) 🚧
+> 🚧 This chapter describes "new-style" trait solving. This is still in the
+> [process of being implemented][wg]; this chapter serves as a kind of
+> in-progress design document. If you would prefer to read about how the
+> current trait solver works, check out
+> [this other chapter](./resolution.html). 🚧
+>
+> By the way, if you would like to help in hacking on the new solver, you will
+> find instructions for getting involved in the
+> [Traits Working Group tracking issue][wg].
 
 [wg]: https://github.com/rust-lang/rust/issues/48416
 
-Trait solving is based around a few key ideas:
+The new-style trait solver is based on the work done in [chalk][chalk]. Chalk
+recasts Rust's trait system explicitly in terms of logic programming. It does
+this by "lowering" Rust code into a kind of logic program we can then execute
+queries against.
+
+You can read more about chalk itself in the
+[Overview of Chalk](./chalk-overview.md) section.
+
+Trait solving in rustc is based around a few key ideas:
 
 - [Lowering to logic](./lowering-to-logic.html), which expresses
   Rust traits in terms of standard logical terms.
@@ -32,17 +41,23 @@ Trait solving is based around a few key ideas:
   `Bar`?") once, and then apply that same result independently in many
   different inference contexts.
 
-Note: this is not a complete list of topics. See the sidebar for more.
+> This is not a complete list of topics. See the sidebar for more.
 
+## Ongoing work
 The design of the new-style trait solving currently happens in two places:
-* The [chalk][chalk] repository is where we experiment with new ideas and
-  designs for the trait system. It basically consists of a unit testing framework
-  for the correctness and feasibility of the logical rules defining the new-style
-  trait system. It also provides the [`chalk_engine`][chalk_engine] crate, which
-  defines the new-style trait solver used both in the unit testing framework and
-  in rustc.
-* Once we are happy with the logical rules, we proceed to implementing them in
-  rustc. This mainly happens in [`librustc_traits`][librustc_traits].
+
+**chalk**. The [chalk][chalk] repository is where we experiment with new ideas
+and designs for the trait system. It primarily consists of two parts:
+* a unit testing framework
+  for the correctness and feasibility of the logical rules defining the
+  new-style trait system.
+* the [`chalk_engine`][chalk_engine] crate, which
+  defines the new-style trait solver used both in the unit testing framework
+  and in rustc.
+
+**rustc**. Once we are happy with the logical rules, we proceed to
+implementing them in rustc. This mainly happens in
+[`librustc_traits`][librustc_traits].
 
 [chalk]: https://github.com/rust-lang-nursery/chalk
 [chalk_engine]: https://github.com/rust-lang-nursery/chalk/tree/master/chalk-engine