Start a code index

Author: mark-i-m

src/SUMMARY.md

@@ -28,3 +28,4 @@
 - [Parameter Environments](./param_env.md)
 - [Generating LLVM IR](./trans.md)
 - [Glossary](./glossary.md)
+- [Code Index](./code-index.md)

src/code-index.md

@@ -0,0 +1,9 @@
+# Code Index
+
+rustc has a lot of important data structures. This is an attempt to give some
+guidance on where to learn more about some of the key data structures of the
+compiler.
+
+Item            |  Kind    | Short description           | Chapter            | Declaration
+----------------|----------|-----------------------------|--------------------|-------------------
+`TyCtxt<'cx, 'tcx, 'tcx>` | type | The "typing context". This is the central data structure in the compiler. It is the context that you use to perform all manner of queries. | [The `ty` modules](ty.html) | [src/librustc/ty/context.rs](https://github.com/rust-lang/rust/blob/master/src/librustc/ty/context.rs)