Flesh out the "representing types" chapter

src/SUMMARY.md

@@ -114,21 +114,25 @@
 # Analysis
 
 - [Prologue](./part-4-intro.md)
+- [Generic parameter definitions](./generic_parameters_summary.md)
+    - [What is `ty::Generics`](./what_is_ty_generics.md)
+    - [Early vs Late bound parameters](./early-late-bound-params/early-late-bound-summary.md)
+        - [Implementation nuances of early/late bound parameters](./early-late-bound-params/early-late-bound-implementation-nuances.md)
+        - [Interactions with turbofishing](./early-late-bound-params/turbofishing-and-early-late-bound.md)
 - [The `ty` module: representing types](./ty.md)
-    - [Generics and substitutions](./generics.md)
-    - [`TypeFolder` and `TypeFoldable`](./ty-fold.md)
-    - [Generic arguments](./generic_arguments.md)
+    - [ADTs and Generic Arguments](./ty_module/generic_arguments.md)
+    - [Parameter types/consts/regions](./ty_module/param_ty_const_regions.md)
+    - [`EarlyBinder` and instantiating parameters](./ty_module/early_binder.md)
+    - [`Binder` and Higher ranked regions](./ty_module/binders.md)
+    - [Instantiating binders](./ty_module/instantiating_binders.md)
     - [Constants in the type system](./constants.md)
-    - [Bound vars and Parameters](./bound-vars-and-params.md)
+- [`TypeFolder` and `TypeFoldable`](./ty-fold.md)
 - [Parameter Environments](./param_env/param_env_summary.md)
     - [What is it?](./param_env/param_env_what_is_it.md)
     - [How are `ParamEnv`'s constructed internally](./param_env/param_env_construction_internals.md)
     - [Which `ParamEnv` do I use?](./param_env/param_env_acquisition.md)
 - [Type inference](./type-inference.md)
 - [Trait solving](./traits/resolution.md)
-    - [Early and Late Bound Parameter Definitions](./early-late-bound-params/early-late-bound-summary.md)
-        - [Implementation nuances of early/late bound parameters](./early-late-bound-params/early-late-bound-implementation-nuances.md)
-        - [Interactions with turbofishing](./early-late-bound-params/turbofishing-and-early-late-bound.md)
     - [Higher-ranked trait bounds](./traits/hrtb.md)
     - [Caching subtleties](./traits/caching.md)
     - [Implied bounds](./traits/implied-bounds.md)

src/generic_parameters_summary.md

@@ -0,0 +1,7 @@
+# Generic parameter definitions
+
+This chapter will discuss how rustc tracks what generic parameters are introduced by an item. For example given some struct defined via `struct Foo<T>` how does rustc track that `Foo` defines some type parameter `T` and nothing else?
+
+This will *not* cover how we track generic parameters introduced via `for<'a>` syntax (i.e. in where clauses or `fn` types), which is covered elsewhere in the [chapter on `Binder`s ][ch_binders].
+
+[ch_binders]: ./ty_module/binders.md

src/ty-fold.md

@@ -1,9 +1,8 @@
 # `TypeFoldable` and `TypeFolder`
 
-How is this `subst` query actually implemented? As you can imagine, we might want to do
-substitutions on a lot of different things. For example, we might want to do a substitution directly
-on a type like we did with `Vec` above. But we might also have a more complex type with other types
-nested inside that also need substitutions.
+In the previous chapter we discussed instantiating binders. This must involves looking at everything inside of a `Early/Binder`
+to find any usages of the bound vars in order to replace them. Binders can wrap an arbitrary rust type `T` not just a `Ty` so
+how do we implement the `instantiate` methods on the `Early/Binder` types.
 
 The answer is a couple of traits:
 [`TypeFoldable`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/fold/trait.TypeFoldable.html)