Add more information and an example to Kind chapter

Author: varkor

src/kinds.md

@@ -1,31 +1,49 @@
 # Kinds
-A `ty::subst::Kind<'tcx>` represents some entity in the type system: currently
-either a type (`Ty<'tcx>`) or a lifetime (`ty::Region<'tcx>`), though in the
-future this will also include constants (`ty::Const<'tcx>`) to facilitate the
-use of const generics. `Kind` is used for type and lifetime substitution (from
-abstract type and lifetime parameters to concrete types and lifetimes).
-
-## `UnpackedKind`
-As `Kind` itself is not type-safe (see [`Kind`](#kind)), the `UnpackedKind` enum
-provides a more convenient and safe interface for dealing with kinds. To
-convert from an `UnpackedKind` to a `Kind`, you can call `Kind::from` (or
-`.into`). It should not be necessary to convert a `Kind` to an `UnpackedKind`:
-instead, you should prefer to deal with `UnpackedKind`, converting it only when
-passing it to `Subst` methods.
-
-## `Kind`
-The actual `Kind` struct is optimised for space, storing the type or lifetime
-as an interned pointer containing a mask identifying its kind (in the lowest
-2 bits).
+A `ty::subst::Kind<'tcx>` represents some entity in the type system: a type
+(`Ty<'tcx>`), lifetime (`ty::Region<'tcx>`) or constant (`ty::Const<'tcx>`).
+`Kind` is used to perform substitutions of generic parameters for concrete
+arguments, such as when calling a function with generic parameters explicitly
+with type arguments. Substitutions are represented using the
+[`Subst` type](#subst) as described below.
 
 ## `Subst`
-`ty::subst::Subst<'tcx>` is simply defined as a slice of `Kind<'tcx>`s
-and acts as an ordered list of substitutions from kind parameters (i.e.
-type and lifetime parameters) to kinds.
+`ty::subst::Subst<'tcx>` is intuitively simply a slice of `Kind<'tcx>`s,
+acting as an ordered list of substitutions from generic parameters to
+concrete arguments (such as types, lifetimes and consts).
 
 For example, given a `HashMap<K, V>` with two type parameters, `K` and `V`, an
 instantiation of the parameters, for example `HashMap<i32, u32>`, would be
 represented by the substitution `&'tcx [tcx.types.i32, tcx.types.u32]`.
 
 `Subst` provides various convenience methods to instantiant substitutions
-given item definitions.
+given item definitions, which should generally be used rather than explicitly
+constructing such substitution slices.
+
+## `Kind`
+The actual `Kind` struct is optimised for space, storing the type, lifetime or
+const as an interned pointer containing a mask identifying its kind (in the
+lowest 2 bits). Unless you are working with the `Subst` implementation
+specifically, you should generally not have to deal with `Kind` and instead
+make use of the safe [`UnpackedKind`](#unpackedkind) abstraction.
+
+## `UnpackedKind`
+As `Kind` itself is not type-safe, the `UnpackedKind` enum provides a more
+convenient and safe interface for dealing with kinds. An `UnpackedKind` can
+be converted to a raw `Kind` using `Kind::from()` (or simply `.into()` when
+the context is clear). As mentioned earlier, substition lists store raw
+`Kind`s, so before dealing with them, it is preferable to convert them to
+`UnpackedKind`s first. This is done by calling the `.unpack()` method.
+
+```rust
+// An example of unpacking and packing a kind.
+fn deal_with_kind<'tcx>(kind: Kind<'tcx>) -> Kind<'tcx> {
+    // Unpack a raw `Kind` to deal with it safely.
+    let new_kind: UnpackedKind<'tcx> = match kind.unpack() {
+        UnpackedKind::Type(ty) => { /* ... */ }
+        UnpackedKind::Lifetime(lt) => { /* ... */ }
+        UnpackedKind::Const(ct) => { /* ... */ }
+    };
+    // Pack the `UnpackedKind` to store it in a substitution list.
+    new_kind.into()
+}
+```