Updated kinds that were renamed to generic arguments.

This change happened in commit bea3d67c77dd643ef1f89c8bd6562e90b373cec4 on
rust-lang/rust.

Author: orium

src/SUMMARY.md

@@ -43,7 +43,7 @@
         - [Debugging](./hir-debugging.md)
     - [Closure expansion](./closure.md)
     - [The `ty` module: representing types](./ty.md)
-    - [Kinds](./kinds.md)
+    - [Generic arguments](./generic_arguments.md)
     - [Type inference](./type-inference.md)
     - [Trait solving (old-style)](./traits/resolution.md)
         - [Higher-ranked trait bounds](./traits/hrtb.md)

src/generic_arguments.md

@@ -0,0 +1,50 @@
+# Generic arguments
+A `ty::subst::GenericArg<'tcx>` represents some entity in the type system: a type
+(`Ty<'tcx>`), lifetime (`ty::Region<'tcx>`) or constant (`ty::Const<'tcx>`).
+`GenericArg` 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 intuitively simply a slice of `GenericArg<'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 instantiate substitutions
+given item definitions, which should generally be used rather than explicitly
+constructing such substitution slices.
+
+## `GenericArg`
+The actual `GenericArg` struct is optimised for space, storing the type, lifetime or
+const as an interned pointer containing a tag 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 `GenericArg` and instead
+make use of the safe [`GenericArgKind`](#genericargkind) abstraction.
+
+## `GenericArgKind`
+As `GenericArg` itself is not type-safe, the `GenericArgKind` enum provides a more
+convenient and safe interface for dealing with generic arguments. An
+`GenericArgKind` can be converted to a raw `GenericArg` using `GenericArg::from()`
+(or simply `.into()` when the context is clear). As mentioned earlier, substitution
+lists store raw `GenericArg`s, so before dealing with them, it is preferable to
+convert them to `GenericArgKind`s first. This is done by calling the `.unpack()`
+method.
+
+```rust,ignore
+// An example of unpacking and packing a generic argument.
+fn deal_with_generic_arg<'tcx>(generic_arg: GenericArg<'tcx>) -> GenericArg<'tcx> {
+    // Unpack a raw `GenericArg` to deal with it safely.
+    let new_generic_arg: GenericArgKind<'tcx> = match generic_arg.unpack() {
+        GenericArgKind::Type(ty) => { /* ... */ }
+        GenericArgKind::Lifetime(lt) => { /* ... */ }
+        GenericArgKind::Const(ct) => { /* ... */ }
+    };
+    // Pack the `GenericArgKind` to store it in a substitution list.
+    new_generic_arg.into()
+}
+```

src/kinds.md

@@ -109,7 +109,7 @@ module. Here are a few examples:
 - `Predicate` defines something the trait system has to prove (see `traits`
   module).
 
-[subst]: ./kinds.html#subst
+[subst]: ./generic_arguments.html#subst
 
 ### Import conventions