Types chapter cleanup and new sections

src/SUMMARY.md

@@ -32,6 +32,8 @@
 
 - [Type system](type-system.md)
     - [Types](types.md)
+    - [Dynamically Sized Types](dynamically-sized-types.md)
+    - [Interior mutability](interior-mutability.md)
     - [Subtyping](subtyping.md)
     - [Type coercions](type-coercions.md)
 

src/dynamically-sized-types.md

@@ -0,0 +1,32 @@
+# Dynamically Sized Types
+
+Most types have a fixed size that is known at compile time and implement the
+trait [`Sized`][sized]. A type with a size that is known only at run-time is
+called a _dynamically sized type_ (_DST_) or (informally) an unsized type.
+[Slices] and [trait objects] are two examples of <abbr title="dynamically sized
+types">DSTs</abbr>. Such types can only be used in certain cases:
+
+* [Pointer types] to <abbr title="dynamically sized types">DSTs</abbr> are
+  sized but have twice the size of pointers to sized types
+    * Pointers to slices also store the number of elements of the slice.
+    * Pointers to trait objects also store a pointer to a vtable.
+* <abbr title="dynamically sized types">DSTs</abbr> can be provided as
+  type arguments when a bound of `?Sized`. By default any type parameter
+  has a `?Sized` bound.
+* Traits may be implemented for <abbr title="dynamically sized
+  types">DSTs</abbr>. Unlike type parameters`Self: ?Sized` by default in trait
+  definitions.
+* Structs may contain a <abbr title="dynamically sized type">DST</abbr> as the
+  last field, this makes the struct itself a
+  <abbr title="dynamically sized type">DST</abbr>.
+
+Notably: [variables], function parameters, [const] and [static] items must be
+`Sized`.
+
+[sized]: the-sized-trait.html
+[Slices]: #array-and-slice-types
+[trait objects]: #trait-objects
+[Pointer types]: #pointer-types
+[variables]: variables.html
+[const]: items.html#constant-items
+[static]: items.html#static-items

src/interior-mutability.md

@@ -0,0 +1,30 @@
+# Interior Mutability
+
+Sometimes a type needs be mutated while having multiple aliases, in Rust this
+is achieved using a pattern called _interior mutability_. A type has interior
+mutability if its internal state can be changed through a [shared reference] to
+it. This goes against the usual [requirement][ub] that the value pointed to by a
+shared reference is not mutated.
+
+[`std::cell::UnsafeCell<T>`] type is the only legal way in Rust to disable this
+requirement. When `UnsafeCell<T>` is immutably aliased, it is still safe to
+mutate, or obtain a mutable reference to, the `T` it contains. As with all
+other types, it is undefined behavior to have multiple `&mut UnsafeCell<T>`
+aliases.
+
+Other types with interior mutability can be created by using `UnsafeCell<T>` as
+a field. The standard library provides a variety of types that provide safe
+interior mutability APIs. For example, [`std::cell::RefCell<T>`] uses run-time
+borrow checks to ensure the usual rules around multiple references. The
+[`std::sync::atomic`] module contains types that wrap a value that is only
+accessed with atomic operations, allowing the value to be shared and mutated
+across threads.
+
+[shared reference]: types.hmtl#shared-references
+[ub]: behavior-considered-undefined.html
+[`std::mem::transmute`]: ../std/mem/fn.transmute.html
+[`std::cell::UnsafeCell<T>`]: ../std/cell/struct.UnsafeCell.html
+[`std::cell::RefCell<T>`]: ../std/cell/struct.RefCell.html
+[`std::sync::atomic`]: ../std/sync/atomic.html
+
+