Add spec identifiers to dynamically-sized-types.md

src/dynamically-sized-types.md

@@ -1,22 +1,35 @@
 # Dynamically Sized Types
 
+r[dynamic-sized]
+
+r[dynamic-sized.intro]
 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:
+types">DSTs</abbr>.
+
+r[dynamic-sized.restriction]
+Such types can only be used in certain cases:
 
+r[dynamic-sized.pointer-types]
 * [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.
+
+r[dynamic-sized.question-sized]
 * <abbr title="dynamically sized types">DSTs</abbr> can be provided as
   type arguments to generic type parameters having the special `?Sized` bound.
   They can also be used for associated type definitions when the corresponding associated type declaration has a `?Sized` bound.
   By default, any type parameter or associated type has a `Sized` bound, unless it is relaxed using `?Sized`.
+
+r[dynamic-sized.trait-impl]
 * Traits may be implemented for <abbr title="dynamically sized
   types">DSTs</abbr>.
   Unlike with generic type parameters, `Self: ?Sized` is the default in trait definitions.
+
+r[dynamic-sized.struct-field]
 * 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>.