Add spec identifier syntax to interior-mutability.md

src/interior-mutability.md

@@ -1,22 +1,38 @@
 # Interior Mutability
 
+r[interior-mut]
+
+r[interior-mut.intro]
 Sometimes a type needs to 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
+is achieved using a pattern called _interior mutability_.
+
+r[interior-mut.shared-ref]
+A type has interior mutability if its internal state can be changed through a [shared reference] to
+it.
+
+r[interior-mut.no-constraint]
+This goes against the usual [requirement][ub] that the value pointed to by a
 shared reference is not mutated.
 
+r[interior-mut.unsafe-cell]
 [`std::cell::UnsafeCell<T>`] type is the only allowed way 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>`
+mutate, or obtain a mutable reference to, the `T` it contains.
+
+r[interior-mut.mut-unsafe-cell]
+As with all other types, it is undefined behavior to have multiple `&mut UnsafeCell<T>`
 aliases.
 
+r[interior-mut.abstraction]
 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
+interior mutability APIs.
+
+r[interior-mut.ref-cell]
+For example, [`std::cell::RefCell<T>`] uses run-time borrow checks to ensure the usual rules around multiple references.
+
+r[interior-mut.atomic]
+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.