SE-0210: improve documentation comments per @natecook1000's comments.

Author: DougGregor

stdlib/public/core/MemoryLayout.swift

@@ -166,26 +166,22 @@ extension MemoryLayout {
   /// Returns the offset of an inline stored property of `T` within the
   /// in-memory representation of `T`.
   ///
-  /// If the given `key` refers to inline storage within the
-  /// in-memory representation of `T`, and the storage is directly
-  /// addressable (meaning that accessing it does not need to trigger any
-  /// `didSet` or `willSet` accessors, perform any representation changes
-  /// such as bridging or closure reabstraction, or mask the value out of
-  /// overlapping storage as for packed bitfields), then the return value
-  /// is a distance in bytes that can be added to a pointer of type `T` to
-  /// get a pointer to the storage accessed by `key`. If the return value is
-  /// non-nil, then these formulations are equivalent:
-  ///
-  ///   var root: T, value: U
-  ///   var key: WritableKeyPath<T, U>
-  ///   // Mutation through the key path...
-  ///   root[keyPath: \.key] = value
-  ///   // ...is exactly equivalent to mutation through the offset pointer...
-  ///   withUnsafePointer(to: &root) {
-  ///     (UnsafeMutableRawPointer($0) + MemoryLayout<T>.offset(of: \.key))
-  ///       // ...which can be assumed to be bound to the target type
-  ///       .assumingMemoryBound(to: U.self).pointee = value
-  ///   }
+  /// If the given `key` refers to inline, directly addressable storage within
+  /// the in-memory representation of `T`, then the return value is a distance
+  /// in bytes that can be added to a pointer of type `T` to get a pointer to
+  /// the storage accessed by `key`. If the return value is non-nil, then these
+  /// formulations are equivalent:
+  ///
+  ///     var root: T, value: U
+  ///     var key: WritableKeyPath<T, U>
+  ///     // Mutation through the key path...
+  ///     root[keyPath: key] = value
+  ///     // ...is exactly equivalent to mutation through the offset pointer...
+  ///     withUnsafeMutablePointer(to: &root) {
+  ///         (UnsafeMutableRawPointer($0) + MemoryLayout<T>.offset(of: key))
+  ///             // ...which can be assumed to be bound to the target type
+  ///             .assumingMemoryBound(to: U.self).pointee = value
+  ///     }
   ///
   /// - Parameter key: A key path referring to storage that can be accessed
   ///   through a value of type `T`.
@@ -194,6 +190,28 @@ extension MemoryLayout {
   ///   such offset is available for the storage referenced by `key`, such as
   ///   because `key` is computed, has observers, requires reabstraction, or
   ///   overlaps storage with other properties.
+  ///
+  /// A property has inline, directly addressable storage when it is a stored
+  /// property for which no additional work is required to extract or set the
+  /// value. For example:
+  ///
+  ///     struct ProductCategory {
+  ///         var name: String           // inline, directly-addressable
+  ///         var updateCounter: Int     // inline, directly-addressable
+  ///         var productCount: Int {    // computed properties are not directly addressable
+  ///             return products.count
+  ///         }
+  ///         var products: [Product] {  // didSet/willSet properties are not directly addressable
+  ///                 didSet { updateCounter += 1 }
+  ///         }
+  ///     }
+  ///
+  /// When using `offset(of:)` with a type imported from a library, don't assume
+  /// that future versions of the library will have the same behavior. If a
+  /// property is converted from a stored property to a computed property, the
+  /// result of `offset(of:)` changes to `nil`. That kind of conversion is
+  /// non-breaking in other contexts, but would trigger a runtime error if the
+  /// result of `offset(of:)` is force-unwrapped.
   @_inlineable // FIXME(sil-serialize-all)
   @_transparent
   public static func offset(of key: PartialKeyPath<T>) -> Int? {