Add reference for Copyable

stdlib/public/core/Misc.swift

@@ -169,6 +169,62 @@ func _rethrowsViaClosure(_ fn: () throws -> ()) rethrows {
   try fn()
 }
 
+/// A type whose values can be implicitly or explicitly copied.
+///
+/// Conforming to this protocol indicates that a type's value can be copied;
+/// this protocol doesn’t have any required methods or properties.
+/// You don't generally need to write an explicit conformance to `Copyable`.
+/// The following places implicitly include `Copyable` conformance:
+///
+/// * Structure declarations,
+///   unless it has a noncopyable stored property
+/// * Enumeration declarations,
+///   unless it has a case whose associated value isn't copyable
+/// * Class declarations
+/// * Actor declarations
+/// * Protocol declarations
+/// * Associated type declarations
+/// * The `Self` type in a protocol extension
+/// * In an extension, the generic parameters of the type being extended
+///
+/// A class or actor can contain noncopyable stored properties,
+/// while still being copyable itself ---
+/// classes and actors are copied by retaining and releasing references.
+///
+/// In a declaration that includes generic type parameters,
+/// each generic type parameter implicitly includes `Copyable`
+/// in its list of requirements.
+/// Metatypes and tuples of copyable types are also implicitly copyable,
+/// as are boxed protocol types.
+/// For example,
+/// all of the following pairs of declarations are equivalent:
+///
+///     struct MyStructure { }
+///     struct MyStructere: Copyable { }
+///
+///     protocol MyProtocol { }
+///     protocol MyProtocol: Copyable { }
+///
+///     protocol AnotherProtocol {
+///         associatedtype MyType
+///         associatedtype MyType: Copyable
+///     }
+///
+///     func genericFunction<T>(t: T) { }
+///     func genericFunction<T>(t: T) where T: Copyable { }
+///
+///     let x: any MyProtocol
+///     let x: any MyProtocol & Copyable
+///
+/// To suppress an implicit conformance to `Copyable` you write `~Copyable`.
+/// For example,
+/// only copyable types can conform to `MyProtocol` in the example above,
+/// but both copyable and noncopyable types
+/// can conform `NoRequirements` in the example below:
+///
+///     protocol NoRequirements: ~Copyable { }
+///
+/// Extensions to the `Copyable` protocol are not allowed.
 @_marker public protocol Copyable {}
 
 @_marker public protocol Escapable {}