Rename @_fixed_layout to @frozen for structs and enums. (#25433)

Author: dan-zheng

stdlib/public/Python/Python.swift

@@ -86,7 +86,7 @@ final class PyReference {
 /// a Python C API `PyObject`.
 @dynamicCallable
 @dynamicMemberLookup
-@_fixed_layout
+@frozen
 public struct PythonObject {
   /// The underlying `PyReference`.
   fileprivate var reference: PyReference
@@ -207,7 +207,7 @@ public extension PythonObject {
 }
 
 /// An error produced by a failable Python operation.
-@_frozen
+@frozen
 public enum PythonError : Error, Equatable {
   /// A Python runtime exception, produced by calling a Python function.
   case exception(PythonObject, traceback: PythonObject?)
@@ -270,7 +270,7 @@ private func throwPythonErrorIfPresent() throws {
 ///   `@dynamicCallable` attribute because the call syntax is unintuitive:
 ///   `x.throwing(arg1, arg2, ...)`. The methods will still be named
 ///   `dynamicallyCall` until further discussion/design.
-@_fixed_layout
+@frozen
 public struct ThrowingPythonObject {
   private var base: PythonObject
 
@@ -417,7 +417,7 @@ public extension PythonObject {
 /// Member access operations return an `Optional` result. When member access
 /// fails, `nil` is returned.
 @dynamicMemberLookup
-@_fixed_layout
+@frozen
 public struct CheckingPythonObject {
   /// The underlying `PythonObject`.
   private var base: PythonObject
@@ -646,7 +646,7 @@ public let Python = PythonInterface()
 /// - Note: It is not intended for `PythonInterface` to be initialized
 ///   directly. Instead, please use the global instance of `PythonInterface`
 ///   called `Python`.
-@_fixed_layout
+@frozen
 @dynamicMemberLookup
 public struct PythonInterface {
   /// A dictionary of the Python builtins.

stdlib/public/core/Array.swift

@@ -1916,15 +1916,15 @@ internal struct _ArrayAnyHashableBox<Element: Hashable>
 extension Array where Element : Differentiable {
   /// The view of an array as the differentiable product manifold of `Element`
   /// multiplied with itself `count` times.
-  @_fixed_layout
+  @frozen
   public struct DifferentiableView : Differentiable {
     private var _base: [Element]
 
     /// The viewed array.
     // I'm implementing this as a computed property instead of directly
     // exposing `_base` because the `@differentiable` annotation does not make
     // the stored property actually differentiable. I think this is a bug.
-    // Maybe it's related to `@_fixed_layout`?
+    // Maybe it's related to `@frozen`?
     // TODO: Determine if that is a bug, and fix.
     public var base: [Element] {
       @differentiable(wrt: self, vjp: _vjpBase)

stdlib/public/core/AutoDiff.swift

@@ -619,7 +619,7 @@ public struct AnyDerivative : Differentiable & AdditiveArithmetic {
   // `AdditiveArithmetic` requirements.
 
   /// Internal struct representing an opaque zero value.
-  @_fixed_layout
+  @frozen
   @usableFromInline
   internal struct OpaqueZero : Differentiable & AdditiveArithmetic {}
 

test/AutoDiff/refcounting.swift

@@ -5,7 +5,7 @@ public class NonTrivialStuff : Equatable {
   public static func == (lhs: NonTrivialStuff, rhs: NonTrivialStuff) -> Bool { return true }
 }
 
-@_fixed_layout
+@frozen
 public struct Vector : AdditiveArithmetic, VectorProtocol, Differentiable, Equatable {
   public var x: Float
   public var y: Float

test/AutoDiff/simple_real_vector.swift

@@ -1,6 +1,6 @@
 // RUN: %target-swift-frontend -emit-silgen %s | %FileCheck %s
 
-@_fixed_layout
+@frozen
 public struct Vector : AdditiveArithmetic, VectorProtocol, Differentiable {
   public var x: Float
   public var y: Float
@@ -54,7 +54,7 @@ public func test1() -> Vector {
 // Should pass verification.
 @_fixed_layout
 public class NonTrivial {}
-@_fixed_layout
+@frozen
 public struct TF189: Differentiable {
   @noDerivative public let x: Double
   @noDerivative public let nonTrivial: NonTrivial