[stdlib] API additions for basic noncopyable primitives

- Add `exchange(_:with:)`
- Add `Optional.take()`
- Add `Unsafe[Mutable]BufferPointer.extracting(_:)`
- Finish generalizing `withExtendedLifetime`
- Radically simplify the implementation of `swap(_:_:)`

stdlib/public/core/LifetimeManager.swift

@@ -20,10 +20,10 @@
 ///     return value for the `withExtendedLifetime(_:_:)` method.
 /// - Returns: The return value, if any, of the `body` closure parameter.
 @_alwaysEmitIntoClient
-public func withExtendedLifetime<T: ~Copyable, Result: ~Copyable>(
+public func withExtendedLifetime<T: ~Copyable, E: Error, Result: ~Copyable>(
   _ x: borrowing T,
-  _ body: () throws -> Result // FIXME: Typed throws rdar://126576356
-) rethrows -> Result {
+  _ body: () throws(E) -> Result
+) throws(E) -> Result {
   defer { _fixLifetime(x) }
   return try body()
 }
@@ -32,8 +32,7 @@ public func withExtendedLifetime<T: ~Copyable, Result: ~Copyable>(
 @_silgen_name("$ss20withExtendedLifetimeyq_x_q_yKXEtKr0_lF")
 @usableFromInline
 internal func __abi_withExtendedLifetime<T, Result>(
-  _ x: T,
-  _ body: () throws -> Result // FIXME: Typed throws rdar://126576356
+  _ x: T, _ body: () throws -> Result
 ) rethrows -> Result {
   defer { _fixLifetime(x) }
   return try body()
@@ -49,9 +48,10 @@ internal func __abi_withExtendedLifetime<T, Result>(
 ///     return value for the `withExtendedLifetime(_:_:)` method.
 /// - Returns: The return value, if any, of the `body` closure parameter.
 @_alwaysEmitIntoClient
-public func withExtendedLifetime<T, Result: ~Copyable>(
-  _ x: T, _ body: (T) throws -> Result // FIXME: Typed throws rdar://126576356
-) rethrows -> Result {
+public func withExtendedLifetime<T: ~Copyable, E: Error, Result: ~Copyable>(
+  _ x: borrowing T,
+  _ body: (borrowing T) throws(E) -> Result
+) throws(E) -> Result {
   defer { _fixLifetime(x) }
   return try body(x)
 }
@@ -60,7 +60,7 @@ public func withExtendedLifetime<T, Result: ~Copyable>(
 @_silgen_name("$ss20withExtendedLifetimeyq_x_q_xKXEtKr0_lF")
 @usableFromInline
 internal func __abi_withExtendedLifetime<T, Result>(
-  _ x: T, _ body: (T) throws -> Result // FIXME: Typed throws rdar://126576356
+  _ x: T, _ body: (T) throws -> Result
 ) rethrows -> Result {
   defer { _fixLifetime(x) }
   return try body(x)

stdlib/public/core/MutableCollection.swift

@@ -529,24 +529,24 @@ extension MutableCollection {
 @inlinable
 @_preInverseGenerics
 public func swap<T: ~Copyable>(_ a: inout T, _ b: inout T) {
-  // Semantically equivalent to (a, b) = (b, a).
-  // Microoptimized to avoid retain/release traffic.
-#if $BuiltinUnprotectedAddressOf
-  let p1 = Builtin.unprotectedAddressOf(&a)
-  let p2 = Builtin.unprotectedAddressOf(&b)
-#else
-  let p1 = Builtin.addressof(&a)
-  let p2 = Builtin.addressof(&b)
-#endif
-  _debugPrecondition(
-    p1 != p2,
-    "swapping a location with itself is not supported")
-
-  // Take from P1.
-  let tmp: T = Builtin.take(p1)
-  // Transfer P2 into P1.
-  Builtin.initialize(Builtin.take(p2) as T, p1)
-  // Initialize P2.
-  Builtin.initialize(tmp, p2)
+  let temp = consume a
+  a = consume b
+  b = consume temp
 }
 
+/// Replaces the value of a mutable value with the supplied new value,
+/// returning the original.
+///
+/// - Parameters:
+///   - item: A mutable binding.
+///   - newValue: The new value of `item`.
+/// - Returns: The original value of `item`.
+@_alwaysEmitIntoClient
+public func exchange<T: ~Copyable>(
+  _ item: inout T,
+  with newValue: consuming T
+) -> T {
+  let oldValue = consume item
+  item = consume newValue
+  return oldValue
+}

stdlib/public/core/NFC.swift

@@ -236,7 +236,7 @@ extension Unicode {
 
       // If we have a leftover composee, make sure to return it.
       // We may still have things in the buffer which are not complete segments.
-      return composee._take() ?? buffer.next()?.scalar
+      return composee.take() ?? buffer.next()?.scalar
     }
   }
 }

stdlib/public/core/NFD.swift

@@ -279,7 +279,7 @@ extension Unicode._NFDNormalizer {
     // The buffer contains the decomposed segment *prior to*
     // any pending starter we might have.
 
-    return buffer.next() ?? pendingStarter._take()
+    return buffer.next() ?? pendingStarter.take()
   }
 
   @inline(__always)
@@ -287,7 +287,7 @@ extension Unicode._NFDNormalizer {
     _ nextFromSource: () -> Unicode.Scalar?
   ) -> ScalarAndNormData? {
 
-    if let pendingStarter = pendingStarter._take() {
+    if let pendingStarter = pendingStarter.take() {
       return pendingStarter
     } else if let nextScalar = nextFromSource() {
       return (nextScalar, Unicode._NormData(nextScalar))

stdlib/public/core/Optional.swift

@@ -232,7 +232,7 @@ extension Optional where Wrapped: ~Copyable {
   ) throws(E) -> U? {
     #if compiler(>=6.0) && $NoncopyableGenerics
     switch self {
-    case .some(_borrowing y):
+    case .some(let y):
       return .some(try transform(y))
     case .none:
       return .none
@@ -310,7 +310,7 @@ extension Optional where Wrapped: ~Copyable {
   ) throws(E) -> U? {
     #if compiler(>=6.0) && $NoncopyableGenerics
     switch self {
-    case .some(_borrowing y):
+    case .some(let y):
       return try transform(y)
     case .none:
       return .none
@@ -418,8 +418,8 @@ extension Optional where Wrapped: ~Copyable {
   ///
   /// - Returns: The wrapped value being stored in this instance. If this
   ///   instance is `nil`, returns `nil`.
-  @_alwaysEmitIntoClient // FIXME(NCG): Make this public.
-  public mutating func _take() -> Self {
+  @_alwaysEmitIntoClient
+  public mutating func take() -> Self {
     let result = consume self
     self = nil
     return result

stdlib/public/core/Result.swift

@@ -99,7 +99,7 @@ extension Result where Success: ~Copyable {
     _ transform: (borrowing Success) -> NewSuccess
   ) -> Result<NewSuccess, Failure> {
     switch self {
-    case .success(borrowing success):
+    case .success(let success):
       return .success(transform(success))
     case let .failure(failure):
       return .failure(failure)
@@ -241,7 +241,7 @@ extension Result where Success: ~Copyable {
     _ transform: (borrowing Success) -> Result<NewSuccess, Failure>
   ) -> Result<NewSuccess, Failure> {
     switch self {
-    case .success(borrowing success):
+    case .success(let success):
       return transform(success)
     case let .failure(failure):
       return .failure(failure)

stdlib/public/core/UnsafeBufferPointer.swift.gyb

@@ -456,6 +456,79 @@ extension Unsafe${Mutable}BufferPointer where Element: ~Copyable {
 
 }
 
+extension Unsafe${Mutable}BufferPointer where Element: ~Copyable {
+  /// Constructs a standalone buffer pointer over the items within the supplied
+  /// range of positions in the memory region addressed by this buffer pointer.
+  ///
+  /// The returned buffer's first item is always at index 0; unlike buffer
+  /// slices, extracted buffers do not generally share their indices with the
+  /// original buffer pointer.
+  ///
+  ///     withUnsafeTemporaryAllocation(of: Int.self, capacity: 5) { buffer in
+  ///       buffer.initialize(repeating: 0)
+  ///       // buffer contains [0, 0, 0, 0, 0]
+  ///       let part = buffer.extracting(2 ..< 4)
+  ///       part[0] = 1
+  ///       part[1] = 2
+  ///       // buffer now contains [0, 0, 1, 2, 0]
+  ///     }
+  ///
+  /// When `Element` is copyable, the `extracting` operation is equivalent to
+  /// slicing the buffer then rebasing the resulting buffer slice:
+  ///
+  ///     let a = buffer.extracting(i ..< j)
+  ///     let b = UnsafeBufferPointer(rebasing: buffer[i ..< j])
+  ///     // `a` and `b` are now holding the same buffer
+  ///
+  /// However, unlike slicing, the `extracting` operation remains available even
+  /// if `Element` happens to be noncopyable.
+  ///
+  /// - Parameter bounds: A valid range of indices within this buffer.
+  /// - Returns: A new buffer pointer over the items at `bounds`.
+  @_alwaysEmitIntoClient
+  public func extracting(_ bounds: Range<Int>) -> Self {
+    _precondition(bounds.lowerBound >= 0 && bounds.upperBound <= count,
+      "Index out of range")
+    guard let start = self.baseAddress else {
+      return Self(start: nil, count: 0)
+    }
+    return Self(start: start + bounds.lowerBound, count: bounds.count)
+  }
+
+  /// Constructs a standalone buffer pointer over the items within the supplied
+  /// range of positions in the memory region addressed by this buffer pointer.
+  ///
+  /// The returned buffer's first item is always at index 0; unlike buffer
+  /// slices, extracted buffers do not generally share their indices with the
+  /// original buffer pointer.
+  ///
+  ///     withUnsafeTemporaryAllocation(of: Int.self, capacity: 5) { buffer in
+  ///       buffer.initialize(repeating: 0)
+  ///       // buffer contains [0, 0, 0, 0, 0]
+  ///       let part = buffer.extracting(2...)
+  ///       part[0] = 1
+  ///       part[1] = 2
+  ///       // buffer now contains [0, 0, 1, 2, 0]
+  ///     }
+  ///
+  /// When `Element` is copyable, the `extracting` operation is equivalent to
+  /// slicing the buffer then rebasing the resulting buffer slice:
+  ///
+  ///     let a = buffer.extracting(i ..< j)
+  ///     let b = UnsafeBufferPointer(rebasing: buffer[i ..< j])
+  ///     // `a` and `b` are now holding the same buffer
+  ///
+  /// However, unlike slicing, the `extracting` operation remains available even
+  /// if `Element` happens to be noncopyable.
+  ///
+  /// - Parameter bounds: A valid range of indices within this buffer.
+  /// - Returns: A new buffer pointer over the items at `bounds`.
+  @_alwaysEmitIntoClient
+  public func extracting(_ bounds: some RangeExpression<Int>) -> Self {
+    extracting(bounds.relative(to: Range(uncheckedBounds: (0, count))))
+  }
+}
+
 @_disallowFeatureSuppression(NoncopyableGenerics)
 extension Unsafe${Mutable}BufferPointer {
   /// Accesses the element at the specified position.

test/IDE/complete_generic_optional.swift

@@ -13,6 +13,6 @@ struct Foo<T> {
 // in an optional
 let x: Foo<Bar>? = Foo<Bar>()
 x.#^FOO_OPTIONAL_1^#
-// FOO_OPTIONAL_1: Begin completions, 7 items
+// FOO_OPTIONAL_1: Begin completions, 8 items
 // FOO_OPTIONAL_1-DAG: Decl[InstanceMethod]/CurrNominal/Erase[1]: ?.myFunction({#(foobar): Bar#})[#Void#]; name=myFunction(:)
 // FOO_OPTIONAL_1-DAG: Keyword[self]/CurrNominal: self[#Foo<Bar>?#]; name=self

test/IDE/complete_unresolved_members.swift

@@ -148,7 +148,7 @@ class C4 {
 // UNRESOLVED_3-DAG: Decl[InstanceMethod]/CurrNominal/TypeRelation[Invalid]: hash({#(self): SomeEnum1#})[#(into: inout Hasher) -> Void#]; name=hash(:)
 
 // Exhaustive to make sure we don't include `init({#(some):` or `init({#nilLiteral:` entries
-// UNRESOLVED_3_OPT: Begin completions, 9 items
+// UNRESOLVED_3_OPT: Begin completions, 10 items
 // UNRESOLVED_3_OPT-DAG: Decl[EnumElement]/CurrNominal/Flair[ExprSpecific]/TypeRelation[Convertible]: North[#SomeEnum1#];
 // UNRESOLVED_3_OPT-DAG: Decl[EnumElement]/CurrNominal/Flair[ExprSpecific]/TypeRelation[Convertible]: South[#SomeEnum1#];
 // UNRESOLVED_3_OPT-DAG: Decl[InstanceMethod]/CurrNominal/TypeRelation[Invalid]: hash({#(self): SomeEnum1#})[#(into: inout Hasher) -> Void#];
@@ -160,7 +160,7 @@ class C4 {
 // UNRESOLVED_3_OPT-DAG: Decl[InstanceMethod]/CurrNominal/IsSystem/TypeRelation[Invalid]: hash({#(self): Optional<SomeEnum1>#})[#(into: inout Hasher) -> Void#];
 
 // Exhaustive to make sure we don't include `init({#(some):` or `init({#nilLiteral:` entries
-// UNRESOLVED_3_OPTOPTOPT: Begin completions, 9 items
+// UNRESOLVED_3_OPTOPTOPT: Begin completions, 10 items
 // UNRESOLVED_3_OPTOPTOPT-DAG: Decl[EnumElement]/CurrNominal/Flair[ExprSpecific]/TypeRelation[Convertible]: North[#SomeEnum1#];
 // UNRESOLVED_3_OPTOPTOPT-DAG: Decl[EnumElement]/CurrNominal/Flair[ExprSpecific]/TypeRelation[Convertible]: South[#SomeEnum1#];
 // UNRESOLVED_3_OPTOPTOPT-DAG: Decl[InstanceMethod]/CurrNominal/TypeRelation[Invalid]: hash({#(self): SomeEnum1#})[#(into: inout Hasher) -> Void#];
@@ -169,6 +169,7 @@ class C4 {
 // UNRESOLVED_3_OPTOPTOPT-DAG: Decl[EnumElement]/CurrNominal/IsSystem/TypeRelation[Convertible]: some({#SomeEnum1??#})[#Optional<SomeEnum1??>#];
 // UNRESOLVED_3_OPTOPTOPT-DAG: Decl[InstanceMethod]/CurrNominal/IsSystem: map({#(self): Optional<SomeEnum1??>#})[#((SomeEnum1??) throws(Error) -> ~Copyable) -> ~Copyable?#];
 // UNRESOLVED_3_OPTOPTOPT-DAG: Decl[InstanceMethod]/CurrNominal/IsSystem: flatMap({#(self): Optional<SomeEnum1??>#})[#((SomeEnum1??) throws(Error) -> ~Copyable?) -> ~Copyable?#];
+// UNRESOLVED_3_OPTOPTOPT-DAG: Decl[InstanceMethod]/CurrNominal/IsSystem: take({#(self): &Optional<SomeEnum1??>#})[#() -> Optional<SomeEnum1??>#];
 // UNRESOLVED_3_OPTOPTOPT-DAG: Decl[InstanceMethod]/CurrNominal/IsSystem/TypeRelation[Invalid]: hash({#(self): Optional<SomeEnum1??>#})[#(into: inout Hasher) -> Void#];
 
 enum Somewhere {
@@ -180,7 +181,7 @@ extension Optional where Wrapped == Somewhere {
 }
 func testOptionalWithCustomExtension() {
   var _: Somewhere? = .#^UNRESOLVED_OPT_4^#
-// UNRESOLVED_OPT_4: Begin completions, 11 items
+// UNRESOLVED_OPT_4: Begin completions, 12 items
 // UNRESOLVED_OPT_4-DAG: Decl[EnumElement]/CurrNominal/Flair[ExprSpecific]/TypeRelation[Convertible]:     earth[#Somewhere#];
 // UNRESOLVED_OPT_4-DAG: Decl[EnumElement]/CurrNominal/Flair[ExprSpecific]/TypeRelation[Convertible]:     mars[#Somewhere#];
 // UNRESOLVED_OPT_4-DAG: Decl[InstanceMethod]/CurrNominal/TypeRelation[Invalid]: hash({#(self): Somewhere#})[#(into: inout Hasher) -> Void#];
@@ -191,6 +192,7 @@ func testOptionalWithCustomExtension() {
 // UNRESOLVED_OPT_4-DAG: Decl[StaticVar]/CurrNominal/TypeRelation[Convertible]: nowhere[#Optional<Somewhere>#]; name=nowhere
 // UNRESOLVED_OPT_4-DAG: Decl[InstanceMethod]/CurrNominal/IsSystem: map({#(self): Optional<Somewhere>#})[#((Somewhere) throws(Error) -> ~Copyable) -> ~Copyable?#];
 // UNRESOLVED_OPT_4-DAG: Decl[InstanceMethod]/CurrNominal/IsSystem: flatMap({#(self): Optional<Somewhere>#})[#((Somewhere) throws(Error) -> ~Copyable?) -> ~Copyable?#];
+// UNRESOLVED_OPT_4-DAG: Decl[InstanceMethod]/CurrNominal/IsSystem: take({#(self): &Optional<Somewhere>#})[#() -> Optional<Somewhere>#];
 // UNRESOLVED_OPT_4-DAG: Decl[InstanceMethod]/CurrNominal/IsSystem/TypeRelation[Invalid]: hash({#(self): Optional<Somewhere>#})[#(into: inout Hasher) -> Void#];
 // UNRESOLVED_OPT_4-NOT: init({#(some):
 // UNRESOLVED_OPT_4-NOT: init({#nilLiteral:
@@ -688,7 +690,7 @@ func testSameType() {
 
   testSugarType(.#^SUGAR_TYPE^#
 // Ensure results aren't duplicated.
-// SUGAR_TYPE: Begin completions, 9 items
+// SUGAR_TYPE: Begin completions, 10 items
 // SUGAR_TYPE-DAG: Decl[EnumElement]/CurrNominal/Flair[ExprSpecific]/TypeRelation[Convertible]: South[#SomeEnum1#];
 // SUGAR_TYPE-DAG: Decl[EnumElement]/CurrNominal/Flair[ExprSpecific]/TypeRelation[Convertible]: North[#SomeEnum1#];
 // SUGAR_TYPE-DAG: Decl[InstanceMethod]/CurrNominal/TypeRelation[Invalid]: hash({#(self): SomeEnum1#})[#(into: inout Hasher) -> Void#];
@@ -697,6 +699,7 @@ func testSameType() {
 // SUGAR_TYPE-DAG: Decl[EnumElement]/CurrNominal/IsSystem/TypeRelation[Convertible]: some({#SomeEnum1#})[#Optional<SomeEnum1>#];
 // SUGAR_TYPE-DAG: Decl[InstanceMethod]/CurrNominal/IsSystem: map({#(self): Optional<SomeEnum1>#})[#((SomeEnum1) throws(Error) -> ~Copyable) -> ~Copyable?#];
 // SUGAR_TYPE-DAG: Decl[InstanceMethod]/CurrNominal/IsSystem: flatMap({#(self): Optional<SomeEnum1>#})[#((SomeEnum1) throws(Error) -> ~Copyable?) -> ~Copyable?#];
+// SUGAR_TYPE-DAG: Decl[InstanceMethod]/CurrNominal/IsSystem: take({#(self): &Optional<SomeEnum1>#})[#() -> Optional<SomeEnum1>#];
 // SUGAR_TYPE-DAG: Decl[InstanceMethod]/CurrNominal/IsSystem/TypeRelation[Invalid]: hash({#(self): Optional<SomeEnum1>#})[#(into: inout Hasher) -> Void#];
 }
 

test/Prototypes/Hypoarray.swift

@@ -0,0 +1,293 @@
+//===--- Algorithms.swift -------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2024 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift -swift-version 6 -o %t/a.out %s
+// RUN: %target-codesign %t/a.out
+// RUN: %target-run %t/a.out
+// REQUIRES: executable_test
+
+import StdlibUnittest
+import Synchronization
+
+struct Hypoarray<Element: ~Copyable>: ~Copyable {
+  private var _storage: UnsafeMutableBufferPointer<Element>
+  private var _count: Int
+
+  var capacity: Int { _storage.count }
+
+  init() {
+    _storage = .init(start: nil, count: 0)
+    _count = 0
+  }
+
+  init(_ element: consuming Element) {
+    _storage = .allocate(capacity: 1)
+    _storage.initializeElement(at: 0, to: element)
+    _count = 1
+  }
+
+  init(count: Int, initializedBy generator: (Int) -> Element) {
+    _storage = .allocate(capacity: count)
+    for i in 0 ..< count {
+      _storage.initializeElement(at: i, to: generator(i))
+    }
+    _count = count
+  }
+
+  deinit {
+    _storage.extracting(0 ..< count).deinitialize()
+    _storage.deallocate()
+  }
+}
+
+extension Hypoarray: @unchecked Sendable where Element: Sendable & ~Copyable {}
+
+extension Hypoarray where Element: ~Copyable {
+  typealias Index = Int
+
+  var isEmpty: Bool { _count == 0 }
+  var count: Int { _count }
+
+  var startIndex: Int { 0 }
+  var endIndex: Int { _count }
+  func index(after i: Int) -> Int { i + 1 }
+  func index(before i: Int) -> Int { i - 1 }
+  func distance(from start: Int, to end: Int) -> Int { end - start }
+  // etc.
+}
+
+extension Hypoarray where Element: ~Copyable {
+  func borrowElement<E: Error, R: ~Copyable> (
+    at index: Int,
+    by body: (borrowing Element) throws(E) -> R
+  ) throws(E) -> R {
+    precondition(index >= 0 && index < _count)
+    return try body(_storage[index])
+  }
+
+  mutating func updateElement<E: Error, R: ~Copyable> (
+    at index: Int,
+    by body: (inout Element) throws(E) -> R
+  ) throws(E) -> R {
+    precondition(index >= 0 && index < _count)
+    return try body(&_storage[index])
+  }
+}
+
+extension Hypoarray where Element: ~Copyable {
+  subscript(position: Int) -> Element {
+    _read {
+      precondition(position >= 0 && position < _count)
+      yield _storage[position]
+    }
+    _modify {
+      precondition(position >= 0 && position < _count)
+      yield &_storage[position]
+    }
+  }
+}
+
+extension Hypoarray where Element: ~Copyable {
+  @discardableResult
+  mutating func remove(at index: Int) -> Element {
+    precondition(index >= 0 && index < count)
+    let old = _storage.moveElement(from: index)
+    let source = _storage.extracting(index + 1 ..< count)
+    let target = _storage.extracting(index ..< count - 1)
+    let i = target.moveInitialize(fromContentsOf: source)
+    assert(i == target.endIndex)
+    _count -= 1
+    return old
+  }
+}
+
+extension Hypoarray where Element: ~Copyable {
+  mutating func reserveCapacity(_ n: Int) {
+    guard capacity < n else { return }
+    let newStorage: UnsafeMutableBufferPointer<Element> = .allocate(capacity: n)
+    let source = _storage.extracting(0 ..< count)
+    let i = newStorage.moveInitialize(fromContentsOf: source)
+    assert(i == count)
+    _storage.deallocate()
+    _storage = newStorage
+  }
+}
+
+extension Hypoarray where Element: ~Copyable {
+  mutating func _ensureFreeCapacity(_ minimumCapacity: Int) {
+    guard capacity < _count + minimumCapacity else { return }
+    reserveCapacity(max(_count + minimumCapacity, 2 * capacity))
+  }
+}
+
+extension Hypoarray where Element: ~Copyable {
+  mutating func append(_ item: consuming Element) {
+    _ensureFreeCapacity(1)
+    _storage.initializeElement(at: _count, to: item)
+    _count += 1
+  }
+}
+
+extension Hypoarray where Element: ~Copyable {
+  mutating func insert(_ item: consuming Element, at index: Int) {
+    precondition(index >= 0 && index <= count)
+    _ensureFreeCapacity(1)
+    if index < count {
+      let source = _storage.extracting(index ..< count)
+      let target = _storage.extracting(index + 1 ..< count + 1)
+      let last = target.moveInitialize(fromContentsOf: source)
+      assert(last == target.endIndex)
+    }
+    _storage.initializeElement(at: index, to: item)
+    _count += 1
+  }
+}
+
+extension Hypoarray {
+  mutating func append(contentsOf items: some Sequence<Element>) {
+    for item in items {
+      append(item)
+    }
+  }
+}
+
+struct Counted: ~Copyable {
+  var value: Int
+  nonisolated(unsafe) static var instances: Int = 0
+
+  init(_ value: Int) {
+    self.value = value
+    Counted.instances += 1
+  }
+
+  deinit {
+    Counted.instances -= 1
+    expectGE(Counted.instances, 0)
+  }
+}
+
+var suite = TestSuite("Hypoarray")
+defer { runAllTests() }
+
+suite.test("basics") {
+  var array = Hypoarray(Counted(42))
+  expectFalse(array.isEmpty)
+  expectEqual(array.count, 1)
+  expectEqual(array[0].value, 42)
+  expectEqual(Counted.instances, 1)
+
+  array.append(Counted(23))
+  expectFalse(array.isEmpty)
+  expectEqual(array.count, 2)
+  expectEqual(array[0].value, 42)
+  expectEqual(array[1].value, 23)
+  expectEqual(Counted.instances, 2)
+
+  let old = array.remove(at: 0)
+  expectEqual(old.value, 42)
+  expectFalse(array.isEmpty)
+  expectEqual(array.count, 1)
+  expectEqual(array[0].value, 23)
+  expectEqual(Counted.instances, 2)
+  _ = consume old
+  expectEqual(Counted.instances, 1)
+
+  let old2 = array.remove(at: 0)
+  expectEqual(old2.value, 23)
+  expectEqual(array.count, 0)
+  expectTrue(array.isEmpty)
+  expectEqual(Counted.instances, 1)
+  _ = consume old2
+  expectEqual(Counted.instances, 0)
+}
+
+suite.test("read access") {
+  let c = 100
+  let array = Hypoarray<Counted>(count: c) { Counted($0) }
+
+  for i in 0 ..< c {
+    expectEqual(array.borrowElement(at: i) { $0.value }, i)
+    expectEqual(array[i].value, i)
+  }
+}
+
+suite.test("update access") {
+  let c = 100
+  var array = Hypoarray<Counted>(count: c) { Counted($0) }
+
+  for i in 0 ..< c {
+    array.updateElement(at: i) { $0.value += 100 }
+    array[i].value += 100
+  }
+
+  for i in 0 ..< c {
+    expectEqual(array[i].value, 200 + i)
+  }
+
+  expectEqual(Counted.instances, c)
+  _ = consume array
+  expectEqual(Counted.instances, 0)
+}
+
+suite.test("append") {
+  var array = Hypoarray<Counted>()
+  let c = 100
+  for i in 0 ..< c {
+    array.append(Counted(100 + i))
+  }
+  expectEqual(Counted.instances, c)
+  expectEqual(array.count, c)
+
+  for i in 0 ..< c {
+    // FIXME: unexpected exclusivity violation (rdar://128441125)
+    //expectEqual(array.borrowElement(at: i) { $0.value }, 100 + i)
+    expectEqual(array[i].value, 100 + i)
+  }
+
+  _ = consume array
+  expectEqual(Counted.instances, 0)
+}
+
+suite.test("insert") {
+  var array = Hypoarray<Counted>()
+  let c = 100
+  for i in 0 ..< c {
+    array.insert(Counted(100 + i), at: 0)
+  }
+  expectEqual(Counted.instances, c)
+  expectEqual(array.count, c)
+
+  for i in 0 ..< c {
+    // FIXME: unexpected exclusivity violation (rdar://128441125)
+    //expectEqual(array.borrowElement(at: i) { $0.value }, c + 99 - i)
+    expectEqual(array[i].value, c + 99 - i)
+  }
+
+  _ = consume array
+  expectEqual(Counted.instances, 0)
+}
+
+suite.test("remove") {
+  let c = 100
+  var array = Hypoarray<Counted>(count: c) { Counted(100 + $0) }
+  expectEqual(Counted.instances, c)
+  expectEqual(array.count, c)
+
+  for i in 0 ..< c {
+    array.remove(at: 0)
+    expectEqual(array.count, c - 1 - i)
+    expectEqual(Counted.instances, c - 1 - i)
+  }
+
+  expectTrue(array.isEmpty)
+  expectEqual(Counted.instances, 0)
+}

test/stdlib/swap.swift

@@ -0,0 +1,78 @@
+// RUN: %target-run-simple-swift
+// REQUIRES: executable_test
+
+import StdlibUnittest
+
+var suite = TestSuite("Unmanaged")
+defer { runAllTests() }
+
+struct Counted: ~Copyable {
+  let value: Int
+  static var instances: Int = 0
+
+  init(_ value: Int) {
+    self.value = value
+    Counted.instances += 1
+  }
+
+  deinit {
+    Counted.instances -= 1
+    expectGE(Counted.instances, 0)
+  }
+}
+
+suite.test("swap.Int") {
+  var a = 1
+  var b = 2
+  swap(&a, &b)
+  expectEqual(a, 2)
+  expectEqual(b, 1)
+}
+
+suite.test("exchange.Int") {
+  var a = 1
+  let old = exchange(&a, with: 2)
+  expectEqual(old, 1)
+  expectEqual(a, 2)
+}
+
+suite.test("swap.class") {
+  var a = LifetimeTracked(10)
+  var b = LifetimeTracked(20)
+  swap(&a, &b)
+  expectEqual(LifetimeTracked.instances, 2)
+  expectEqual(a.value, 20)
+  expectEqual(b.value, 10)
+}
+
+suite.test("exchange.class") {
+  var a = LifetimeTracked(10)
+  let old = exchange(&a, with: LifetimeTracked(20))
+  expectEqual(LifetimeTracked.instances, 2)
+  expectEqual(old.value, 10)
+  expectEqual(a.value, 20)
+}
+
+suite.test("swap.noncopyable") {
+  var a = Counted(10)
+  var b = Counted(20)
+  expectEqual(Counted.instances, 2)
+  swap(&a, &b)
+  expectEqual(Counted.instances, 2)
+  expectEqual(a.value, 20)
+  expectEqual(b.value, 10)
+  _ = consume a
+  _ = consume b
+  expectEqual(Counted.instances, 0)
+}
+
+suite.test("exchange.noncopyable") {
+  var a = Counted(10)
+  let old = exchange(&a, with: Counted(20))
+  expectEqual(Counted.instances, 2)
+  expectEqual(a.value, 20)
+  expectEqual(old.value, 10)
+  _ = consume old
+  _ = consume a
+  expectEqual(Counted.instances, 0)
+}