[cxx-interop] Add CxxConvertibleToCollection protocol

stdlib/public/Cxx/CMakeLists.txt

@@ -1,4 +1,5 @@
 add_swift_target_library(swiftCxx ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_SDK_OVERLAY
+    CxxConvertibleToCollection.swift
     CxxRandomAccessCollection.swift
     CxxSequence.swift
 

stdlib/public/Cxx/CxxConvertibleToCollection.swift

@@ -0,0 +1,70 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2022 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
+//
+//===----------------------------------------------------------------------===//
+
+/// A C++ type that can be converted to a Swift collection.
+public protocol CxxConvertibleToCollection {
+  associatedtype RawIterator: UnsafeCxxInputIterator
+
+  /// Do not implement this function manually in Swift.
+  mutating func __beginUnsafe() -> RawIterator
+
+  /// Do not implement this function manually in Swift.
+  mutating func __endUnsafe() -> RawIterator
+}
+
+@inlinable @inline(__always)
+internal func forEachElement<C: CxxConvertibleToCollection>(
+  of c: C,
+  body: (C.RawIterator.Pointee) -> Void
+) {
+  var mutableC = c
+  withExtendedLifetime(mutableC) {
+    var rawIterator = mutableC.__beginUnsafe()
+    let endIterator = mutableC.__endUnsafe()
+    while rawIterator != endIterator {
+      body(rawIterator.pointee)
+      rawIterator = rawIterator.successor()
+    }
+  }
+}
+
+extension Array {
+  /// Creates an array containing the elements of a C++ collection.
+  ///
+  /// This initializer copies each element of the C++ collection to a new Swift
+  /// array.
+  ///
+  /// - Complexity: O(*n*), where *n* is the number of elements in the C++
+  ///   collection.
+  public init<C: CxxConvertibleToCollection>(_ c: C)
+    where C.RawIterator.Pointee == Element {
+
+    self.init()
+    forEachElement(of: c) { self.append($0) }
+  }
+}
+
+extension Set {
+  /// Creates an set containing the elements of a C++ collection.
+  ///
+  /// This initializer copies each element of the C++ collection to a new Swift
+  /// set.
+  ///
+  /// - Complexity: O(*n*), where *n* is the number of elements in the C++
+  ///   collection.
+  public init<C: CxxConvertibleToCollection>(_ c: C)
+    where C.RawIterator.Pointee == Element {
+
+    self.init()
+    forEachElement(of: c) { self.insert($0) }
+  }
+}

test/Interop/Cxx/stdlib/overlay/custom-convertible-to-collection.swift

@@ -0,0 +1,40 @@
+// RUN: %target-run-simple-swift(-I %S/Inputs -Xfrontend -enable-experimental-cxx-interop)
+//
+// REQUIRES: executable_test
+// REQUIRES: OS=macosx || OS=linux-gnu
+
+import StdlibUnittest
+import CustomSequence
+import Cxx
+
+var CxxSequenceTestSuite = TestSuite("CxxConvertibleToCollection")
+
+extension SimpleSequence: CxxConvertibleToCollection {}
+
+CxxSequenceTestSuite.test("SimpleSequence to Swift.Array") {
+  let seq = SimpleSequence()
+  let array = Array(seq)
+  expectEqual([1, 2, 3, 4] as [Int32], array)
+}
+
+CxxSequenceTestSuite.test("SimpleSequence to Swift.Set") {
+  let seq = SimpleSequence()
+  let set = Set(seq)
+  expectEqual(Set([1, 2, 3, 4] as [Int32]), set)
+}
+
+extension SimpleEmptySequence: CxxConvertibleToCollection {}
+
+CxxSequenceTestSuite.test("SimpleEmptySequence to Swift.Array") {
+  let seq = SimpleEmptySequence()
+  let array = Array(seq)
+  expectTrue(array.isEmpty)
+}
+
+CxxSequenceTestSuite.test("SimpleEmptySequence to Swift.Set") {
+  let seq = SimpleEmptySequence()
+  let set = Set(seq)
+  expectTrue(set.isEmpty)
+}
+
+runAllTests()