[cxx-interop] Add UnsafeCxxContiguousIterator & UnsafeCxxMutableContiguousIterator protocols

This adds a pair of Swift protocols that represents C++ iterator types conforming to `std::contiguous_iterator_tag` requirements. These are random access iterators that guarantee that the values are stored in consequent memory addresses.

This will be used to optimize usage of C++ containers such as `std::vector` from Swift, for instance, by providing an overload of `withContiguousStorageIfAvailable` for contiguous containers.

rdar://137877849

include/swift/AST/KnownProtocols.def

@@ -142,6 +142,8 @@ PROTOCOL(UnsafeCxxInputIterator)
 PROTOCOL(UnsafeCxxMutableInputIterator)
 PROTOCOL(UnsafeCxxRandomAccessIterator)
 PROTOCOL(UnsafeCxxMutableRandomAccessIterator)
+PROTOCOL(UnsafeCxxContiguousIterator)
+PROTOCOL(UnsafeCxxMutableContiguousIterator)
 
 PROTOCOL(AsyncSequence)
 PROTOCOL(AsyncIteratorProtocol)

lib/AST/ASTContext.cpp

@@ -1444,6 +1444,8 @@ ProtocolDecl *ASTContext::getProtocol(KnownProtocolKind kind) const {
   case KnownProtocolKind::UnsafeCxxMutableInputIterator:
   case KnownProtocolKind::UnsafeCxxRandomAccessIterator:
   case KnownProtocolKind::UnsafeCxxMutableRandomAccessIterator:
+  case KnownProtocolKind::UnsafeCxxContiguousIterator:
+  case KnownProtocolKind::UnsafeCxxMutableContiguousIterator:
     M = getLoadedModule(Id_Cxx);
     break;
   case KnownProtocolKind::Copyable:

lib/ClangImporter/ClangDerivedConformances.cpp

@@ -462,12 +462,16 @@ void swift::conformToCxxIteratorIfNeeded(
   auto isRandomAccessIteratorDecl = [&](const clang::CXXRecordDecl *base) {
     return isIteratorCategoryDecl(base, "random_access_iterator_tag");
   };
+  auto isContiguousIteratorDecl = [&](const clang::CXXRecordDecl *base) {
+    return isIteratorCategoryDecl(base, "contiguous_iterator_tag"); // C++20
+  };
 
   // Traverse all transitive bases of `underlyingDecl` to check if
   // it inherits from `std::input_iterator_tag`.
   bool isInputIterator = isInputIteratorDecl(underlyingCategoryDecl);
   bool isRandomAccessIterator =
       isRandomAccessIteratorDecl(underlyingCategoryDecl);
+  bool isContiguousIterator = isContiguousIteratorDecl(underlyingCategoryDecl);
   underlyingCategoryDecl->forallBases([&](const clang::CXXRecordDecl *base) {
     if (isInputIteratorDecl(base)) {
       isInputIterator = true;
@@ -475,6 +479,11 @@ void swift::conformToCxxIteratorIfNeeded(
     if (isRandomAccessIteratorDecl(base)) {
       isRandomAccessIterator = true;
       isInputIterator = true;
+    }
+    if (isContiguousIteratorDecl(base)) {
+      isContiguousIterator = true;
+      isRandomAccessIterator = true;
+      isInputIterator = true;
       return false;
     }
     return true;
@@ -594,6 +603,15 @@ void swift::conformToCxxIteratorIfNeeded(
   else
     impl.addSynthesizedProtocolAttrs(
         decl, {KnownProtocolKind::UnsafeCxxRandomAccessIterator});
+
+  if (isContiguousIterator) {
+    if (pointeeSettable)
+      impl.addSynthesizedProtocolAttrs(
+          decl, {KnownProtocolKind::UnsafeCxxMutableContiguousIterator});
+    else
+      impl.addSynthesizedProtocolAttrs(
+          decl, {KnownProtocolKind::UnsafeCxxContiguousIterator});
+  }
 }
 
 void swift::conformToCxxConvertibleToBoolIfNeeded(

lib/IRGen/GenMeta.cpp

@@ -6967,6 +6967,8 @@ SpecialProtocol irgen::getSpecialProtocolID(ProtocolDecl *P) {
   case KnownProtocolKind::UnsafeCxxMutableInputIterator:
   case KnownProtocolKind::UnsafeCxxRandomAccessIterator:
   case KnownProtocolKind::UnsafeCxxMutableRandomAccessIterator:
+  case KnownProtocolKind::UnsafeCxxContiguousIterator:
+  case KnownProtocolKind::UnsafeCxxMutableContiguousIterator:
   case KnownProtocolKind::Executor:
   case KnownProtocolKind::SerialExecutor:
   case KnownProtocolKind::TaskExecutor:

stdlib/public/Cxx/UnsafeCxxIterators.swift

@@ -87,3 +87,15 @@ public protocol UnsafeCxxMutableRandomAccessIterator:
 UnsafeCxxRandomAccessIterator, UnsafeCxxMutableInputIterator {}
 
 extension UnsafeMutablePointer: UnsafeCxxMutableRandomAccessIterator {}
+
+/// Bridged C++ iterator that allows traversing elements of a random access
+/// collection that are stored in contiguous memory segments.
+///
+/// Mostly useful for optimizing operations with containers that conform to
+/// `CxxRandomAccessCollection` and should not generally be used directly.
+///
+/// - SeeAlso: https://en.cppreference.com/w/cpp/named_req/ContiguousIterator
+public protocol UnsafeCxxContiguousIterator: UnsafeCxxRandomAccessIterator {}
+
+public protocol UnsafeCxxMutableContiguousIterator:
+UnsafeCxxContiguousIterator, UnsafeCxxMutableRandomAccessIterator {}

test/Interop/Cxx/stdlib/overlay/Inputs/custom-iterator.h

@@ -286,6 +286,229 @@ struct HasTypedefIteratorTag {
   }
 };
 
+struct MutableRACIterator {
+private:
+  int *value;
+
+public:
+  struct iterator_category : std::random_access_iterator_tag,
+                             std::output_iterator_tag {};
+  using value_type = int;
+  using pointer = int *;
+  using reference = const int &;
+  using difference_type = int;
+
+  MutableRACIterator(int *value) : value(value) {}
+  MutableRACIterator(const MutableRACIterator &other) = default;
+
+  const int &operator*() const { return *value; }
+  int &operator*() { return *value; }
+
+  MutableRACIterator &operator++() {
+    value++;
+    return *this;
+  }
+  MutableRACIterator operator++(int) {
+    auto tmp = MutableRACIterator(value);
+    value++;
+    return tmp;
+  }
+
+  void operator+=(difference_type v) { value += v; }
+  void operator-=(difference_type v) { value -= v; }
+  MutableRACIterator operator+(difference_type v) const {
+    return MutableRACIterator(value + v);
+  }
+  MutableRACIterator operator-(difference_type v) const {
+    return MutableRACIterator(value - v);
+  }
+  friend MutableRACIterator operator+(difference_type v,
+                                      const MutableRACIterator &it) {
+    return it + v;
+  }
+  int operator-(const MutableRACIterator &other) const {
+    return value - other.value;
+  }
+
+  bool operator<(const MutableRACIterator &other) const {
+    return value < other.value;
+  }
+
+  bool operator==(const MutableRACIterator &other) const {
+    return value == other.value;
+  }
+  bool operator!=(const MutableRACIterator &other) const {
+    return value != other.value;
+  }
+};
+
+#if __cplusplus >= 202002L
+struct ConstContiguousIterator {
+private:
+  const int *value;
+
+public:
+  using iterator_category = std::contiguous_iterator_tag;
+  using value_type = int;
+  using pointer = int *;
+  using reference = const int &;
+  using difference_type = int;
+
+  ConstContiguousIterator(const int *value) : value(value) {}
+  ConstContiguousIterator(const ConstContiguousIterator &other) = default;
+
+  const int &operator*() const { return *value; }
+
+  ConstContiguousIterator &operator++() {
+    value++;
+    return *this;
+  }
+  ConstContiguousIterator operator++(int) {
+    auto tmp = ConstContiguousIterator(value);
+    value++;
+    return tmp;
+  }
+
+  void operator+=(difference_type v) { value += v; }
+  void operator-=(difference_type v) { value -= v; }
+  ConstContiguousIterator operator+(difference_type v) const {
+    return ConstContiguousIterator(value + v);
+  }
+  ConstContiguousIterator operator-(difference_type v) const {
+    return ConstContiguousIterator(value - v);
+  }
+  friend ConstContiguousIterator operator+(difference_type v,
+                                           const ConstContiguousIterator &it) {
+    return it + v;
+  }
+  int operator-(const ConstContiguousIterator &other) const {
+    return value - other.value;
+  }
+
+  bool operator<(const ConstContiguousIterator &other) const {
+    return value < other.value;
+  }
+
+  bool operator==(const ConstContiguousIterator &other) const {
+    return value == other.value;
+  }
+  bool operator!=(const ConstContiguousIterator &other) const {
+    return value != other.value;
+  }
+};
+
+struct HasCustomContiguousIteratorTag {
+private:
+  const int *value;
+
+public:
+  struct CustomTag : std::contiguous_iterator_tag {};
+  using iterator_category = CustomTag;
+  using value_type = int;
+  using pointer = int *;
+  using reference = const int &;
+  using difference_type = int;
+
+  HasCustomContiguousIteratorTag(const int *value) : value(value) {}
+  HasCustomContiguousIteratorTag(const HasCustomContiguousIteratorTag &other) =
+      default;
+
+  const int &operator*() const { return *value; }
+
+  HasCustomContiguousIteratorTag &operator++() {
+    value++;
+    return *this;
+  }
+  HasCustomContiguousIteratorTag operator++(int) {
+    auto tmp = HasCustomContiguousIteratorTag(value);
+    value++;
+    return tmp;
+  }
+
+  void operator+=(difference_type v) { value += v; }
+  void operator-=(difference_type v) { value -= v; }
+  HasCustomContiguousIteratorTag operator+(difference_type v) const {
+    return HasCustomContiguousIteratorTag(value + v);
+  }
+  HasCustomContiguousIteratorTag operator-(difference_type v) const {
+    return HasCustomContiguousIteratorTag(value - v);
+  }
+  friend HasCustomContiguousIteratorTag
+  operator+(difference_type v, const HasCustomContiguousIteratorTag &it) {
+    return it + v;
+  }
+  int operator-(const HasCustomContiguousIteratorTag &other) const {
+    return value - other.value;
+  }
+
+  bool operator<(const HasCustomContiguousIteratorTag &other) const {
+    return value < other.value;
+  }
+
+  bool operator==(const HasCustomContiguousIteratorTag &other) const {
+    return value == other.value;
+  }
+  bool operator!=(const HasCustomContiguousIteratorTag &other) const {
+    return value != other.value;
+  }
+};
+
+struct MutableContiguousIterator {
+private:
+  int *value;
+
+public:
+  using iterator_category = std::contiguous_iterator_tag;
+  using value_type = int;
+  using pointer = int *;
+  using reference = const int &;
+  using difference_type = int;
+
+  MutableContiguousIterator(int *value) : value(value) {}
+  MutableContiguousIterator(const MutableContiguousIterator &other) = default;
+
+  const int &operator*() const { return *value; }
+  int &operator*() { return *value; }
+
+  MutableContiguousIterator &operator++() {
+    value++;
+    return *this;
+  }
+  MutableContiguousIterator operator++(int) {
+    auto tmp = MutableContiguousIterator(value);
+    value++;
+    return tmp;
+  }
+
+  void operator+=(difference_type v) { value += v; }
+  void operator-=(difference_type v) { value -= v; }
+  MutableContiguousIterator operator+(difference_type v) const {
+    return MutableContiguousIterator(value + v);
+  }
+  MutableContiguousIterator operator-(difference_type v) const {
+    return MutableContiguousIterator(value - v);
+  }
+  friend MutableContiguousIterator
+  operator+(difference_type v, const MutableContiguousIterator &it) {
+    return it + v;
+  }
+  int operator-(const MutableContiguousIterator &other) const {
+    return value - other.value;
+  }
+
+  bool operator<(const MutableContiguousIterator &other) const {
+    return value < other.value;
+  }
+
+  bool operator==(const MutableContiguousIterator &other) const {
+    return value == other.value;
+  }
+  bool operator!=(const MutableContiguousIterator &other) const {
+    return value != other.value;
+  }
+};
+#endif
+
 // MARK: Types that are not actually iterators
 
 struct HasNoIteratorCategory {
@@ -916,62 +1139,6 @@ public:
   }
 };
 
-struct MutableRACIterator {
-private:
-  int *value;
-
-public:
-  struct iterator_category : std::random_access_iterator_tag,
-                             std::output_iterator_tag {};
-  using value_type = int;
-  using pointer = int *;
-  using reference = const int &;
-  using difference_type = int;
-
-  MutableRACIterator(int *value) : value(value) {}
-  MutableRACIterator(const MutableRACIterator &other) = default;
-
-  const int &operator*() const { return *value; }
-  int &operator*() { return *value; }
-
-  MutableRACIterator &operator++() {
-    value++;
-    return *this;
-  }
-  MutableRACIterator operator++(int) {
-    auto tmp = MutableRACIterator(value);
-    value++;
-    return tmp;
-  }
-
-  void operator+=(difference_type v) { value += v; }
-  void operator-=(difference_type v) { value -= v; }
-  MutableRACIterator operator+(difference_type v) const {
-    return MutableRACIterator(value + v);
-  }
-  MutableRACIterator operator-(difference_type v) const {
-    return MutableRACIterator(value - v);
-  }
-  friend MutableRACIterator operator+(difference_type v,
-                                      const MutableRACIterator &it) {
-    return it + v;
-  }
-  int operator-(const MutableRACIterator &other) const {
-    return value - other.value;
-  }
-
-  bool operator<(const MutableRACIterator &other) const {
-    return value < other.value;
-  }
-
-  bool operator==(const MutableRACIterator &other) const {
-    return value == other.value;
-  }
-  bool operator!=(const MutableRACIterator &other) const {
-    return value != other.value;
-  }
-};
-
 /// clang::StmtIteratorBase
 class ProtectedIteratorBase {
 protected:

test/Interop/Cxx/stdlib/overlay/custom-contiguous-iterator-module-interface.swift

@@ -0,0 +1,28 @@
+// RUN: %target-swift-ide-test -print-module -module-to-print=CustomIterator -source-filename=x -I %S/Inputs -cxx-interoperability-mode=swift-6 -Xcc -std=c++20 | %FileCheck %s
+// RUN: %target-swift-ide-test -print-module -module-to-print=CustomIterator -source-filename=x -I %S/Inputs -cxx-interoperability-mode=upcoming-swift -Xcc -std=c++20 | %FileCheck %s 
+
+// Ubuntu 20.04 ships with an old version of libstdc++, which does not provide
+// std::contiguous_iterator_tag from C++20.
+// UNSUPPORTED: LinuxDistribution=ubuntu-20.04
+// UNSUPPORTED: LinuxDistribution=amzn-2
+
+// CHECK: struct ConstContiguousIterator : UnsafeCxxContiguousIterator, UnsafeCxxRandomAccessIterator, UnsafeCxxInputIterator {
+// CHECK:   func successor() -> ConstContiguousIterator
+// CHECK:   var pointee: Int32
+// CHECK:   typealias Pointee = Int32
+// CHECK:   typealias Distance = Int32
+// CHECK: }
+
+// CHECK: struct HasCustomContiguousIteratorTag : UnsafeCxxContiguousIterator, UnsafeCxxRandomAccessIterator, UnsafeCxxInputIterator {
+// CHECK:   func successor() -> HasCustomContiguousIteratorTag
+// CHECK:   var pointee: Int32
+// CHECK:   typealias Pointee = Int32
+// CHECK:   typealias Distance = Int32
+// CHECK: }
+
+// CHECK: struct MutableContiguousIterator : UnsafeCxxMutableContiguousIterator, UnsafeCxxMutableRandomAccessIterator, UnsafeCxxMutableInputIterator {
+// CHECK:   func successor() -> MutableContiguousIterator
+// CHECK:   var pointee: Int32
+// CHECK:   typealias Pointee = Int32
+// CHECK:   typealias Distance = Int32
+// CHECK: }

test/Interop/Cxx/stdlib/overlay/custom-iterator-module-interface.swift

@@ -80,6 +80,13 @@
 // CHECK:   static func == (lhs: HasTypedefIteratorTag, other: HasTypedefIteratorTag) -> Bool
 // CHECK: }
 
+// CHECK: struct MutableRACIterator : UnsafeCxxMutableRandomAccessIterator, UnsafeCxxMutableInputIterator {
+// CHECK:   func successor() -> MutableRACIterator
+// CHECK:   var pointee: Int32
+// CHECK:   typealias Pointee = Int32
+// CHECK:   typealias Distance = Int32
+// CHECK: }
+
 // CHECK-NOT: struct HasNoIteratorCategory : UnsafeCxxInputIterator
 // CHECK-NOT: struct HasInvalidIteratorCategory : UnsafeCxxInputIterator
 // CHECK-NOT: struct HasNoEqualEqual : UnsafeCxxInputIterator
@@ -139,10 +146,3 @@
 // CHECK:   var pointee: Int32 { get nonmutating set }
 // CHECK:   typealias Pointee = Int32
 // CHECK: }
-
-// CHECK: struct MutableRACIterator : UnsafeCxxMutableRandomAccessIterator, UnsafeCxxMutableInputIterator {
-// CHECK:   func successor() -> MutableRACIterator
-// CHECK:   var pointee: Int32
-// CHECK:   typealias Pointee = Int32
-// CHECK:   typealias Distance = Int32
-// CHECK: }