[cxx-interop] Add CxxVector protocol

This makes it possible to initialize `std::vector` from a Swift Sequence. This also conforms C++ vectors to `ExpressibleByArrayLiteral`, making it possible, for instance, to pass a Swift array to a C++ function that takes a vector of strings as a parameter.

rdar://104826995

Author: egorzhdan

include/swift/AST/KnownProtocols.def

@@ -115,6 +115,7 @@ PROTOCOL(CxxSet)
 PROTOCOL(CxxRandomAccessCollection)
 PROTOCOL(CxxSequence)
 PROTOCOL(CxxUniqueSet)
+PROTOCOL(CxxVector)
 PROTOCOL(UnsafeCxxInputIterator)
 PROTOCOL(UnsafeCxxMutableInputIterator)
 PROTOCOL(UnsafeCxxRandomAccessIterator)

lib/AST/ASTContext.cpp

@@ -1139,6 +1139,7 @@ ProtocolDecl *ASTContext::getProtocol(KnownProtocolKind kind) const {
   case KnownProtocolKind::CxxSet:
   case KnownProtocolKind::CxxSequence:
   case KnownProtocolKind::CxxUniqueSet:
+  case KnownProtocolKind::CxxVector:
   case KnownProtocolKind::UnsafeCxxInputIterator:
   case KnownProtocolKind::UnsafeCxxMutableInputIterator:
   case KnownProtocolKind::UnsafeCxxRandomAccessIterator:

lib/ClangImporter/ClangDerivedConformances.cpp

@@ -927,3 +927,47 @@ void swift::conformToCxxDictionaryIfNeeded(
                                insert->getResultInterfaceType());
   impl.addSynthesizedProtocolAttrs(decl, {KnownProtocolKind::CxxDictionary});
 }
+
+void swift::conformToCxxVectorIfNeeded(ClangImporter::Implementation &impl,
+                                       NominalTypeDecl *decl,
+                                       const clang::CXXRecordDecl *clangDecl) {
+  PrettyStackTraceDecl trace("conforming to CxxVector", decl);
+
+  assert(decl);
+  assert(clangDecl);
+  ASTContext &ctx = decl->getASTContext();
+
+  // Only auto-conform types from the C++ standard library. Custom user types
+  // might have a similar interface but different semantics.
+  if (!isStdDecl(clangDecl, {"vector"}))
+    return;
+
+  auto valueType = lookupDirectSingleWithoutExtensions<TypeAliasDecl>(
+      decl, ctx.getIdentifier("value_type"));
+  auto iterType = lookupDirectSingleWithoutExtensions<TypeAliasDecl>(
+      decl, ctx.getIdentifier("const_iterator"));
+  if (!valueType || !iterType)
+    return;
+
+  ProtocolDecl *cxxRandomAccessIteratorProto =
+      ctx.getProtocol(KnownProtocolKind::UnsafeCxxRandomAccessIterator);
+  if (!cxxRandomAccessIteratorProto)
+    return;
+
+  auto rawIteratorTy = iterType->getUnderlyingType();
+
+  // Check if RawIterator conforms to UnsafeCxxRandomAccessIterator.
+  ModuleDecl *module = decl->getModuleContext();
+  auto rawIteratorConformanceRef =
+      module->lookupConformance(rawIteratorTy, cxxRandomAccessIteratorProto);
+  if (!isConcreteAndValid(rawIteratorConformanceRef, module))
+    return;
+
+  impl.addSynthesizedTypealias(decl, ctx.Id_Element,
+                               valueType->getUnderlyingType());
+  impl.addSynthesizedTypealias(decl, ctx.Id_ArrayLiteralElement,
+                               valueType->getUnderlyingType());
+  impl.addSynthesizedTypealias(decl, ctx.getIdentifier("RawIterator"),
+                               rawIteratorTy);
+  impl.addSynthesizedProtocolAttrs(decl, {KnownProtocolKind::CxxVector});
+}

lib/ClangImporter/ClangDerivedConformances.h

@@ -59,6 +59,12 @@ void conformToCxxDictionaryIfNeeded(ClangImporter::Implementation &impl,
                                     NominalTypeDecl *decl,
                                     const clang::CXXRecordDecl *clangDecl);
 
+/// If the decl is an instantiation of C++ `std::vector`, synthesize a
+/// conformance to CxxVector, which is defined in the Cxx module.
+void conformToCxxVectorIfNeeded(ClangImporter::Implementation &impl,
+                                NominalTypeDecl *decl,
+                                const clang::CXXRecordDecl *clangDecl);
+
 } // namespace swift
 
 #endif // SWIFT_CLANG_DERIVED_CONFORMANCES_H

lib/ClangImporter/ImportDecl.cpp

@@ -2797,6 +2797,7 @@ namespace {
         conformToCxxDictionaryIfNeeded(Impl, nominalDecl, decl);
         conformToCxxPairIfNeeded(Impl, nominalDecl, decl);
         conformToCxxOptionalIfNeeded(Impl, nominalDecl, decl);
+        conformToCxxVectorIfNeeded(Impl, nominalDecl, decl);
       }
 
       if (auto *ntd = dyn_cast<NominalTypeDecl>(result))

lib/IRGen/GenMeta.cpp

@@ -6310,6 +6310,7 @@ SpecialProtocol irgen::getSpecialProtocolID(ProtocolDecl *P) {
   case KnownProtocolKind::CxxSet:
   case KnownProtocolKind::CxxSequence:
   case KnownProtocolKind::CxxUniqueSet:
+  case KnownProtocolKind::CxxVector:
   case KnownProtocolKind::UnsafeCxxInputIterator:
   case KnownProtocolKind::UnsafeCxxMutableInputIterator:
   case KnownProtocolKind::UnsafeCxxRandomAccessIterator:

stdlib/public/Cxx/CMakeLists.txt

@@ -19,6 +19,7 @@ add_swift_target_library(swiftCxx ${SWIFT_CXX_LIBRARY_KIND} NO_LINK_NAME IS_STDL
     CxxSet.swift
     CxxRandomAccessCollection.swift
     CxxSequence.swift
+    CxxVector.swift
     UnsafeCxxIterators.swift
 
     SWIFT_COMPILE_FLAGS ${SWIFT_RUNTIME_SWIFT_COMPILE_FLAGS} ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}

stdlib/public/Cxx/CxxVector.swift

@@ -0,0 +1,47 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2023 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 represents a vector of values.
+///
+/// C++ standard library type `std::vector` conforms to this protocol.
+public protocol CxxVector<Element>: ExpressibleByArrayLiteral {
+  associatedtype Element
+  associatedtype RawIterator: UnsafeCxxRandomAccessIterator
+    where RawIterator.Pointee == Element
+
+  init()
+
+  mutating func push_back(_ element: Element)
+}
+
+extension CxxVector {
+  /// Creates a C++ vector containing the elements of a Swift Sequence.
+  ///
+  /// This initializes the vector by copying every element of the sequence.
+  ///
+  /// - Complexity: O(*n*), where *n* is the number of elements in the Swift
+  ///   sequence
+  @inlinable
+  public init<S: Sequence>(_ sequence: __shared S) where S.Element == Element {
+    self.init()
+    for item in sequence {
+      self.push_back(item)
+    }
+  }
+}
+
+extension CxxVector {
+  @inlinable
+  public init(arrayLiteral elements: Element...) {
+    self.init(elements)
+  }
+}

test/Interop/Cxx/stdlib/Inputs/std-vector.h

@@ -9,4 +9,8 @@ using VectorOfString = std::vector<std::string>;
 
 inline Vector initVector() { return {}; }
 
+inline std::string takesVectorOfString(const VectorOfString &v) {
+  return v.front();
+}
+
 #endif // TEST_INTEROP_CXX_STDLIB_INPUTS_STD_VECTOR_H

test/Interop/Cxx/stdlib/use-std-vector.swift

@@ -19,6 +19,48 @@ StdVectorTestSuite.test("VectorOfInt.init") {
     expectTrue(v.empty())
 }
 
+StdVectorTestSuite.test("VectorOfInt.init(sequence)") {
+    let v = Vector([])
+    expectEqual(v.size(), 0)
+    expectTrue(v.empty())
+
+    let v2 = Vector([1, 2, 3])
+    expectEqual(v2.size(), 3)
+    expectFalse(v2.empty())
+    expectEqual(v2[0], 1)
+    expectEqual(v2[1], 2)
+    expectEqual(v2[2], 3)
+}
+
+StdVectorTestSuite.test("VectorOfString.init(sequence)") {
+    let v = VectorOfString([])
+    expectEqual(v.size(), 0)
+    expectTrue(v.empty())
+
+    let v2 = VectorOfString(["", "ab", "abc"])
+    expectEqual(v2.size(), 3)
+    expectFalse(v2.empty())
+    expectEqual(v2[0], "")
+    expectEqual(v2[1], "ab")
+    expectEqual(v2[2], "abc")
+
+    let first = takesVectorOfString(["abc", "qwe"])
+    expectEqual(first, "abc")
+}
+
+StdVectorTestSuite.test("VectorOfInt as ExpressibleByArrayLiteral") {
+    let v: Vector = []
+    expectEqual(v.size(), 0)
+    expectTrue(v.empty())
+
+    let v2: Vector = [1, 2, 3]
+    expectEqual(v2.size(), 3)
+    expectFalse(v2.empty())
+    expectEqual(v2[0], 1)
+    expectEqual(v2[1], 2)
+    expectEqual(v2[2], 3)
+}
+
 StdVectorTestSuite.test("VectorOfInt.push_back") {
     var v = Vector()
     let _42: CInt = 42