[cxx-interop] Allow initializing std::function from Swift closures

This adds a Swift initializer to instantiations of `std::function` that accepts a Swift closure with `@convention(c)`.

rdar://103979602

Author: egorzhdan

lib/ClangImporter/ClangDerivedConformances.cpp

@@ -1019,3 +1019,90 @@ void swift::conformToCxxVectorIfNeeded(ClangImporter::Implementation &impl,
                                rawIteratorTy);
   impl.addSynthesizedProtocolAttrs(decl, {KnownProtocolKind::CxxVector});
 }
+
+void swift::conformToCxxFunctionIfNeeded(
+    ClangImporter::Implementation &impl, NominalTypeDecl *decl,
+    const clang::CXXRecordDecl *clangDecl) {
+  PrettyStackTraceDecl trace("conforming to CxxFunction", decl);
+
+  assert(decl);
+  assert(clangDecl);
+  ASTContext &ctx = decl->getASTContext();
+  clang::ASTContext &clangCtx = impl.getClangASTContext();
+  clang::Sema &clangSema = impl.getClangSema();
+
+  // Only auto-conform types from the C++ standard library. Custom user types
+  // might have a similar interface but different semantics.
+  if (!isStdDecl(clangDecl, {"function"}))
+    return;
+
+  // There is no typealias for the argument types on the C++ side, so to
+  // retrieve the argument types we look at the overload of `operator()` that
+  // got imported into Swift.
+
+  auto callAsFunctionDecl = lookupDirectSingleWithoutExtensions<FuncDecl>(
+      decl, ctx.getIdentifier("callAsFunction"));
+  if (!callAsFunctionDecl)
+    return;
+
+  auto operatorCallDecl = dyn_cast_or_null<clang::CXXMethodDecl>(
+      callAsFunctionDecl->getClangDecl());
+  if (!operatorCallDecl)
+    return;
+
+  std::vector<clang::QualType> operatorCallParamTypes;
+  llvm::transform(
+      operatorCallDecl->parameters(),
+      std::back_inserter(operatorCallParamTypes),
+      [](const clang::ParmVarDecl *paramDecl) { return paramDecl->getType(); });
+
+  auto funcPointerType = clangCtx.getPointerType(clangCtx.getFunctionType(
+      operatorCallDecl->getReturnType(), operatorCallParamTypes,
+      clang::FunctionProtoType::ExtProtoInfo()));
+
+  // Create a fake variable with a function type that matches the type of
+  // `operator()`.
+  auto fakeFuncPointerVarDecl = clang::VarDecl::Create(
+      clangCtx, /*DC*/ clangCtx.getTranslationUnitDecl(),
+      clang::SourceLocation(), clang::SourceLocation(), /*Id*/ nullptr,
+      funcPointerType, clangCtx.getTrivialTypeSourceInfo(funcPointerType),
+      clang::StorageClass::SC_None);
+  auto fakeFuncPointerRefExpr = new (clangCtx) clang::DeclRefExpr(
+      clangCtx, fakeFuncPointerVarDecl, false, funcPointerType,
+      clang::ExprValueKind::VK_LValue, clang::SourceLocation());
+
+  auto clangDeclTyInfo = clangCtx.getTrivialTypeSourceInfo(
+      clang::QualType(clangDecl->getTypeForDecl(), 0));
+  SmallVector<clang::Expr *, 1> constructExprArgs = {fakeFuncPointerRefExpr};
+
+  // Instantiate the templated constructor that would accept this fake variable.
+  auto constructExprResult = clangSema.BuildCXXTypeConstructExpr(
+      clangDeclTyInfo, clangDecl->getLocation(), constructExprArgs,
+      clangDecl->getLocation(), /*ListInitialization*/ false);
+  if (!constructExprResult.isUsable())
+    return;
+
+  auto castExpr = dyn_cast_or_null<clang::CastExpr>(constructExprResult.get());
+  if (!castExpr)
+    return;
+
+  auto bindTempExpr =
+      dyn_cast_or_null<clang::CXXBindTemporaryExpr>(castExpr->getSubExpr());
+  if (!bindTempExpr)
+    return;
+
+  auto constructExpr =
+      dyn_cast_or_null<clang::CXXConstructExpr>(bindTempExpr->getSubExpr());
+  if (!constructExpr)
+    return;
+
+  auto constructorDecl = constructExpr->getConstructor();
+
+  auto importedConstructor =
+      impl.importDecl(constructorDecl, impl.CurrentVersion);
+  if (!importedConstructor)
+    return;
+  decl->addMember(importedConstructor);
+
+  // TODO: actually conform to some form of CxxFunction protocol
+}

lib/ClangImporter/ClangDerivedConformances.h

@@ -71,6 +71,12 @@ void conformToCxxVectorIfNeeded(ClangImporter::Implementation &impl,
                                 NominalTypeDecl *decl,
                                 const clang::CXXRecordDecl *clangDecl);
 
+/// If the decl is an instantiation of C++ `std::function`, synthesize a
+/// conformance to CxxFunction, which is defined in the Cxx module.
+void conformToCxxFunctionIfNeeded(ClangImporter::Implementation &impl,
+                                  NominalTypeDecl *decl,
+                                  const clang::CXXRecordDecl *clangDecl);
+
 } // namespace swift
 
 #endif // SWIFT_CLANG_DERIVED_CONFORMANCES_H

lib/ClangImporter/ImportDecl.cpp

@@ -2818,6 +2818,7 @@ namespace {
           conformToCxxPairIfNeeded(Impl, nominalDecl, decl);
           conformToCxxOptionalIfNeeded(Impl, nominalDecl, decl);
           conformToCxxVectorIfNeeded(Impl, nominalDecl, decl);
+          conformToCxxFunctionIfNeeded(Impl, nominalDecl, decl);
         }
       }
 

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

@@ -2,8 +2,10 @@
 #define TEST_INTEROP_CXX_STDLIB_INPUTS_STD_FUNCTION_H
 
 #include <functional>
+#include <string>
 
 using FunctionIntToInt = std::function<int(int)>;
+using FunctionStringToString = std::function<std::string(const std::string&)>;
 
 FunctionIntToInt getIdentityFunction() {
   return [](int x) { return x; };
@@ -13,4 +15,8 @@ bool isEmptyFunction(FunctionIntToInt f) { return !(bool)f; }
 
 int invokeFunction(FunctionIntToInt f, int x) { return f(x); }
 
+std::string invokeFunctionTwice(FunctionStringToString f, std::string s) {
+  return f(f(s));
+}
+
 #endif // TEST_INTEROP_CXX_STDLIB_INPUTS_STD_FUNCTION_H

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

@@ -5,25 +5,50 @@
 
 import StdlibUnittest
 import StdFunction
+import CxxStdlib
 
 var StdFunctionTestSuite = TestSuite("StdFunction")
 
-StdFunctionTestSuite.test("init empty") {
+StdFunctionTestSuite.test("FunctionIntToInt init empty") {
   let f = FunctionIntToInt()
   expectTrue(isEmptyFunction(f))
 
   let copied = f
   expectTrue(isEmptyFunction(copied))
 }
 
-StdFunctionTestSuite.test("call") {
+StdFunctionTestSuite.test("FunctionIntToInt call") {
   let f = getIdentityFunction()
   expectEqual(123, f(123))
 }
 
-StdFunctionTestSuite.test("retrieve and pass back as parameter") {
+StdFunctionTestSuite.test("FunctionIntToInt retrieve and pass back as parameter") {
   let res = invokeFunction(getIdentityFunction(), 456)
   expectEqual(456, res)
 }
 
+StdFunctionTestSuite.test("FunctionIntToInt init from closure and call") {
+  let cClosure: @convention(c) (Int32) -> Int32 = { $0 + 1 }
+  let f = FunctionIntToInt(cClosure)
+  expectEqual(1, f(0))
+  expectEqual(124, f(123))
+  expectEqual(0, f(-1))
+
+  let f2 = FunctionIntToInt({ $0 * 2 })
+  expectEqual(0, f2(0))
+  expectEqual(246, f2(123))
+}
+
+StdFunctionTestSuite.test("FunctionIntToInt init from closure and pass as parameter") {
+  let res = invokeFunction(.init({ $0 * 2 }), 111)
+  expectEqual(222, res)
+}
+
+// FIXME: assertion for address-only closure params
+//StdFunctionTestSuite.test("FunctionStringToString init from closure and pass as parameter") {
+//  let res = invokeFunctionTwice(.init({ $0 + std.string("abc") }),
+//                                std.string("prefix"))
+//  expectEqual(std.string("prefixabcabc"), res)
+//}
+
 runAllTests()