[interop][SwiftToCxx] move std::string -> Swift String constructor to the overlay header

Author: hyp

lib/PrintAsClang/PrintClangValueType.cpp

@@ -159,14 +159,6 @@ static void addCppExtensionsToStdlibType(const NominalTypeDecl *typeDecl,
       os << "    return result;\n";
       os << "  }\n";
     });
-    os << "#ifndef SWIFT_CXX_INTEROP_HIDE_STL_OVERLAY\n";
-    ClangSyntaxPrinter(os).printInlineForThunk();
-    os << "String(const std::string &str) noexcept {\n";
-    os << "    auto res = "
-          "_impl::$sSS7cStringSSSPys4Int8VG_tcfC(str.c_str());\n";
-    os << "    memcpy(_getOpaquePointer(), &res, sizeof(res));\n";
-    os << "}\n";
-    os << "#endif\n";
     // Add additional methods for the `String` declaration.
     printer.printDefine("SWIFT_CXX_INTEROP_STRING_MIXIN");
     printer.printIncludeForShimHeader("_SwiftStdlibCxxOverlay.h");

lib/PrintAsClang/_SwiftStdlibCxxOverlay.h

@@ -35,6 +35,12 @@ SWIFT_INLINE_THUNK T_0_0 get() const
 
 #ifndef SWIFT_CXX_INTEROP_HIDE_STL_OVERLAY
 
+/// Constructs a Swift string from a C++ string.
+SWIFT_INLINE_THUNK String(const std::string &str) noexcept {
+  auto res = _impl::$sSS7cStringSSSPys4Int8VG_tcfC(str.c_str());
+  memcpy(_getOpaquePointer(), &res, sizeof(res));
+}
+
 /// Casts the Swift String value to a C++ std::string.
 SWIFT_INLINE_THUNK operator std::string() const;
 

test/Interop/SwiftToCxx/stdlib/swift-stdlib-in-cxx.swift

@@ -90,12 +90,6 @@
 // CHECK-NEXT:  }
 // CHECK-EMPTY:
 // CHECK-NEXT:  #endif
-// CHECK-NEXT: #ifndef SWIFT_CXX_INTEROP_HIDE_STL_OVERLAY
-// CHECK-NEXT: inline __attribute__((always_inline)) String(const std::string &str) noexcept {
-// CHECK-NEXT:      auto res = _impl::$sSS7cStringSSSPys4Int8VG_tcfC(str.c_str());
-// CHECK-NEXT:      memcpy(_getOpaquePointer(), &res, sizeof(res));
-// CHECK-NEXT: }
-// CHECK-NEXT: #endif
 // CHECK-NEXT: #define SWIFT_CXX_INTEROP_STRING_MIXIN
 // CHECK-NEXT: // Look for the C++ interop support header relative to clang's resource dir:
 // CHECK-NEXT: //  '<toolchain>/usr/lib/clang/<version>/include/../../../swift/swiftToCxx'.