Replace SWIFT_SWIFT_PARSER_ROUNDTRIP with an experimental feature.

Remove the CMake configuration option `SWIFT_SWIFT_PARSER_ROUNDTRIP`.
Instead, whenever the "early" SwiftSyntax module is built, link the
Swift Swift parser into the compiler and related tools.

Introduce a new experimental feature `ParserRoundTrip` that can be
enabled to perform round-trip testing.

Author: DougGregor

CMakeLists.txt

@@ -870,18 +870,8 @@ endif()
 
 # When we have the early SwiftSyntax build, we can include its parser.
 if(SWIFT_PATH_TO_EARLYSWIFTSYNTAX_BUILD_DIR)
-  # An option that specifies how the SwiftSyntax-based Swift parser is
-  # integrated.
-  option(SWIFT_SWIFT_PARSER_MODE [=[
-How to use the SwiftSyntax-based Swift parser. Possible values are
-    OFF:           do not use this parser at all
-    ROUNDTRIP:     ensure that the SwiftSyntax-based Swift parser round-trips
-]=] OFF)
-
-  if(SWIFT_SWIFT_PARSER_MODE)
-    set(SWIFT_SWIFT_PARSER TRUE)
-    include(${SWIFT_PATH_TO_EARLYSWIFTSYNTAX_BUILD_DIR}/cmake/SwiftSyntaxTargets.cmake)
-  endif()
+  set(SWIFT_SWIFT_PARSER TRUE)
+  include(${SWIFT_PATH_TO_EARLYSWIFTSYNTAX_BUILD_DIR}/cmake/SwiftSyntaxTargets.cmake)
 endif()
 
 
@@ -1091,7 +1081,7 @@ if(SWIFT_INCLUDE_TOOLS)
   message(STATUS "  Assertions:     ${LLVM_ENABLE_ASSERTIONS}")
   message(STATUS "  LTO:            ${SWIFT_TOOLS_ENABLE_LTO}")
   message(STATUS "  Bootstrapping:  ${BOOTSTRAPPING_MODE}")
-  message(STATUS "  Swift parser:   ${SWIFT_SWIFT_PARSER_MODE}")
+  message(STATUS "  Swift parser:   ${SWIFT_SWIFT_PARSER}")
   message(STATUS "")
 else()
   message(STATUS "Not building host Swift tools")

include/swift/Basic/Features.def

@@ -126,6 +126,9 @@ EXPERIMENTAL_FEATURE(OpaqueTypeErasure)
 /// wrapped type.
 EXPERIMENTAL_FEATURE(TypeWrappers)
 
+/// Whether to perform round-trip testing of the Swift Swift parser.
+EXPERIMENTAL_FEATURE(ParserRoundTrip)
+
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE
 #undef SUPPRESSIBLE_LANGUAGE_FEATURE

lib/AST/ASTPrinter.cpp

@@ -2935,6 +2935,10 @@ static bool usesFeatureTypeWrappers(Decl *decl) {
   return decl->getAttrs().hasAttribute<TypeWrapperAttr>();
 }
 
+static bool usesFeatureParserRoundTrip(Decl *decl) {
+  return false;
+}
+
 static void suppressingFeatureSpecializeAttributeWithAvailability(
                                         PrintOptions &options,
                                         llvm::function_ref<void()> action) {

lib/Parse/CMakeLists.txt

@@ -59,7 +59,7 @@ if (SWIFT_SWIFT_PARSER)
 
   target_compile_definitions(swiftParse
     PRIVATE
-    SWIFT_SWIFT_PARSER_ROUNDTRIP
+    SWIFT_SWIFT_PARSER
   )
 endif()
 

lib/Parse/ParseRequests.cpp

@@ -26,7 +26,7 @@
 #include "swift/Syntax/SyntaxNodes.h"
 #include "swift/SyntaxParse/SyntaxTreeCreator.h"
 
-#ifdef SWIFT_SWIFT_PARSER_ROUNDTRIP
+#ifdef SWIFT_SWIFT_PARSER
 #include "SwiftParserCompilerSupport.h"
 #endif
 
@@ -193,8 +193,9 @@ SourceFileParsingResult ParseSourceFileRequest::evaluate(Evaluator &evaluator,
   if (auto tokens = parser.takeTokenReceiver()->finalize())
     tokensRef = ctx.AllocateCopy(*tokens);
 
-#ifdef SWIFT_SWIFT_PARSER_ROUNDTRIP
-  if (ctx.SourceMgr.getCodeCompletionBufferID() != bufferID) {
+#ifdef SWIFT_SWIFT_PARSER
+  if (ctx.LangOpts.hasFeature(Feature::ParserRoundTrip) &&
+      ctx.SourceMgr.getCodeCompletionBufferID() != bufferID) {
     auto bufferRange = ctx.SourceMgr.getRangeForBuffer(*bufferID);
     unsigned int flags = SPCC_RoundTrip;