[New parser] Move round-trip checking logic into ASTGen

This removes one bit of code dependency on SwiftCompilerSupport, which
we want to eliminate.

Author: DougGregor

include/swift/AST/DiagnosticsParse.def

@@ -2019,5 +2019,8 @@ ERROR(macro_expansion_expr_expected_macro_identifier,none,
 ERROR(macro_expansion_decl_expected_macro_identifier,none,
       "expected a macro identifier for a pound literal declaration", ())
 
+ERROR(parser_round_trip_error,none,
+      "source file did not round-trip through the Swift Swift parser", ())
+
 #define UNDEFINE_DIAGNOSTIC_MACROS
 #include "DefineDiagnosticMacros.h"

lib/ASTGen/Sources/ASTGen/SourceFile.swift

@@ -48,3 +48,14 @@ public func destroySourceFile(
     sourceFile.deallocate()
   }
 }
+
+/// Check for whether the given source file round-trips
+@_cdecl("swift_ASTGen_roundTripCheck")
+public func roundTripCheck(
+  sourceFilePtr: UnsafeMutablePointer<UInt8>
+) -> CInt {
+  sourceFilePtr.withMemoryRebound(to: ExportedSourceFile.self, capacity: 1) { sourceFile in
+    let sf = sourceFile.pointee
+    return sf.syntax.syntaxTextBytes.elementsEqual(sf.buffer) ? 0 : 1
+  }
+}

lib/Parse/ParseDecl.cpp

@@ -178,6 +178,11 @@ extern "C" void *swift_ASTGen_parseSourceFile(const char *buffer,
 /// Destroy a source file parsed with swift_ASTGen_parseSourceFile.
 extern "C" void swift_ASTGen_destroySourceFile(void *sourceFile);
 
+/// Check whether the given source file round-trips correctly. Returns 0 if
+/// round-trip succeeded, non-zero otherwise.
+extern "C" int swift_ASTGen_roundTripCheck(void *sourceFile);
+
+
 // Build AST nodes for the top-level entities in the syntax.
 extern "C" void swift_ASTGen_buildTopLevelASTNodes(void *sourceFile,
                                                    void *declContext,
@@ -197,6 +202,7 @@ void Parser::parseTopLevelItems(SmallVectorImpl<ASTNode> &items) {
 #if SWIFT_SWIFT_PARSER
   if ((Context.LangOpts.hasFeature(Feature::Macros) ||
        Context.LangOpts.hasFeature(Feature::BuiltinMacros) ||
+       Context.LangOpts.hasFeature(Feature::ParserRoundTrip) ||
        Context.LangOpts.hasFeature(Feature::ParserASTGen)) &&
       !SourceMgr.hasIDEInspectionTargetBuffer() &&
       SF.Kind != SourceFileKind::SIL) {
@@ -273,6 +279,20 @@ void Parser::parseTopLevelItems(SmallVectorImpl<ASTNode> &items) {
       consumeToken();
     }
   }
+
+#if SWIFT_SWIFT_PARSER
+  // Perform round-trip checking.
+  if (Context.LangOpts.hasFeature(Feature::ParserRoundTrip) &&
+      SF.exportedSourceFile &&
+      !L->lexingCutOffOffset() &&
+      swift_ASTGen_roundTripCheck(SF.exportedSourceFile)) {
+    SourceLoc loc;
+    if (auto bufferID = SF.getBufferID()) {
+      loc = Context.SourceMgr.getLocForBufferStart(*bufferID);
+    }
+    diagnose(loc, diag::parser_round_trip_error);
+  }
+#endif
 }
 
 bool Parser::parseTopLevelSIL() {

lib/Parse/ParseRequests.cpp

@@ -176,18 +176,12 @@ SourceFileParsingResult ParseSourceFileRequest::evaluate(Evaluator &evaluator,
     tokensRef = ctx.AllocateCopy(*tokens);
 
 #if SWIFT_SWIFT_PARSER
-  if ((ctx.LangOpts.hasFeature(Feature::ParserRoundTrip) ||
-       ctx.LangOpts.hasFeature(Feature::ParserValidation)) &&
+  if (ctx.LangOpts.hasFeature(Feature::ParserValidation) &&
       ctx.SourceMgr.getIDEInspectionTargetBufferID() != bufferID &&
       SF->Kind != SourceFileKind::SIL) {
     auto bufferRange = ctx.SourceMgr.getRangeForBuffer(*bufferID);
     unsigned int flags = 0;
 
-    if (ctx.LangOpts.hasFeature(Feature::ParserRoundTrip) &&
-        !parser.L->lexingCutOffOffset()) {
-      flags |= SCC_RoundTrip;
-    }
-
     if (!ctx.Diags.hadAnyError() &&
         ctx.LangOpts.hasFeature(Feature::ParserValidation))
       flags |= SCC_ParseDiagnostics;