Experimentally emit diagnostics from the new Swift parser

Introduce the experimental feature `ParserDiagnostics`, which emits
diagnostics from the new Swift parser *first* for a source file. If
that produces any errors, we suppress any diagnostics emitted from the
C++ parser.

Author: DougGregor

include/swift/Basic/Features.def

@@ -138,9 +138,10 @@ EXPERIMENTAL_FEATURE(ParserRoundTrip, false)
 /// Swift parser.
 EXPERIMENTAL_FEATURE(ParserValidation, false)
 
-/// Whether to fold sequence expressions in the syntax tree produced by the
-/// Swift Swift parser.
-EXPERIMENTAL_FEATURE(ParserSequenceFolding, false)
+/// Whether to emit diagnostics from the new parser first, and only emit
+/// diagnostics from the existing parser when there are none from the new
+/// parser.
+EXPERIMENTAL_FEATURE(ParserDiagnostics, false)
 
 /// Enables implicit some while also enabling existential `any`
 EXPERIMENTAL_FEATURE(ImplicitSome, false)

lib/AST/ASTPrinter.cpp

@@ -2974,7 +2974,7 @@ static bool usesFeatureParserValidation(Decl *decl) {
   return false;
 }
 
-static bool usesFeatureParserSequenceFolding(Decl *decl) {
+static bool usesFeatureParserDiagnostics(Decl *decl) {
   return false;
 }
 

lib/Parse/ParseDecl.cpp

@@ -205,9 +205,11 @@ extern "C" void swift_ASTGen_buildTopLevelASTNodes(void *sourceFile,
 /// \endverbatim
 void Parser::parseTopLevelItems(SmallVectorImpl<ASTNode> &items) {
 #if SWIFT_SWIFT_PARSER
+  Optional<DiagnosticTransaction> existingParsingTransaction;
   if ((Context.LangOpts.hasFeature(Feature::Macros) ||
        Context.LangOpts.hasFeature(Feature::BuiltinMacros) ||
        Context.LangOpts.hasFeature(Feature::ParserRoundTrip) ||
+       Context.LangOpts.hasFeature(Feature::ParserDiagnostics) ||
        Context.LangOpts.hasFeature(Feature::ParserValidation) ||
        Context.LangOpts.hasFeature(Feature::ParserASTGen)) &&
       !SourceMgr.hasIDEInspectionTargetBuffer() &&
@@ -225,6 +227,18 @@ void Parser::parseTopLevelItems(SmallVectorImpl<ASTNode> &items) {
       swift_ASTGen_destroySourceFile(exportedSourceFile);
     });
 
+    // If we're supposed to emit diagnostics from the parser, do so now.
+    if ((Context.LangOpts.hasFeature(Feature::ParserDiagnostics) ||
+         Context.LangOpts.hasFeature(Feature::ParserASTGen)) &&
+        swift_ASTGen_emitParserDiagnostics(
+            &Context.Diags, SF.exportedSourceFile) &&
+        Context.Diags.hadAnyError() &&
+        !Context.LangOpts.hasFeature(Feature::ParserASTGen)) {
+      // Errors were emitted, and we're still using the C++ parser, so
+      // disable diagnostics from the C++ parser.
+      existingParsingTransaction.emplace(Context.Diags);
+    }
+
     // If we want to do ASTGen, do so now.
     if (Context.LangOpts.hasFeature(Feature::ParserASTGen)) {
       swift_ASTGen_buildTopLevelASTNodes(
@@ -287,6 +301,9 @@ void Parser::parseTopLevelItems(SmallVectorImpl<ASTNode> &items) {
   }
 
 #if SWIFT_SWIFT_PARSER
+  if (existingParsingTransaction)
+    existingParsingTransaction->abort();
+
   // Perform round-trip and/or validation checking.
   if ((Context.LangOpts.hasFeature(Feature::ParserRoundTrip) ||
        Context.LangOpts.hasFeature(Feature::ParserValidation)) &&

test/Parse/new_parser_diagnostics.swift

@@ -0,0 +1,8 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-feature ParserDiagnostics
+
+// FIXME: Swift parser is not enabled on Linux CI yet.
+// REQUIRES: OS=macosx
+// REQUIRES: asserts
+
+_ = [(Int) -> async throws Int]()
+// expected-error@-1{{'async throws' may only occur before '->'}}