[Dependency Scanning] Disable Swift Parser ASTGen during dependency scan

On both input moduel source-files and interface files.
This currently yields dramatic scanning performance improvements at no cost - we do not require an AST during scan.

Author: artemcm

include/swift/AST/SourceFile.h

@@ -90,6 +90,10 @@ class SourceFile final : public FileUnit {
     /// Whether to suppress warnings when parsing. This is set for secondary
     /// files, as they get parsed multiple times.
     SuppressWarnings = 1 << 4,
+
+    /// Whether to disable the Swift Parser ASTGen
+    /// e.g. in dependency scanning, where an AST is not needed.
+    DisableSwiftParserASTGen = 1 << 5,
   };
   using ParsingOptions = OptionSet<ParsingFlags>;
 

lib/Frontend/Frontend.cpp

@@ -1369,6 +1369,12 @@ CompilerInstance::getSourceFileParsingOptions(bool forPrimary) const {
     opts |= SourceFile::ParsingFlags::SuppressWarnings;
   }
 
+  // Dependency scanning does not require an AST, so disable Swift Parser
+  // ASTGen parsing completely.
+  if (frontendOpts.RequestedAction ==
+      FrontendOptions::ActionType::ScanDependencies)
+    opts |= SourceFile::ParsingFlags::DisableSwiftParserASTGen;
+
   // Enable interface hash computation for primaries or emit-module-separately,
   // but not in WMO, as it's only currently needed for incremental mode.
   if (forPrimary ||

lib/Parse/ParseDecl.cpp

@@ -209,7 +209,9 @@ extern "C" void swift_ASTGen_buildTopLevelASTNodes(void *sourceFile,
 void Parser::parseTopLevelItems(SmallVectorImpl<ASTNode> &items) {
 #if SWIFT_SWIFT_PARSER
   Optional<DiagnosticTransaction> existingParsingTransaction;
-  parseSourceFileViaASTGen(items, existingParsingTransaction);
+  if (!SF.getParsingOptions()
+      .contains(SourceFile::ParsingFlags::DisableSwiftParserASTGen))
+    parseSourceFileViaASTGen(items, existingParsingTransaction);
 #endif
 
   // Prime the lexer.
@@ -259,36 +261,39 @@ 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)) &&
-      SF.exportedSourceFile) {
-    if (Context.LangOpts.hasFeature(Feature::ParserRoundTrip) &&
-        swift_ASTGen_roundTripCheck(SF.exportedSourceFile)) {
-      SourceLoc loc;
-      if (auto bufferID = SF.getBufferID()) {
-        loc = Context.SourceMgr.getLocForBufferStart(*bufferID);
-      }
-      diagnose(loc, diag::parser_round_trip_error);
-    } else if (Context.LangOpts.hasFeature(Feature::ParserValidation) &&
-               !Context.Diags.hadAnyError() &&
-               swift_ASTGen_emitParserDiagnostics(
-                   &Context.Diags, SF.exportedSourceFile,
-                   /*emitOnlyErrors=*/true,
-                   /*downgradePlaceholderErrorsToWarnings=*/
-                       Context.LangOpts.Playground ||
-                       Context.LangOpts.WarnOnEditorPlaceholder)) {
-      // We might have emitted warnings in the C++ parser but no errors, in
-      // which case we still have `hadAnyError() == false`. To avoid emitting
-      // the same warnings from SwiftParser, only emit errors from SwiftParser
-      SourceLoc loc;
-      if (auto bufferID = SF.getBufferID()) {
-        loc = Context.SourceMgr.getLocForBufferStart(*bufferID);
-      }
-      diagnose(loc, diag::parser_new_parser_errors);
+  if (!SF.getParsingOptions().contains(
+          SourceFile::ParsingFlags::DisableSwiftParserASTGen)) {
+    if (existingParsingTransaction)
+      existingParsingTransaction->abort();
+
+    // Perform round-trip and/or validation checking.
+    if ((Context.LangOpts.hasFeature(Feature::ParserRoundTrip) ||
+         Context.LangOpts.hasFeature(Feature::ParserValidation)) &&
+        SF.exportedSourceFile) {
+      if (Context.LangOpts.hasFeature(Feature::ParserRoundTrip) &&
+          swift_ASTGen_roundTripCheck(SF.exportedSourceFile)) {
+        SourceLoc loc;
+        if (auto bufferID = SF.getBufferID()) {
+          loc = Context.SourceMgr.getLocForBufferStart(*bufferID);
+        }
+        diagnose(loc, diag::parser_round_trip_error);
+      } else if (Context.LangOpts.hasFeature(Feature::ParserValidation) &&
+                 !Context.Diags.hadAnyError() &&
+                 swift_ASTGen_emitParserDiagnostics(
+                     &Context.Diags, SF.exportedSourceFile,
+                     /*emitOnlyErrors=*/true,
+                     /*downgradePlaceholderErrorsToWarnings=*/
+                     Context.LangOpts.Playground ||
+                         Context.LangOpts.WarnOnEditorPlaceholder)) {
+        // We might have emitted warnings in the C++ parser but no errors, in
+        // which case we still have `hadAnyError() == false`. To avoid emitting
+        // the same warnings from SwiftParser, only emit errors from SwiftParser
+        SourceLoc loc;
+        if (auto bufferID = SF.getBufferID()) {
+          loc = Context.SourceMgr.getLocForBufferStart(*bufferID);
+        }
+        diagnose(loc, diag::parser_new_parser_errors);
+      }
     }
   }
 #endif

lib/Serialization/ModuleDependencyScanner.cpp

@@ -16,6 +16,7 @@
 #include "swift/AST/ModuleDependencies.h"
 #include "swift/AST/SourceFile.h"
 #include "swift/AST/TypeCheckRequests.h"
+#include "swift/AST/SourceFile.h"
 #include "swift/Basic/FileTypes.h"
 #include "swift/Frontend/ModuleInterfaceLoader.h"
 #include "swift/Serialization/SerializedModuleLoader.h"
@@ -157,8 +158,13 @@ ErrorOr<ModuleDependencyInfo> ModuleDependencyScanner::scanInterfaceFile(
     // Create a source file.
     unsigned bufferID = Ctx.SourceMgr.addNewSourceBuffer(std::move(interfaceBuf.get()));
     auto moduleDecl = ModuleDecl::create(realModuleName, Ctx);
+
+    // Dependency scanning does not require an AST, so disable Swift Parser
+    // ASTGen parsing completely.
+    SourceFile::ParsingOptions parsingOpts;
+    parsingOpts |= SourceFile::ParsingFlags::DisableSwiftParserASTGen;
     auto sourceFile = new (Ctx) SourceFile(
-        *moduleDecl, SourceFileKind::Interface, bufferID);
+        *moduleDecl, SourceFileKind::Interface, bufferID, parsingOpts);
     moduleDecl->addAuxiliaryFile(*sourceFile);
 
     // Walk the source file to find the import declarations.