[Frontend] Only enable request reference tracking when needed

Only enable when we have a reference dependency output, or are
validating dependencies or collecting statistics. This avoids enabling
for SourceKit.

Author: hamishknight

lib/Frontend/Frontend.cpp

@@ -308,6 +308,16 @@ void CompilerInstance::recordPrimaryInputBuffer(unsigned BufID) {
   PrimaryBufferIDs.insert(BufID);
 }
 
+static bool shouldEnableRequestReferenceTracking(const CompilerInstance &CI) {
+  // Enable request reference dependency tracking when we're either writing
+  // dependencies for incremental mode, verifying dependencies, or collecting
+  // stats.
+  auto &opts = CI.getInvocation().getFrontendOptions();
+  return opts.InputsAndOutputs.hasReferenceDependenciesFilePath() ||
+         opts.EnableIncrementalDependencyVerifier ||
+         !opts.StatsOutputDir.empty();
+}
+
 bool CompilerInstance::setUpASTContextIfNeeded() {
   if (FrontendOptions::doesActionBuildModuleFromInterface(
           Invocation.getFrontendOptions().RequestedAction) &&
@@ -318,10 +328,8 @@ bool CompilerInstance::setUpASTContextIfNeeded() {
     return false;
   }
 
-  // For the time being, we only need to record dependencies in batch mode
-  // and single file builds.
-  Invocation.getLangOptions().RecordRequestReferences
-    = !isWholeModuleCompilation();
+  Invocation.getLangOptions().RecordRequestReferences =
+      shouldEnableRequestReferenceTracking(*this);
 
   Context.reset(ASTContext::get(
       Invocation.getLangOptions(), Invocation.getTypeCheckerOptions(),

test/Driver/createCompilerInvocation.swift

@@ -40,3 +40,10 @@
 // NOOUTPUT_ARGS-DAG: -typecheck
 // NOOUTPUT_ARGS-DAG: -module-name
 // NOOUTPUT_ARGS: Frontend Arguments END
+
+// Make sure that '-incremental' is ignored, we don't want SourceKit to run with
+// reference dependency tracking enabled. The legacy driver simply doesn't
+// implement incremental compilation, but make sure when we switch to the new
+// driver this doesn't start failing.
+// RUN: %swift-ide-test_plain -test-createCompilerInvocation -incremental %S/Input/main.swift | %FileCheck --check-prefix INCREMENTAL %s
+// INCREMENTAL-NOT: emit-reference-dependencies