Frontend: Emit reference dependencies at the end of the frontend pipeline

As part of eliminating cascading dependencies, we need to make sure to
capture any dependencies discovered during SILGen and IRGen.

This is a little bit tricky because we exit early in various places in
the pipeline, so we want to do it after performCompileStepsPostSema()
returns, but we can't do it too late, because the ASTContext might
already have been deallocated.

So emit the dependencies manually before freeing the ASTContext if we
decide to do that, or just do it before returning from performCompile()
using a SWIFT_DEFER otherwise.

This should have no observable effect until we start recording dependencies
for lookups done on behalf of declarations in secondary files.

Author: slavapestov

lib/FrontendTool/FrontendTool.cpp

@@ -1290,10 +1290,16 @@ static bool performCompile(CompilerInstance &Instance,
   if (opts.PrintClangStats && Context.getClangModuleLoader())
     Context.getClangModuleLoader()->printStatistics();
 
-  emitReferenceDependenciesForAllPrimaryInputsIfNeeded(Instance);
   emitSwiftRangesForAllPrimaryInputsIfNeeded(Instance);
   emitCompiledSourceForAllPrimaryInputsIfNeeded(Instance);
 
+  SWIFT_DEFER {
+    // We might have freed the ASTContext already, but in that case we must have
+    // emitted the dependencies first.
+    if (Instance.hasASTContext())
+      emitReferenceDependenciesForAllPrimaryInputsIfNeeded(Instance);
+  };
+
   emitIndexData(Instance);
 
   if (Context.hadError())
@@ -1508,6 +1514,11 @@ static void freeDeallocatableResourcesIfPossible(CompilerInstance &Instance) {
     break;
   case DeallocatableResources::SILModuleAndASTContext:
     Instance.freeSILModule();
+
+    // Make sure we emit dependencies now, because we can't do it after the
+    // context is gone.
+    emitReferenceDependenciesForAllPrimaryInputsIfNeeded(Instance);
+
     Instance.freeASTContext();
     break;
   }
@@ -2184,14 +2195,6 @@ int swift::performFrontend(ArrayRef<const char *> Args,
                        Invocation.getFrontendOptions().DumpAPIPath);
   }
 
-  // If we're asked to enable private intransitive dependencies, we need to
-  // write over the dependency files we just emitted because we need to
-  // get the dependencies written post-Sema down on disk.
-  // FIXME: Evaluate the impact turning this on universally has.
-  if (Invocation.getLangOptions().EnableExperientalPrivateIntransitiveDependencies) {
-    emitReferenceDependenciesForAllPrimaryInputsIfNeeded(*Instance);
-  }
-
   // Verify reference dependencies of the current compilation job *before*
   // verifying diagnostics so that the former can be tested via the latter.
   if (Invocation.getFrontendOptions().EnableIncrementalDependencyVerifier) {

test/Incremental/PrivateDependencies/reference-dependencies-dynamic-lookup-fine.swift

@@ -29,10 +29,12 @@ import Foundation
   // CHECK-DAG:  dynamicLookup implementation  '' bar true
   // CHECK-DAG:  dynamicLookup interface  '' bar true
 
+  // DUPLICATE-NOT:  dynamicLookup implementation  '' bar true
   // DUPLICATE:  dynamicLookup implementation  '' bar true
-  // DUPLICATE:  dynamicLookup implementation  '' bar true
-  // DUPLICATE:  dynamicLookup interface  '' bar true
+  // DUPLICATE-NOT:  dynamicLookup implementation  '' bar true
+  // DUPLICATE-NOT:  dynamicLookup interface  '' bar true
   // DUPLICATE:  dynamicLookup interface  '' bar true
+  // DUPLICATE-NOT:  dynamicLookup interface  '' bar true
   func bar(_ x: Int, y: Int) {}
   func bar(_ str: String) {}