Address comments

Author: David Ungar

Sources/SwiftDriver/IncrementalCompilation/IncrementalCompilationState.swift

@@ -564,8 +564,7 @@ extension IncrementalCompilationState {
     var externalDependencySources = Set<DependencySource>()
     for extDepAndPrint in moduleDependencyGraph.fingerprintedExternalDependencies {
       let extDep = extDepAndPrint.externalDependency
-      let extModTime = (try? fileSystem.getFileInfo(extDep.file).modTime)
-        ?? Date.distantFuture
+      let extModTime = extDep.modTime(fileSystem) ?? Date.distantFuture
       if extModTime >= buildTime {
         for dependent in moduleDependencyGraph.untracedDependents(of: extDepAndPrint) {
           guard let dependencySource = dependent.dependencySource else {

Tests/SwiftDriverTests/ModuleDependencyGraphTests.swift

@@ -1212,10 +1212,11 @@ fileprivate struct SourceFileDependencyGraphMocker {
   }
 
   private mutating func addArc(def: Node, use: Node) {
+    dependencyAccumulator.reserveCapacity(use.sequenceNumber)
     while dependencyAccumulator.count <= use.sequenceNumber {
       dependencyAccumulator.append(nil)
     }
-    var dh = dependencyAccumulator[use.sequenceNumber] ?? {
+    let dh = dependencyAccumulator[use.sequenceNumber] ?? {
       let newOne = DependencyHolder()
       dependencyAccumulator[use.sequenceNumber] = newOne
       return newOne