Add options for debug printing of the explicit module dependency graph.

Sources/SwiftDriver/ExplicitModuleBuilds/InterModuleDependencies/InterModuleDependencyGraph.swift

@@ -10,6 +10,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+import class Foundation.JSONEncoder
+
 /// A map from a module identifier to its info
 public typealias ModuleInfoMap = [ModuleDependencyId: ModuleInfo]
 
@@ -272,6 +274,21 @@ public struct InterModuleDependencyGraph: Codable {
   public var mainModule: ModuleInfo { modules[.swift(mainModuleName)]! }
 }
 
+internal extension InterModuleDependencyGraph {
+  func toJSONString() throws -> String {
+    let encoder = JSONEncoder()
+#if os(Linux) || os(Android)
+    encoder.outputFormatting = [.prettyPrinted]
+#else
+    if #available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *) {
+      encoder.outputFormatting = [.prettyPrinted, .withoutEscapingSlashes]
+    }
+#endif
+    let data = try encoder.encode(self)
+    return String(data: data, encoding: .utf8)!
+  }
+}
+
 public struct InterModuleDependencyImports: Codable {
   public var imports: [String]
 

Sources/SwiftDriver/Jobs/Planning.swift

@@ -637,6 +637,11 @@ extension Driver {
   throws -> InterModuleDependencyGraph {
     var dependencyGraph = try performDependencyScan()
 
+    if parsedOptions.hasArgument(.printPreprocessedExplicitDependencyGraph) {
+      try stdoutStream <<< dependencyGraph.toJSONString()
+      stdoutStream.flush()
+    }
+
     if let externalTargetDetails = externalTargetModuleDetailsMap {
       // Resolve external dependencies in the dependency graph, if any.
       try dependencyGraph.resolveExternalDependencies(for: externalTargetDetails)
@@ -648,6 +653,11 @@ extension Driver {
     // Set dependency modules' paths to be saved in the module cache.
     try resolveDependencyModulePaths(dependencyGraph: &dependencyGraph)
 
+    if parsedOptions.hasArgument(.printExplicitDependencyGraph) {
+      try stdoutStream <<< dependencyGraph.toJSONString()
+      stdoutStream.flush()
+    }
+
     return dependencyGraph
   }
 

Sources/SwiftOptions/ExtraOptions.swift

@@ -22,6 +22,8 @@ extension Option {
   public static let emitModuleSerializeDiagnosticsPath: Option = Option("-emit-module-serialize-diagnostics-path", .separate, attributes: [.argumentIsPath, .supplementaryOutput], metaVar: "<path>", helpText: "Emit a serialized diagnostics file for the emit-module task to <path>")
   public static let emitModuleDependenciesPath: Option = Option("-emit-module-dependencies-path", .separate, attributes: [.argumentIsPath, .supplementaryOutput], metaVar: "<path>", helpText: "Emit a discovered dependencies file for the emit-module task to <path>")
   public static let useFrontendParseableOutput: Option = Option("-use-frontend-parseable-output", .flag, attributes: [.helpHidden], helpText: "Emit parseable-output from swift-frontend jobs instead of from the driver")
+  public static let printExplicitDependencyGraph: Option = Option("-print-explicit-dependency-graph", .flag, attributes: [.helpHidden], helpText: "Print the result of module dependency scanning after external module resolution to output")
+  public static let printPreprocessedExplicitDependencyGraph: Option = Option("-print-preprocessed-explicit-dependency-graph", .flag, attributes: [.helpHidden], helpText: "Print the result of module dependency scanning to output")
 
   // API digester operations
   public static let emitDigesterBaseline: Option = Option("-emit-digester-baseline", .flag, attributes: [.noInteractive, .supplementaryOutput], helpText: "Emit a baseline file for the module using the API digester")
@@ -42,6 +44,8 @@ extension Option {
       Option.emitModuleSeparatelyWMO,
       Option.noEmitModuleSeparatelyWMO,
       Option.useFrontendParseableOutput,
+      Option.printExplicitDependencyGraph,
+      Option.printPreprocessedExplicitDependencyGraph,
       Option.emitDigesterBaseline,
       Option.emitDigesterBaselinePath,
       Option.compareToBaselinePath,