[Explicit Module Builds] Simplify -explain-module-dependency behavior and introduce -explain-module-dependency-detailed

Sources/SwiftDriver/Driver/Driver.swift

@@ -1474,6 +1474,38 @@ extension Diagnostic.Message {
   }
 }
 
+extension Driver {
+  func explainModuleDependency(_ explainModuleName: String, allPaths: Bool) throws {
+    guard let dependencyPlanner = explicitDependencyBuildPlanner else {
+      fatalError("Cannot explain dependency without Explicit Build Planner")
+    }
+    guard let dependencyPaths = try dependencyPlanner.explainDependency(explainModuleName, allPaths: allPaths) else {
+      diagnosticEngine.emit(.remark("No such module dependency found: '\(explainModuleName)'"))
+      return
+    }
+    diagnosticEngine.emit(.remark("Module '\(moduleOutputInfo.name)' depends on '\(explainModuleName)'"))
+    for path in dependencyPaths {
+      var pathString:String = ""
+      for (index, moduleId) in path.enumerated() {
+        switch moduleId {
+        case .swift(let moduleName):
+          pathString = pathString + "[" + moduleName + "]"
+        case .swiftPrebuiltExternal(let moduleName):
+          pathString = pathString + "[" + moduleName + "]"
+        case .clang(let moduleName):
+          pathString = pathString + "[" + moduleName + "](ObjC)"
+        case .swiftPlaceholder(_):
+          fatalError("Unexpected unresolved Placeholder module")
+        }
+        if index < path.count - 1 {
+          pathString = pathString + " -> "
+        }
+      }
+      diagnosticEngine.emit(.note(pathString))
+    }
+  }
+}
+
 extension Driver {
   /// Determine the driver kind based on the command-line arguments, consuming the arguments
   /// conveying this information.
@@ -1520,34 +1552,10 @@ extension Driver {
     }
 
     // If we're only supposed to explain a dependency on a given module, do so now.
-    if let explainModuleName = parsedOptions.getLastArgument(.explainModuleDependency) {
-      guard let dependencyPlanner = explicitDependencyBuildPlanner else {
-        fatalError("Cannot explain dependency without Explicit Build Planner")
-      }
-      guard let dependencyPaths = try dependencyPlanner.explainDependency(explainModuleName.asSingle) else {
-        diagnosticEngine.emit(.remark("No such module dependency found: '\(explainModuleName.asSingle)'"))
-        return
-      }
-      diagnosticEngine.emit(.remark("Module '\(moduleOutputInfo.name)' depends on '\(explainModuleName.asSingle)'"))
-      for path in dependencyPaths {
-        var pathString:String = ""
-        for (index, moduleId) in path.enumerated() {
-          switch moduleId {
-          case .swift(let moduleName):
-            pathString = pathString + "[" + moduleName + "]"
-          case .swiftPrebuiltExternal(let moduleName):
-            pathString = pathString + "[" + moduleName + "]"
-          case .clang(let moduleName):
-            pathString = pathString + "[" + moduleName + "](ObjC)"
-          case .swiftPlaceholder(_):
-            fatalError("Unexpected unresolved Placeholder module")
-          }
-          if index < path.count - 1 {
-            pathString = pathString + " -> "
-          }
-        }
-        diagnosticEngine.emit(.note(pathString))
-      }
+    if let explainModuleName = parsedOptions.getLastArgument(.explainModuleDependencyDetailed) {
+      try explainModuleDependency(explainModuleName.asSingle, allPaths: true)
+    } else if let explainModuleNameDetailed = parsedOptions.getLastArgument(.explainModuleDependency) {
+      try explainModuleDependency(explainModuleNameDetailed.asSingle, allPaths: false)
     }
 
     if parsedOptions.contains(.driverPrintOutputFileMap) {

Sources/SwiftDriver/ExplicitModuleBuilds/ExplicitDependencyBuildPlanner.swift

@@ -618,8 +618,19 @@ public typealias ExternalTargetModuleDetailsMap = [ModuleDependencyId: ExternalT
 }
 
 internal extension ExplicitDependencyBuildPlanner {
-  func explainDependency(_ dependencyModuleName: String) throws -> [[ModuleDependencyId]]? {
-    return try dependencyGraph.explainDependency(dependencyModuleName: dependencyModuleName)
+  func explainDependency(_ dependencyModuleName: String, allPaths: Bool) throws -> [[ModuleDependencyId]]? {
+    return try dependencyGraph.explainDependency(dependencyModuleName: dependencyModuleName, allPaths: allPaths)
+  }
+
+  func findPath(from source: ModuleDependencyId, to destination: ModuleDependencyId) throws -> [ModuleDependencyId]? {
+    guard dependencyGraph.modules.contains(where: { $0.key == destination }) else { return nil }
+    var result: [ModuleDependencyId]? = nil
+    var visited: Set<ModuleDependencyId> = []
+    try dependencyGraph.findAPath(source: source,
+                                  pathSoFar: [source],
+                                  visited: &visited,
+                                  result: &result) { $0 == destination }
+    return result
   }
 }
 

Sources/SwiftDriver/ExplicitModuleBuilds/InterModuleDependencies/CommonDependencyOperations.swift

@@ -415,46 +415,97 @@ internal extension InterModuleDependencyGraph {
 }
 
 internal extension InterModuleDependencyGraph {
-  func explainDependency(dependencyModuleName: String) throws -> [[ModuleDependencyId]]? {
+  func explainDependency(dependencyModuleName: String, allPaths: Bool) throws -> [[ModuleDependencyId]]? {
     guard modules.contains(where: { $0.key.moduleName == dependencyModuleName }) else { return nil }
-    var results = Set<[ModuleDependencyId]>()
-    try findAllPaths(source: .swift(mainModuleName),
-                     to: dependencyModuleName,
-                     pathSoFar: [.swift(mainModuleName)],
-                     results: &results)
-    return results.sorted(by: { $0.count < $1.count })
+    var result: Set<[ModuleDependencyId]> = []
+    if allPaths {
+      try findAllPaths(source: .swift(mainModuleName),
+                       pathSoFar: [.swift(mainModuleName)],
+                       results: &result,
+                       destinationMatch: { $0.moduleName == dependencyModuleName })
+    } else {
+      var visited: Set<ModuleDependencyId> = []
+      var singlePathResult: [ModuleDependencyId]? = nil
+      if try findAPath(source: .swift(mainModuleName),
+                       pathSoFar: [.swift(mainModuleName)],
+                       visited: &visited,
+                       result: &singlePathResult,
+                       destinationMatch: { $0.moduleName == dependencyModuleName }),
+         let resultingPath = singlePathResult {
+        result = [resultingPath]
+      }
+    }
+    return Array(result)
   }
 
+  @discardableResult
+  func findAPath(source: ModuleDependencyId,
+                 pathSoFar: [ModuleDependencyId],
+                 visited: inout Set<ModuleDependencyId>,
+                 result: inout [ModuleDependencyId]?,
+                 destinationMatch: (ModuleDependencyId) -> Bool) throws -> Bool {
+    // Mark this node as visited
+    visited.insert(source)
+    let sourceInfo = try moduleInfo(of: source)
+    if destinationMatch(source) {
+      // If the source is a target Swift module, also check if it
+      // depends on a corresponding Clang module with the same name.
+      // If it does, add it to the path as well.
+      var completePath = pathSoFar
+      if let dependencies = sourceInfo.directDependencies,
+         dependencies.contains(.clang(source.moduleName)) {
+        completePath.append(.clang(source.moduleName))
+      }
+      result = completePath
+      return true
+    }
+
+    var allDependencies = sourceInfo.directDependencies ?? []
+    if case .swift(let swiftModuleDetails) = sourceInfo.details,
+          let overlayDependencies = swiftModuleDetails.swiftOverlayDependencies {
+      allDependencies.append(contentsOf: overlayDependencies)
+    }
+
+    for dependency in allDependencies {
+      if try findAPath(source: dependency,
+                       pathSoFar: pathSoFar + [dependency],
+                       visited: &visited,
+                       result: &result,
+                       destinationMatch: destinationMatch) {
+        return true
+      }
+    }
+    return false
+  }
 
   private func findAllPaths(source: ModuleDependencyId,
-                            to moduleName: String,
                             pathSoFar: [ModuleDependencyId],
-                            results: inout Set<[ModuleDependencyId]>) throws {
+                            results: inout Set<[ModuleDependencyId]>,
+                            destinationMatch: (ModuleDependencyId) -> Bool) throws {
     let sourceInfo = try moduleInfo(of: source)
-    // If the source is our target, we are done
-    if source.moduleName == moduleName {
+    if destinationMatch(source) {
       // If the source is a target Swift module, also check if it
       // depends on a corresponding Clang module with the same name.
       // If it does, add it to the path as well.
       var completePath = pathSoFar
       if let dependencies = sourceInfo.directDependencies,
-         dependencies.contains(.clang(moduleName)) {
-        completePath.append(.clang(moduleName))
+         dependencies.contains(.clang(source.moduleName)) {
+        completePath.append(.clang(source.moduleName))
       }
       results.insert(completePath)
+      return
     }
 
     var allDependencies = sourceInfo.directDependencies ?? []
     if case .swift(let swiftModuleDetails) = sourceInfo.details,
           let overlayDependencies = swiftModuleDetails.swiftOverlayDependencies {
       allDependencies.append(contentsOf: overlayDependencies)
     }
-
     for dependency in allDependencies {
       try findAllPaths(source: dependency,
-                       to: moduleName,
                        pathSoFar: pathSoFar + [dependency],
-                       results: &results)
+                       results: &results,
+                       destinationMatch: destinationMatch)
     }
   }
 }

Sources/SwiftDriver/Jobs/FrontendJobHelpers.swift

@@ -51,6 +51,7 @@ extension Driver {
     /// If the driver is in Explicit Module Build mode, the dependency graph has been computed
     case computed
   }
+
   /// Add frontend options that are common to different frontend invocations.
   mutating func addCommonFrontendOptions(
     commandLine: inout [Job.ArgTemplate],

Sources/SwiftOptions/Options.swift

@@ -499,7 +499,8 @@ extension Option {
   public static let experimentalSpiImports: Option = Option("-experimental-spi-imports", .flag, attributes: [.helpHidden, .frontend, .noDriver], helpText: "Enable experimental support for SPI imports")
   public static let experimentalSpiOnlyImports: Option = Option("-experimental-spi-only-imports", .flag, attributes: [.helpHidden, .frontend, .noDriver], helpText: "Enable use of @_spiOnly imports")
   public static let enableExperimentalSwiftBasedClosureSpecialization: Option = Option("-experimental-swift-based-closure-specialization", .flag, attributes: [.helpHidden, .frontend, .noDriver], helpText: "Use the experimental Swift based closure-specialization optimization pass instead of the existing C++ one")
-  public static let explainModuleDependency: Option = Option("-explain-module-dependency", .separate, attributes: [], helpText: "Emit remark/notes describing why compilation may depend on a module with a given name.")
+  public static let explainModuleDependencyDetailed: Option = Option("-explain-module-dependency-detailed", .separate, attributes: [], helpText: "Emit remarks describing every possible dependency path that explains why compilation may depend on a module with a given name.")
+  public static let explainModuleDependency: Option = Option("-explain-module-dependency", .separate, attributes: [], helpText: "Emit remark describing why compilation may depend on a module with a given name.")
   public static let explicitAutoLinking: Option = Option("-explicit-auto-linking", .flag, attributes: [], helpText: "Instead of linker-load directives, have the driver specify all link dependencies on the linker invocation. Requires '-explicit-module-build'.")
   public static let explicitDependencyGraphFormat: Option = Option("-explicit-dependency-graph-format=", .joined, attributes: [.helpHidden, .doesNotAffectIncrementalBuild], helpText: "Specify the explicit dependency graph output format to either 'json' or 'dot'")
   public static let explicitInterfaceModuleBuild: Option = Option("-explicit-interface-module-build", .flag, attributes: [.helpHidden, .frontend, .noDriver], helpText: "Use the specified command-line to build the module from interface, instead of flags specified in the interface")
@@ -1371,6 +1372,7 @@ extension Option {
       Option.experimentalSpiImports,
       Option.experimentalSpiOnlyImports,
       Option.enableExperimentalSwiftBasedClosureSpecialization,
+      Option.explainModuleDependencyDetailed,
       Option.explainModuleDependency,
       Option.explicitAutoLinking,
       Option.explicitDependencyGraphFormat,

Tests/SwiftDriverTests/ExplicitModuleBuildTests.swift

@@ -2248,30 +2248,61 @@ final class ExplicitModuleBuildTests: XCTestCase {
           try testInputsPath.appending(component: "ExplicitModuleBuilds")
                             .appending(component: "Swift")
       let sdkArgumentsForTesting = (try? Driver.sdkArgumentsForTesting()) ?? []
-      var driver = try Driver(args: ["swiftc",
-                                     "-I", cHeadersPath.nativePathString(escaped: true),
-                                     "-I", swiftModuleInterfacesPath.nativePathString(escaped: true),
-                                     "-explicit-module-build", "-v",
-                                     "-module-cache-path", moduleCachePath.nativePathString(escaped: true),
-                                     "-working-directory", path.nativePathString(escaped: true),
-                                     "-explain-module-dependency", "A",
-                                     main.nativePathString(escaped: true)] + sdkArgumentsForTesting,
-                              env: ProcessEnv.vars)
-      let jobs = try driver.planBuild()
-      try driver.run(jobs: jobs)
-      XCTAssertTrue(!driver.diagnosticEngine.diagnostics.isEmpty)
-      XCTAssertTrue(driver.diagnosticEngine.diagnostics.contains { $0.behavior == .remark &&
-                                                                   $0.message.text == "Module 'testTraceDependency' depends on 'A'"})
 
-      for diag in driver.diagnosticEngine.diagnostics {
-        print(diag.behavior)
-        print(diag.message)
+      // Detailed explain (all possible paths)
+      do {
+        var driver = try Driver(args: ["swiftc",
+                                       "-I", cHeadersPath.nativePathString(escaped: true),
+                                       "-I", swiftModuleInterfacesPath.nativePathString(escaped: true),
+                                       "-explicit-module-build", "-v",
+                                       "-module-cache-path", moduleCachePath.nativePathString(escaped: true),
+                                       "-working-directory", path.nativePathString(escaped: true),
+                                       "-explain-module-dependency-detailed", "A",
+                                       main.nativePathString(escaped: true)] + sdkArgumentsForTesting,
+                                env: ProcessEnv.vars)
+        let jobs = try driver.planBuild()
+        try driver.run(jobs: jobs)
+        XCTAssertTrue(!driver.diagnosticEngine.diagnostics.isEmpty)
+        XCTAssertTrue(driver.diagnosticEngine.diagnostics.contains { $0.behavior == .remark &&
+          $0.message.text == "Module 'testTraceDependency' depends on 'A'"})
+
+        for diag in driver.diagnosticEngine.diagnostics {
+          print(diag.behavior)
+          print(diag.message)
+        }
+        XCTAssertEqual(driver.diagnosticEngine.diagnostics.filter { $0.behavior == .note}.count, 2)
+        XCTAssertTrue(driver.diagnosticEngine.diagnostics.contains { $0.behavior == .note &&
+          $0.message.text == "[testTraceDependency] -> [A] -> [A](ObjC)"})
+        XCTAssertTrue(driver.diagnosticEngine.diagnostics.contains { $0.behavior == .note &&
+          $0.message.text == "[testTraceDependency] -> [C](ObjC) -> [B](ObjC) -> [A](ObjC)"})
+      }
+
+      // Simple explain (first available path)
+      do {
+        var driver = try Driver(args: ["swiftc",
+                                       "-I", cHeadersPath.nativePathString(escaped: true),
+                                       "-I", swiftModuleInterfacesPath.nativePathString(escaped: true),
+                                       "-explicit-module-build", "-v",
+                                       "-module-cache-path", moduleCachePath.nativePathString(escaped: true),
+                                       "-working-directory", path.nativePathString(escaped: true),
+                                       "-explain-module-dependency", "A",
+                                       main.nativePathString(escaped: true)] + sdkArgumentsForTesting,
+                                env: ProcessEnv.vars)
+        let jobs = try driver.planBuild()
+        try driver.run(jobs: jobs)
+        XCTAssertTrue(!driver.diagnosticEngine.diagnostics.isEmpty)
+        XCTAssertTrue(driver.diagnosticEngine.diagnostics.contains { $0.behavior == .remark &&
+          $0.message.text == "Module 'testTraceDependency' depends on 'A'"})
+
+        for diag in driver.diagnosticEngine.diagnostics {
+          print(diag.behavior)
+          print(diag.message)
+        }
+        XCTAssertEqual(driver.diagnosticEngine.diagnostics.filter { $0.behavior == .note}.count, 1)
+        XCTAssertTrue(driver.diagnosticEngine.diagnostics.contains { $0.behavior == .note &&
+          ($0.message.text == "[testTraceDependency] -> [A] -> [A](ObjC)" ||
+           $0.message.text == "[testTraceDependency] -> [C](ObjC) -> [B](ObjC) -> [A](ObjC)")})
       }
-      XCTAssertEqual(driver.diagnosticEngine.diagnostics.filter { $0.behavior == .note}.count, 2)
-      XCTAssertTrue(driver.diagnosticEngine.diagnostics.contains { $0.behavior == .note &&
-                                                                   $0.message.text == "[testTraceDependency] -> [A] -> [A](ObjC)"})
-      XCTAssertTrue(driver.diagnosticEngine.diagnostics.contains { $0.behavior == .note &&
-                                                                   $0.message.text == "[testTraceDependency] -> [C](ObjC) -> [B](ObjC) -> [A](ObjC)"})
     }
   }