[Explicit Module Builds] Pass external module dependencies as placeholders to the scanning action

Author: artemcm

Sources/SwiftDriver/Driver/Driver.swift

@@ -194,7 +194,7 @@ public struct Driver {
   @_spi(Testing) public var explicitModuleBuildHandler: ExplicitModuleBuildHandler? = nil
 
   /// A collection describing external dependencies for the current main module that may be invisible to
-  /// the driver itself, but visible to its clients (e.g. build systems like SwiftPM). Along with the external dependenceis'
+  /// the driver itself, but visible to its clients (e.g. build systems like SwiftPM). Along with the external dependencies'
   /// module dependency graphs.
   internal var externalDependencyArtifactMap: ExternalDependencyArtifactMap? = nil
 

Sources/SwiftDriver/Explicit Module Builds/ExplicitModuleBuildHandler.swift

@@ -13,13 +13,13 @@ import TSCBasic
 import TSCUtility
 import Foundation
 
+/// A map from a module identifier to a pair consisting of a path to its .swiftmodule file and its module dependency graph.
 public typealias ExternalDependencyArtifactMap =
     [ModuleDependencyId: (AbsolutePath, InterModuleDependencyGraph)]
 
 /// In Explicit Module Build mode, this handler is responsible for generating and providing
 /// build jobs for all module dependencies and providing compile command options
 /// that specify said explicit module dependencies.
-
 @_spi(Testing) public struct ExplicitModuleBuildHandler {
   /// The module dependency graph.
   public var dependencyGraph: InterModuleDependencyGraph
@@ -119,13 +119,27 @@ public typealias ExternalDependencyArtifactMap =
   /// - Generate Job: S1
   ///
   mutating public func generateExplicitModuleDependenciesBuildJobs() throws -> [Job] {
+    // Resolve placeholder dependencies in the dependency graph, if any.
+    try resolvePlaceholderDependencies()
+
+    // Compute jobs for all main module dependencies
     var mainModuleInputs: [TypedVirtualPath] = []
     var mainModuleCommandLine: [Job.ArgTemplate] = []
     try resolveMainModuleDependencies(inputs: &mainModuleInputs,
                                       commandLine: &mainModuleCommandLine)
     return Array(swiftModuleBuildCache.values) + clangTargetModuleBuildCache.allJobs
   }
 
+  /// TODO: Explain
+  mutating public func resolvePlaceholderDependencies() throws {
+    print("Hello, World")
+    for moduleId in dependencyGraph.modules.keys {
+      if case .swiftPlaceholder(let moduleName) = moduleId {
+        print(moduleName)
+      }
+    }
+  }
+
   /// Resolve all module dependencies of the main module and add them to the lists of
   /// inputs and command line flags.
   mutating public func resolveMainModuleDependencies(inputs: inout [TypedVirtualPath],
@@ -306,7 +320,7 @@ public typealias ExternalDependencyArtifactMap =
                                               clangDependencyArtifacts: inout [ClangModuleArtifactInfo],
                                               swiftDependencyArtifacts: inout [SwiftModuleArtifactInfo]
   ) throws {
-    for dependencyId in try dependencyGraph.moduleInfo(of: moduleId).directDependencies {
+    for dependencyId in try dependencyGraph.moduleInfo(of: moduleId).directDependencies! {
       guard addedDependenciesSet.insert(dependencyId).inserted else {
         continue
       }
@@ -323,6 +337,8 @@ public typealias ExternalDependencyArtifactMap =
                                        addedDependenciesSet: &addedDependenciesSet,
                                        clangDependencyArtifacts: &clangDependencyArtifacts,
                                        swiftDependencyArtifacts: &swiftDependencyArtifacts)
+        case .swiftPlaceholder:
+          fatalError("Unresolved placeholder dependencies at planning stage.")
       }
     }
   }

Sources/SwiftDriver/Explicit Module Builds/ModuleDependencyScanning.swift

@@ -31,6 +31,15 @@ extension Driver {
                                  moduleDependencyGraphUse: .dependencyScan)
     // FIXME: MSVC runtime flags
 
+    // Pass in external dependencies to be treated as placeholder dependencies by the scanner
+    if let externalDependencyArtifactMap = externalDependencyArtifactMap {
+      let dependencyPlaceholderMapFile =
+        try serializeExternalDependencyArtifacts(externalDependencyArtifactMap:
+                                                  externalDependencyArtifactMap)
+      commandLine.appendFlag("-placeholder-dependency-module-map-file")
+      commandLine.appendPath(dependencyPlaceholderMapFile)
+    }
+
     // Pass on the input files
     commandLine.append(contentsOf: inputFiles.map { .path($0.file)})
 
@@ -44,4 +53,24 @@ extension Driver {
                outputs: [TypedVirtualPath(file: .standardOutput, type: .jsonDependencies)],
                supportsResponseFiles: true)
   }
+
+  /// Serialize a map of placeholder
+  func serializeExternalDependencyArtifacts(externalDependencyArtifactMap: ExternalDependencyArtifactMap)
+  throws -> AbsolutePath {
+    let temporaryDirectory = try determineTempDirectory()
+    let placeholderMapFilePath =
+      temporaryDirectory.appending(component: "\(moduleOutputInfo.name)-placeholder-modules.json")
+
+    var placeholderArtifacts: [SwiftModuleArtifactInfo] = []
+    for (moduleId, dependencyInfo) in externalDependencyArtifactMap {
+      placeholderArtifacts.append(
+          SwiftModuleArtifactInfo(name: moduleId.moduleName,
+                                  modulePath: dependencyInfo.0.description))
+    }
+    let encoder = JSONEncoder()
+    encoder.outputFormatting = [.prettyPrinted]
+    let contents = try encoder.encode(placeholderArtifacts)
+    try fileSystem.writeFileContents(placeholderMapFilePath, bytes: ByteString(contents))
+    return placeholderMapFilePath
+  }
 }