Generate Swift module maps during the build

Sources/Build/BuildDescription/SwiftTargetBuildDescription.swift

@@ -220,7 +220,13 @@ public final class SwiftTargetBuildDescription {
     let fileSystem: FileSystem
 
     /// The modulemap file for this target, if any.
-    private(set) var moduleMap: AbsolutePath?
+    var moduleMap: AbsolutePath? {
+        if self.shouldGenerateModuleMap {
+            return self.tempsPath.appending(component: moduleMapFilename)
+        } else {
+            return nil
+        }
+    }
 
     /// The results of applying any build tool plugins to this target.
     public let buildToolPluginInvocationResults: [BuildToolPluginInvocationResult]
@@ -237,6 +243,11 @@ public final class SwiftTargetBuildDescription {
     /// Whether or not to generate code for test observation.
     private let shouldGenerateTestObservation: Bool
 
+    /// Whether or not to generate a module map for this target.
+    var shouldGenerateModuleMap: Bool {
+        return self.shouldEmitObjCCompatibilityHeader
+    }
+
     /// Create a new target description with target and build parameters.
     init(
         package: ResolvedPackage,
@@ -286,10 +297,6 @@ public final class SwiftTargetBuildDescription {
             observabilityScope: observabilityScope
         )
 
-        if self.shouldEmitObjCCompatibilityHeader {
-            self.moduleMap = try self.generateModuleMap()
-        }
-
         // Do nothing if we're not generating a bundle.
         if self.bundlePath != nil {
             try self.generateResourceAccessor()
@@ -723,31 +730,6 @@ public final class SwiftTargetBuildDescription {
         return path
     }
 
-    /// Generates the module map for the Swift target and returns its path.
-    private func generateModuleMap() throws -> AbsolutePath {
-        let path = self.tempsPath.appending(component: moduleMapFilename)
-
-        let bytes = ByteString(
-            #"""
-            module \#(self.target.c99name) {
-                header "\#(self.objCompatibilityHeaderPath.pathString)"
-                requires objc
-            }
-
-            """#.utf8
-        )
-
-        // Return early if the contents are identical.
-        if self.fileSystem.isFile(path), try self.fileSystem.readFileContents(path) == bytes {
-            return path
-        }
-
-        try self.fileSystem.createDirectory(path.parentDirectory, recursive: true)
-        try self.fileSystem.writeFileContents(path, bytes: bytes)
-
-        return path
-    }
-
     /// Returns the path to the ObjC compatibility header for this Swift target.
     var objCompatibilityHeaderPath: AbsolutePath {
         self.tempsPath.appending("\(self.target.name)-Swift.h")

Sources/Build/BuildManifest/LLBuildManifestBuilder+Swift.swift

@@ -37,8 +37,26 @@ extension LLBuildManifestBuilder {
     func createSwiftCompileCommand(
         _ target: SwiftTargetBuildDescription
     ) throws {
+        var modulesReadyInputs = [Node]()
+
+        // If the given target needs a generated module map, set up the dependency and required task to write out the module map.
+        if target.shouldGenerateModuleMap, let moduleMapPath = target.moduleMap {
+            modulesReadyInputs.append(.file(moduleMapPath))
+
+            self.manifest.addWriteSwiftModuleMapCommand(
+                moduleName: target.target.c99name,
+                objCompatibilityHeaderPath: target.objCompatibilityHeaderPath,
+                outputPath: moduleMapPath
+            )
+        }
+
+        let modulesReady = self.addPhonyCommand(
+            targetName: target.target.getLLBuildModulesReadyCmdName(config: self.buildConfig),
+            inputs: modulesReadyInputs
+        )
+
         // Inputs.
-        let inputs = try self.computeSwiftCompileCmdInputs(target)
+        let inputs = try self.computeSwiftCompileCmdInputs(target) + [modulesReady]
 
         // Outputs.
         let objectNodes = try target.objects.map(Node.file)

Sources/LLBuildManifest/LLBuildManifest.swift

@@ -29,6 +29,7 @@ public enum WriteAuxiliary {
         LinkFileList.self,
         SourcesFileList.self,
         SwiftGetVersion.self,
+        SwiftModuleMap.self,
         XCTestInfoPlist.self
     ]
 
@@ -56,6 +57,39 @@ public enum WriteAuxiliary {
         }
     }
 
+    public struct SwiftModuleMap: AuxiliaryFileType {
+        public static let name = "swift-modulemap"
+
+        public static func computeInputs(
+            moduleName: String,
+            objCompatibilityHeaderPath: AbsolutePath
+        ) -> [Node] {
+            return [
+                .virtual(Self.name),
+                .virtual(moduleName),
+                // this can't be a path since we don't create any rule to generate this file, it's a byproduct of the compilation
+                .virtual(objCompatibilityHeaderPath.pathString)
+            ]
+        }
+
+        public static func getFileContents(inputs: [Node]) throws -> String {
+            guard inputs.count == 2 else {
+                throw StringError("invalid module map generation task, inputs: \(inputs)")
+            }
+
+            let moduleName = inputs[0].extractedVirtualNodeName
+            let objCompatibilityHeaderPath = try AbsolutePath(validating: inputs[1].extractedVirtualNodeName)
+
+            return #"""
+                module \#(moduleName) {
+                    header "\#(objCompatibilityHeaderPath.pathString)"
+                    requires objc
+                }
+
+                """#
+        }
+    }
+
     public struct ClangModuleMap: AuxiliaryFileType {
         public static let name = "modulemap"
 
@@ -334,6 +368,20 @@ public struct LLBuildManifest {
         commands[name] = Command(name: name, tool: tool)
     }
 
+    public mutating func addWriteSwiftModuleMapCommand(
+        moduleName: String,
+        objCompatibilityHeaderPath: AbsolutePath,
+        outputPath: AbsolutePath
+    ) {
+        let inputs = WriteAuxiliary.SwiftModuleMap.computeInputs(
+            moduleName: moduleName,
+            objCompatibilityHeaderPath: objCompatibilityHeaderPath
+        )
+        let tool = WriteAuxiliaryFile(inputs: inputs, outputFilePath: outputPath)
+        let name = outputPath.pathString
+        commands[name] = Command(name: name, tool: tool)
+    }
+
     public mutating func addWriteClangModuleMapCommand(
         targetName: String,
         moduleName: String,

Tests/BuildTests/BuildPlanTests.swift

@@ -874,7 +874,7 @@ final class BuildPlanTests: XCTestCase {
             let contents: String = try fs.readFileContents(yaml)
             let swiftGetVersionFilePath = try XCTUnwrap(llbuild.swiftGetVersionFiles.first?.value)
             XCTAssertMatch(contents, .contains("""
-                    inputs: ["\(Pkg.appending(components: "Sources", "exe", "main.swift").escapedPathString)","\(swiftGetVersionFilePath.escapedPathString)","\(buildPath.appending(components: "PkgLib.swiftmodule").escapedPathString)","\(buildPath.appending(components: "exe.build", "sources").escapedPathString)"]
+                    inputs: ["\(Pkg.appending(components: "Sources", "exe", "main.swift").escapedPathString)","\(swiftGetVersionFilePath.escapedPathString)","\(buildPath.appending(components: "PkgLib.swiftmodule").escapedPathString)","<exe-release.modules-ready>","\(buildPath.appending(components: "exe.build", "sources").escapedPathString)"]
                 """))
 
         }
@@ -904,7 +904,7 @@ final class BuildPlanTests: XCTestCase {
             let buildPath = plan.buildParameters.dataPath.appending(components: "debug")
             let swiftGetVersionFilePath = try XCTUnwrap(llbuild.swiftGetVersionFiles.first?.value)
             XCTAssertMatch(contents, .contains("""
-                    inputs: ["\(Pkg.appending(components: "Sources", "exe", "main.swift").escapedPathString)","\(swiftGetVersionFilePath.escapedPathString)","\(buildPath.appending(components: "exe.build", "sources").escapedPathString)"]
+                    inputs: ["\(Pkg.appending(components: "Sources", "exe", "main.swift").escapedPathString)","\(swiftGetVersionFilePath.escapedPathString)","<exe-debug.modules-ready>","\(buildPath.appending(components: "exe.build", "sources").escapedPathString)"]
                 """))
         }
     }
@@ -3972,7 +3972,7 @@ final class BuildPlanTests: XCTestCase {
         let suffix = ""
 #endif
         XCTAssertMatch(contents, .contains("""
-                inputs: ["\(PkgA.appending(components: "Sources", "swiftlib", "lib.swift").escapedPathString)","\(swiftGetVersionFilePath.escapedPathString)","\(buildPath.appending(components: "exe\(suffix)").escapedPathString)","\(buildPath.appending(components: "swiftlib.build", "sources").escapedPathString)"]
+                inputs: ["\(PkgA.appending(components: "Sources", "swiftlib", "lib.swift").escapedPathString)","\(swiftGetVersionFilePath.escapedPathString)","\(buildPath.appending(components: "exe\(suffix)").escapedPathString)","<swiftlib-debug.modules-ready>","\(buildPath.appending(components: "swiftlib.build", "sources").escapedPathString)"]
                 outputs: ["\(buildPath.appending(components: "swiftlib.build", "lib.swift.o").escapedPathString)","\(buildPath.escapedPathString)
             """))
     }