[Xcodeproj] Assign stable object IDs to critical objects.

 - Objects where can be referenced across projects should have stable IDs,
   because Xcode will embed the references to these targets by their ID. We want
   those references to remain stable even as the project evolves under
   regeneration.

 - <rdar://problem/31019219> Generated .xcodeproj needs to use stable object IDs

Author: ddunbar

Sources/Xcodeproj/XcodeProjectModel.swift

@@ -75,8 +75,8 @@ public struct Xcode {
 
         /// Creates and adds a new target (which does not initially have any
         /// build phases).
-        public func addTarget(productType: Target.ProductType?, name: String) -> Target {
-            let target = Target(productType: productType, name: name)
+        public func addTarget(objectID: String? = nil, productType: Target.ProductType?, name: String) -> Target {
+            let target = Target(objectID: objectID, productType: productType, name: name)
             targets.append(target)
             return target
         }
@@ -119,10 +119,12 @@ public struct Xcode {
 
     /// A reference to a file system entity (a file, folder, etc).
     public class FileReference: Reference {
+        var objectID: String?
         var fileType: String?
 
-        init(path: String, pathBase: RefPathBase = .groupDir, name: String? = nil, fileType: String? = nil) {
+        init(path: String, pathBase: RefPathBase = .groupDir, name: String? = nil, fileType: String? = nil, objectID: String? = nil) {
             super.init(path: path, pathBase: pathBase, name: name)
+            self.objectID = objectID
             self.fileType = fileType
         }
     }
@@ -150,16 +152,18 @@ public struct Xcode {
             path: String,
             pathBase: RefPathBase = .groupDir,
             name: String? = nil,
-            fileType: String? = nil
+            fileType: String? = nil,
+            objectID: String? = nil
         ) -> FileReference {
-            let fref = FileReference(path: path, pathBase: pathBase, name: name, fileType: fileType)
+            let fref = FileReference(path: path, pathBase: pathBase, name: name, fileType: fileType, objectID: objectID)
             subitems.append(fref)
             return fref
         }
     }
 
     /// An Xcode target, representing a single entity to build.
     public class Target {
+        var objectID: String?
         var name: String
         var productName: String
         var productType: ProductType?
@@ -176,7 +180,8 @@ public struct Xcode {
             case unitTest = "com.apple.product-type.bundle.unit-test"
             var asString: String { return rawValue }
         }
-        init(productType: ProductType?, name: String) {
+        init(objectID: String?, productType: ProductType?, name: String) {
+            self.objectID = objectID
             self.name = name
             self.productType = productType
             self.productName = name

Sources/Xcodeproj/XcodeProjectModelSerialization.swift

@@ -470,19 +470,19 @@ fileprivate class PropertyListSerializer {
     /// dictionary is what gets written to the property list.
     var idsToDicts = [String: PropertyList]()
 
-    /// Returns the identifier for the object, assigning one if needed.
+    /// Returns the quoted identifier for the object, assigning one if needed.
     func id(of object: PropertyListSerializable) -> String {
         // We need a "serialized object ref" wrapper for the `objsToIds` map.
         let serObjRef = SerializedObjectRef(object)
         if let id = objsToIds[serObjRef] {
-            return id
+            return "\"\(id)\""
         }
         // We currently always assign identifiers starting at 1 and going up.
         // FIXME: This is a suboptimal format for object identifier strings;
         // for debugging purposes they should at least sort in numeric order.
-        let id = "OBJ_\(objsToIds.count + 1)"
+        let id = object.objectID ?? "OBJ_\(objsToIds.count + 1)"
         objsToIds[serObjRef] = id
-        return id
+        return "\"\(id)\""
     }
 
     /// Serializes `object` by asking it to construct a plist dictionary and
@@ -546,13 +546,24 @@ fileprivate protocol PropertyListSerializable: class {
     /// matters.  So this is acceptable for now in the interest of getting it
     /// done.
 
+    /// A custom ID to use for the instance, if enabled.
+    ///
+    /// This ID must be unique across the entire serialized graph.
+    var objectID: String? { get }
+    
     /// Should create and return a property list dictionary of the object's
     /// attributes.  This function may also use the serializer's `serialize()`
     /// function to serialize other objects, and may use `id(of:)` to access
     /// ids of objects that either have or will be serialized.
     func serialize(to serializer: PropertyListSerializer) -> [String: PropertyList]
 }
 
+extension PropertyListSerializable {
+    var objectID: String? {
+        return nil
+    }
+}
+
 /// A very simple representation of a property list.  Note that the `identifier`
 /// enum is not strictly necessary, but useful to semantically distinguish the
 /// strings that represents object identifiers from those that are just data.

Sources/Xcodeproj/pbxproj().swift

@@ -194,6 +194,20 @@ func xcodeProject(
     // Determine the list of external package dependencies, if any.
     let externalPackages = graph.packages.filter({ !graph.rootPackages.contains($0) })
 
+    // Build a backmap of targets and products to packages.
+    var packagesByTarget = [ResolvedTarget: ResolvedPackage]()
+    for package in graph.packages {
+        for target in package.targets {
+            packagesByTarget[target] = package
+        }
+    }
+    var packagesByProduct = [ResolvedProduct: ResolvedPackage]()
+    for package in graph.packages {
+        for product in package.products {
+            packagesByProduct[product] = package
+        }
+    }
+    
     // To avoid creating multiple groups for the same path, we keep a mapping
     // of the paths we've seen and the corresponding groups we've created.
     var srcPathsToGroups: [AbsolutePath: Xcode.Group] = [:]
@@ -351,7 +365,10 @@ func xcodeProject(
         }
 
         // Create a Xcode target for the target.
-        let xcodeTarget = project.addTarget(productType: productType, name: target.name)
+        let package = packagesByTarget[target]!
+        let xcodeTarget = project.addTarget(
+            objectID: "\(package.name)::\(target.name)",
+            productType: productType, name: target.name)
 
         // Set the product name to the C99-mangled form of the target name.
         xcodeTarget.productName = target.c99name
@@ -427,7 +444,7 @@ func xcodeProject(
         targetSettings.common.FRAMEWORK_SEARCH_PATHS = ["$(inherited)", "$(PLATFORM_DIR)/Developer/Library/Frameworks"]
 
         // Add a file reference for the target's product.
-        let productRef = productsGroup.addFileReference(path: target.productPath.asString, pathBase: .buildDir)
+        let productRef = productsGroup.addFileReference(path: target.productPath.asString, pathBase: .buildDir, objectID: "\(package.name)::\(target.name)::Product")
 
         // Set that file reference as the target's product reference.
         xcodeTarget.productReference = productRef
@@ -556,7 +573,10 @@ func xcodeProject(
         // Go on to next product if we already have a target with the same name.
         if targetNames.contains(product.name) { continue }
         // Otherwise, create an aggreate target.
-        let aggregateTarget = project.addTarget(productType: nil, name: product.name)
+        let package = packagesByProduct[product]!
+        let aggregateTarget = project.addTarget(
+            objectID: "\(package.name)::\(product.name)::ProductTarget",
+            productType: nil, name: product.name)
         // Add dependencies on the targets created for each of the dependencies.
         for target in product.targets {
             // Find the target that corresponds to the target.  There might not