Read the extended module's name from the SymbolGraph.

Sources/SymbolKit/SymbolGraph/Module.swift

@@ -19,6 +19,12 @@ extension SymbolGraph {
         /// Optional bystander module names.
         public var bystanders: [String]?
 
+        /// Optional extended module name.
+        ///
+        /// Extended module name is set when the symbol graph contains symbols that are all
+        /// extensions from a specific module.
+        public var extended: String?
+
         /// The platform intended for deployment.
         public var platform: Platform
 
@@ -29,18 +35,20 @@ extension SymbolGraph {
         /// but one created implicitly to hold relationships.
         public var isVirtual: Bool = false
 
-        public init(name: String, platform: Platform, version: SemanticVersion? = nil, bystanders: [String]? = nil, isVirtual: Bool = false) {
+        public init(name: String, platform: Platform, version: SemanticVersion? = nil, bystanders: [String]? = nil, extended: String? = nil, isVirtual: Bool = false) {
             self.name = name
             self.platform = platform
             self.version = version
             self.bystanders = bystanders
+            self.extended = extended
             self.isVirtual = isVirtual
         }
 
         public init(from decoder: Decoder) throws {
             let container = try decoder.container(keyedBy: CodingKeys.self)
             self.name = try container.decode(String.self, forKey: .name)
             self.bystanders = try container.decodeIfPresent([String].self, forKey: .bystanders)
+            self.extended = try container.decodeIfPresent(String.self, forKey: .extended)
             self.platform = try container.decode(Platform.self, forKey: .platform)
             self.version = try container.decodeIfPresent(SemanticVersion.self, forKey: .version)
             self.isVirtual = try container.decodeIfPresent(Bool.self, forKey: .isVirtual) ?? false

Sources/SymbolKit/UnifiedSymbolGraph/GraphCollector.swift

@@ -136,8 +136,15 @@ extension GraphCollector {
         // extension to another modules.
         // The snippet extractor generates symbol graph files without an `@` symbol and with the
         // the metadata that indicates the module `isVirtual`.
-        let isMainSymbolGraph = !url.lastPathComponent.contains("@") && !graph.module.isVirtual
-
+        let isMainSymbolGraph: Bool
+        if let extended = graph.module.extended {
+            // The "extended" field was added in a revision to the symbolgraph format. When that
+            // field is not set, the symbolgraph data was emitted by a previous compiler version.
+            isMainSymbolGraph = extended == graph.module.name
+        } else {
+            isMainSymbolGraph = !url.lastPathComponent.contains("@") && !graph.module.isVirtual
+        }
+
         let moduleName: String
         if isMainSymbolGraph && graph.module.bystanders == nil {
             // When bystander modules are present, the symbol graph is a cross-import overlay, and
@@ -158,6 +165,10 @@ extension GraphCollector {
             if url.lastPathComponent.contains("-snippets.symbols.json") {
                 return (graph.module.name, isMainSymbolGraph)
             }
+
+            if let extended = graph.module.extended {
+                return (extended, isMainSymbolGraph)
+            }
 
             // For extension symbol graphs, derive the extended module's name from the file name.
             //

Tests/SymbolKitTests/UnifiedGraph/GraphCollectorTests.swift

@@ -127,8 +127,26 @@ class GraphCollectorTests: XCTestCase {
                                            mixins: [:])
                                  ],
                                  relationships: [])
+
+        var (extensionName, extensionIsMain) = GraphCollector.moduleNameFor(a_At_B, at: .init(fileURLWithPath: "A@B.symbols.json"))
+        XCTAssertFalse(extensionIsMain)
+        XCTAssertEqual("B", extensionName)
+
+        // "A@B" style graph, with extension of B in module A, using shortened name scheme.
+        let a_At_B_Shortened = SymbolGraph(metadata: .init(formatVersion: .init(major: 1, minor: 0, patch: 0), generator: "unit-test"),
+                                 module: .init(name: "A", platform: .init(), extended: "B"),
+                                 symbols: [
+                                     .init(identifier: .init(precise: "s:BBAatB", interfaceLanguage: "swift"),
+                                           names: .init(title: "AatB", navigator: nil, subHeading: nil, prose: nil),
+                                           pathComponents: ["B", "AatB"],
+                                           docComment: nil,
+                                           accessLevel: .init(rawValue: "public"),
+                                           kind: .init(parsedIdentifier: .class, displayName: "Class"),
+                                           mixins: [:])
+                                 ],
+                                 relationships: [])
 
-        let (extensionName, extensionIsMain) = GraphCollector.moduleNameFor(a_At_B, at: .init(fileURLWithPath: "A@B.symbols.json"))
+        (extensionName, extensionIsMain) = GraphCollector.moduleNameFor(a_At_B_Shortened, at: .init(fileURLWithPath: "ABCDEF123456.symbols.json"))
         XCTAssertFalse(extensionIsMain)
         XCTAssertEqual("B", extensionName)
 
@@ -186,5 +204,12 @@ class GraphCollectorTests: XCTestCase {
         let (extendedB, extendedBIsMain) = GraphCollector.moduleNameFor(a_At_B, at: .init(fileURLWithPath: "_A_B@B.symbols.json"))
         XCTAssertFalse(extendedBIsMain)
         XCTAssertEqual("B", extendedB)
+
+        // "A@B" style graph, with extension of B in module A, using shortened name scheme.
+        var a_At_B_ShortenedFileName = a_At_B
+        a_At_B_ShortenedFileName.module.extended = "B"
+        let (extendedBShortForm, extendedBIsMainShortForm) = GraphCollector.moduleNameFor(a_At_B, at: .init(fileURLWithPath: "_A_B@B.symbols.json"))
+        XCTAssertFalse(extendedBIsMainShortForm)
+        XCTAssertEqual("B", extendedBShortForm)
     }
 }