Add generated interface support for SDK types in inlay hints

Sources/SwiftLanguageService/InlayHintResolve.swift

@@ -62,6 +62,7 @@ extension SwiftLanguageService {
   /// Looks up the definition location for the type at the given position.
   ///
   /// This is used by inlay hint resolution to enable go-to-definition on type hints.
+  /// For SDK types, this returns a location in the generated interface.
   func lookupTypeDefinitionLocation(
     snapshot: DocumentSnapshot,
     position: Position
@@ -82,12 +83,32 @@ extension SwiftLanguageService {
       return nil
     }
 
-    if let typeInfo = try await cursorInfoFromTypeUSR(typeUsr, in: snapshot),
-      let location = typeInfo.symbolInfo.bestLocalDeclaration
-    {
+    guard let typeInfo = try await cursorInfoFromTypeUSR(typeUsr, in: snapshot) else {
+      return nil
+    }
+
+    // For local types, return the local declaration
+    if let location = typeInfo.symbolInfo.bestLocalDeclaration {
       return location
     }
 
+    // For SDK types, fall back to generated interface
+    if typeInfo.symbolInfo.isSystem ?? false,
+      let systemModule = typeInfo.symbolInfo.systemModule
+    {
+      let interfaceDetails = try await self.openGeneratedInterface(
+        document: snapshot.uri,
+        moduleName: systemModule.moduleName,
+        groupName: systemModule.groupName,
+        symbolUSR: typeInfo.symbolInfo.usr
+      )
+      if let details = interfaceDetails {
+        let position = details.position ?? Position(line: 0, utf16index: 0)
+        return Location(uri: details.uri, range: Range(position))
+      }
+    }
+
     return nil
   }
 }
+

Tests/SourceKitLSPTests/InlayHintTests.swift

@@ -372,4 +372,37 @@ final class InlayHintTests: SourceKitLSPTestCase {
     XCTAssertEqual(location.uri, try project.uri(for: "MyType.swift"))
     XCTAssertEqual(location.range, try Range(project.position(of: "1️⃣", in: "MyType.swift")))
   }
+
+  func testInlayHintResolveSDKType() async throws {
+    let project = try await IndexedSingleSwiftFileTestProject(
+      """
+      let 1️⃣x = "hello"
+      """,
+      indexSystemModules: true
+    )
+
+    let request = InlayHintRequest(textDocument: TextDocumentIdentifier(project.fileURI), range: nil)
+    let hints = try await project.testClient.send(request)
+
+    guard let typeHint = hints.first(where: { $0.kind == .type }) else {
+      XCTFail("Expected type hint for String")
+      return
+    }
+
+    let resolvedHint = try await project.testClient.send(InlayHintResolveRequest(inlayHint: typeHint))
+
+    guard case .parts(let parts) = resolvedHint.label,
+      let location = parts.only?.location
+    else {
+      XCTFail("Expected label part to have location for go-to-definition")
+      return
+    }
+
+    // Should point to generated Swift interface
+    XCTAssertTrue(
+      location.uri.pseudoPath.hasSuffix(".swiftinterface"),
+      "Expected .swiftinterface file, got: \(location.uri.pseudoPath)"
+    )
+  }
 }
+