Add signature help LSP request support (#2250)

Depends on swiftlang/swift#83378

---

Adds support for the LSP signature help request.

> [!NOTE]
> As of swiftlang/swift#83378, SourceKitD still
doesn't separate parameter documentation from the signature
documentation and thus parameters don't have their own separate
documentation. This should just work once SourceKitD implements this
functionality and we'll only need to modify the tests.

Author: a7medev

Sources/ClangLanguageService/ClangLanguageService.swift

@@ -491,6 +491,10 @@ extension ClangLanguageService {
     return try await forwardRequestToClangd(req)
   }
 
+  package func signatureHelp(_ req: SignatureHelpRequest) async throws -> SignatureHelp? {
+    return try await forwardRequestToClangd(req)
+  }
+
   package func hover(_ req: HoverRequest) async throws -> HoverResponse? {
     return try await forwardRequestToClangd(req)
   }

Sources/LanguageServerProtocol/SupportTypes/RegistrationOptions.swift

@@ -90,6 +90,46 @@ public struct CompletionRegistrationOptions: RegistrationOptions, TextDocumentRe
   }
 }
 
+/// Signature help registration options.
+public struct SignatureHelpRegistrationOptions: RegistrationOptions, TextDocumentRegistrationOptionsProtocol, Hashable {
+  public var textDocumentRegistrationOptions: TextDocumentRegistrationOptions
+  public var signatureHelpOptions: SignatureHelpOptions
+
+  public init(documentSelector: DocumentSelector? = nil, signatureHelpOptions: SignatureHelpOptions) {
+    self.textDocumentRegistrationOptions =
+      TextDocumentRegistrationOptions(documentSelector: documentSelector)
+    self.signatureHelpOptions = signatureHelpOptions
+  }
+
+  public init?(fromLSPDictionary dictionary: [String: LSPAny]) {
+    guard let signatureHelpOptions = SignatureHelpOptions(fromLSPDictionary: dictionary) else {
+      return nil
+    }
+
+    self.signatureHelpOptions = signatureHelpOptions
+
+    guard let textDocumentRegistrationOptions = TextDocumentRegistrationOptions(fromLSPDictionary: dictionary) else {
+      return nil
+    }
+
+    self.textDocumentRegistrationOptions = textDocumentRegistrationOptions
+  }
+
+  public func encodeToLSPAny() -> LSPAny {
+    var dict: [String: LSPAny] = [:]
+
+    if case .dictionary(let dictionary) = signatureHelpOptions.encodeToLSPAny() {
+      dict.merge(dictionary) { (current, _) in current }
+    }
+
+    if case .dictionary(let dictionary) = textDocumentRegistrationOptions.encodeToLSPAny() {
+      dict.merge(dictionary) { (current, _) in current }
+    }
+
+    return .dictionary(dict)
+  }
+}
+
 /// Folding range registration options.
 public struct FoldingRangeRegistrationOptions: RegistrationOptions, TextDocumentRegistrationOptionsProtocol, Hashable {
   public var textDocumentRegistrationOptions: TextDocumentRegistrationOptions

Sources/LanguageServerProtocol/SupportTypes/ServerCapabilities.swift

@@ -612,6 +612,38 @@ public struct SignatureHelpOptions: WorkDoneProgressOptions, Codable, Hashable,
     self.triggerCharacters = triggerCharacters
     self.retriggerCharacters = retriggerCharacters
   }
+
+  public init?(fromLSPDictionary dictionary: [String: LSPAny]) {
+    if let arrayAny = dictionary["triggerCharacters"] {
+      triggerCharacters = [String](fromLSPArray: arrayAny)
+    }
+
+    if let arrayAny = dictionary["retriggerCharacters"] {
+      retriggerCharacters = [String](fromLSPArray: arrayAny)
+    }
+
+    if case .bool(let value) = dictionary["workDoneProgress"] {
+      workDoneProgress = value
+    }
+  }
+
+  public func encodeToLSPAny() -> LSPAny {
+    var dict: [String: LSPAny] = [:]
+
+    if let triggerCharacters {
+      dict["triggerCharacters"] = triggerCharacters.encodeToLSPAny()
+    }
+
+    if let retriggerCharacters {
+      dict["retriggerCharacters"] = retriggerCharacters.encodeToLSPAny()
+    }
+
+    if let workDoneProgress {
+      dict["workDoneProgress"] = .bool(workDoneProgress)
+    }
+
+    return .dictionary(dict)
+  }
 }
 
 public struct DocumentFilter: Codable, Hashable, Sendable {

Sources/SourceKitD/sourcekitd_uids.swift

@@ -103,6 +103,14 @@ package struct sourcekitd_api_keys {
   package let fullyAnnotatedDecl: sourcekitd_api_uid_t
   /// `key.fully_annotated_generic_signature`
   package let fullyAnnotatedGenericSignature: sourcekitd_api_uid_t
+  /// `key.signatures`
+  package let signatures: sourcekitd_api_uid_t
+  /// `key.active_signature`
+  package let activeSignature: sourcekitd_api_uid_t
+  /// `key.parameters`
+  package let parameters: sourcekitd_api_uid_t
+  /// `key.active_parameter`
+  package let activeParameter: sourcekitd_api_uid_t
   /// `key.doc.brief`
   package let docBrief: sourcekitd_api_uid_t
   /// `key.context`
@@ -411,6 +419,8 @@ package struct sourcekitd_api_keys {
   package let indexUnitOutputPath: sourcekitd_api_uid_t
   /// `key.include_locals`
   package let includeLocals: sourcekitd_api_uid_t
+  /// `key.compress`
+  package let compress: sourcekitd_api_uid_t
   /// `key.ignore_clang_modules`
   package let ignoreClangModules: sourcekitd_api_uid_t
   /// `key.include_system_modules`
@@ -566,6 +576,10 @@ package struct sourcekitd_api_keys {
     annotatedDecl = api.uid_get_from_cstr("key.annotated_decl")!
     fullyAnnotatedDecl = api.uid_get_from_cstr("key.fully_annotated_decl")!
     fullyAnnotatedGenericSignature = api.uid_get_from_cstr("key.fully_annotated_generic_signature")!
+    signatures = api.uid_get_from_cstr("key.signatures")!
+    activeSignature = api.uid_get_from_cstr("key.active_signature")!
+    parameters = api.uid_get_from_cstr("key.parameters")!
+    activeParameter = api.uid_get_from_cstr("key.active_parameter")!
     docBrief = api.uid_get_from_cstr("key.doc.brief")!
     context = api.uid_get_from_cstr("key.context")!
     typeRelation = api.uid_get_from_cstr("key.typerelation")!
@@ -720,6 +734,7 @@ package struct sourcekitd_api_keys {
     indexStorePath = api.uid_get_from_cstr("key.index_store_path")!
     indexUnitOutputPath = api.uid_get_from_cstr("key.index_unit_output_path")!
     includeLocals = api.uid_get_from_cstr("key.include_locals")!
+    compress = api.uid_get_from_cstr("key.compress")!
     ignoreClangModules = api.uid_get_from_cstr("key.ignore_clang_modules")!
     includeSystemModules = api.uid_get_from_cstr("key.include_system_modules")!
     ignoreStdlib = api.uid_get_from_cstr("key.ignore_stdlib")!
@@ -809,6 +824,8 @@ package struct sourcekitd_api_requests {
   package let codeCompleteSetPopularAPI: sourcekitd_api_uid_t
   /// `source.request.codecomplete.setcustom`
   package let codeCompleteSetCustom: sourcekitd_api_uid_t
+  /// `source.request.signaturehelp`
+  package let signatureHelp: sourcekitd_api_uid_t
   /// `source.request.typecontextinfo`
   package let typeContextInfo: sourcekitd_api_uid_t
   /// `source.request.conformingmethods`
@@ -907,6 +924,7 @@ package struct sourcekitd_api_requests {
     codeCompleteCacheOnDisk = api.uid_get_from_cstr("source.request.codecomplete.cache.ondisk")!
     codeCompleteSetPopularAPI = api.uid_get_from_cstr("source.request.codecomplete.setpopularapi")!
     codeCompleteSetCustom = api.uid_get_from_cstr("source.request.codecomplete.setcustom")!
+    signatureHelp = api.uid_get_from_cstr("source.request.signaturehelp")!
     typeContextInfo = api.uid_get_from_cstr("source.request.typecontextinfo")!
     conformingMethodList = api.uid_get_from_cstr("source.request.conformingmethods")!
     activeRegions = api.uid_get_from_cstr("source.request.activeregions")!

Sources/SourceKitLSP/CapabilityRegistry.swift

@@ -27,6 +27,9 @@ package final actor CapabilityRegistry {
   /// Dynamically registered completion options.
   private var completion: [CapabilityRegistration: CompletionRegistrationOptions] = [:]
 
+  /// Dynamically registered signature help options.
+  private var signatureHelp: [CapabilityRegistration: SignatureHelpRegistrationOptions] = [:]
+
   /// Dynamically registered folding range options.
   private var foldingRange: [CapabilityRegistration: FoldingRangeRegistrationOptions] = [:]
 
@@ -51,6 +54,10 @@ package final actor CapabilityRegistry {
     clientCapabilities.textDocument?.completion?.dynamicRegistration == true
   }
 
+  package var clientHasDynamicSignatureHelpRegistration: Bool {
+    clientCapabilities.textDocument?.signatureHelp?.dynamicRegistration == true
+  }
+
   package var clientHasDynamicFoldingRangeRegistration: Bool {
     clientCapabilities.textDocument?.foldingRange?.dynamicRegistration == true
   }
@@ -220,6 +227,26 @@ package final actor CapabilityRegistry {
     )
   }
 
+  package func registerSignatureHelpIfNeeded(
+    options: SignatureHelpOptions,
+    for languages: [Language],
+    server: SourceKitLSPServer
+  ) async {
+    guard clientHasDynamicCompletionRegistration else { return }
+
+    await registerLanguageSpecificCapability(
+      options: SignatureHelpRegistrationOptions(
+        documentSelector: DocumentSelector(for: languages),
+        signatureHelpOptions: options
+      ),
+      forMethod: SignatureHelpRequest.method,
+      languages: languages,
+      in: server,
+      registrationDict: signatureHelp,
+      setRegistrationDict: { signatureHelp[$0] = $1 }
+    )
+  }
+
   package func registerDidChangeWatchedFiles(
     watchers: [FileSystemWatcher],
     server: SourceKitLSPServer

Sources/SourceKitLSP/LanguageService.swift

@@ -196,6 +196,7 @@ package protocol LanguageService: AnyObject, Sendable {
 
   func completion(_ req: CompletionRequest) async throws -> CompletionList
   func completionItemResolve(_ req: CompletionItemResolveRequest) async throws -> CompletionItem
+  func signatureHelp(_ req: SignatureHelpRequest) async throws -> SignatureHelp?
   func hover(_ req: HoverRequest) async throws -> HoverResponse?
   func doccDocumentation(_ req: DoccDocumentationRequest) async throws -> DoccDocumentationResponse
   func symbolInfo(_ request: SymbolInfoRequest) async throws -> [SymbolDetails]
@@ -370,6 +371,10 @@ package extension LanguageService {
     throw ResponseError.requestNotImplemented(CompletionItemResolveRequest.self)
   }
 
+  func signatureHelp(_ req: SignatureHelpRequest) async throws -> SignatureHelp? {
+    throw ResponseError.requestNotImplemented(SignatureHelpRequest.self)
+  }
+
   func hover(_ req: HoverRequest) async throws -> HoverResponse? {
     throw ResponseError.requestNotImplemented(HoverRequest.self)
   }

Sources/SourceKitLSP/SourceKitLSPServer.swift

@@ -780,6 +780,8 @@ extension SourceKitLSPServer: QueueBasedMessageHandler {
       await self.handleRequest(for: request, requestHandler: self.completion)
     case let request as RequestAndReply<CompletionItemResolveRequest>:
       await request.reply { try await completionItemResolve(request: request.params) }
+    case let request as RequestAndReply<SignatureHelpRequest>:
+      await self.handleRequest(for: request, requestHandler: self.signatureHelp)
     case let request as RequestAndReply<DeclarationRequest>:
       await self.handleRequest(for: request, requestHandler: self.declaration)
     case let request as RequestAndReply<DefinitionRequest>:
@@ -1075,6 +1077,15 @@ extension SourceKitLSPServer {
         triggerCharacters: [".", "("]
       )
 
+    let signatureHelpOptions =
+      await registry.clientHasDynamicSignatureHelpRegistration
+      ? nil
+      : LanguageServerProtocol.SignatureHelpOptions(
+        triggerCharacters: ["(", "["],
+        // We retrigger on `:` as it's potentially after an argument label which can change the active parameter or signature.
+        retriggerCharacters: [",", ":"]
+      )
+
     let onTypeFormattingOptions =
       options.hasExperimentalFeature(.onTypeFormatting)
       ? DocumentOnTypeFormattingOptions(triggerCharacters: ["\n", "\r\n", "\r", "{", "}", ";", ".", ":", "#"])
@@ -1129,6 +1140,7 @@ extension SourceKitLSPServer {
       ),
       hoverProvider: .bool(true),
       completionProvider: completionOptions,
+      signatureHelpProvider: signatureHelpOptions,
       definitionProvider: .bool(true),
       implementationProvider: .bool(true),
       referencesProvider: .bool(true),
@@ -1177,6 +1189,9 @@ extension SourceKitLSPServer {
     if let completionOptions = server.completionProvider {
       await registry.registerCompletionIfNeeded(options: completionOptions, for: languages, server: self)
     }
+    if let signatureHelpOptions = server.signatureHelpProvider {
+      await registry.registerSignatureHelpIfNeeded(options: signatureHelpOptions, for: languages, server: self)
+    }
     if server.foldingRangeProvider?.isSupported == true {
       await registry.registerFoldingRangeIfNeeded(options: FoldingRangeOptions(), for: languages, server: self)
     }
@@ -1638,6 +1653,14 @@ extension SourceKitLSPServer {
     return try await languageService.hover(req)
   }
 
+  func signatureHelp(
+    _ req: SignatureHelpRequest,
+    workspace: Workspace,
+    languageService: LanguageService
+  ) async throws -> SignatureHelp? {
+    return try await languageService.signatureHelp(req)
+  }
+
   /// Handle a workspace/symbol request, returning the SymbolInformation.
   /// - returns: An array with SymbolInformation for each matching symbol in the workspace.
   func workspaceSymbols(_ req: WorkspaceSymbolsRequest) async throws -> [WorkspaceSymbolItem]? {

Sources/SwiftLanguageService/AdjustPositionToStartOfArgument.swift

@@ -0,0 +1,82 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import LanguageServerProtocol
+import SKLogging
+import SourceKitLSP
+import SwiftSyntax
+
+private class StartOfArgumentFinder: SyntaxAnyVisitor {
+  let requestedPosition: AbsolutePosition
+  var resolvedPosition: AbsolutePosition?
+
+  init(position: AbsolutePosition) {
+    self.requestedPosition = position
+    super.init(viewMode: .sourceAccurate)
+  }
+
+  override func visitAny(_ node: Syntax) -> SyntaxVisitorContinueKind {
+    if node.range.contains(requestedPosition) {
+      return .visitChildren
+    }
+    return .skipChildren
+  }
+
+  override func visit(_ node: FunctionCallExprSyntax) -> SyntaxVisitorContinueKind {
+    return visit(arguments: node.arguments)
+  }
+
+  override func visit(_ node: SubscriptCallExprSyntax) -> SyntaxVisitorContinueKind {
+    return visit(arguments: node.arguments)
+  }
+
+  private func visit(arguments: LabeledExprListSyntax) -> SyntaxVisitorContinueKind {
+    guard (arguments.position...arguments.endPosition).contains(requestedPosition) else {
+      return .skipChildren
+    }
+
+    guard !arguments.isEmpty else {
+      self.resolvedPosition = arguments.position
+      return .skipChildren
+    }
+
+    for argument in arguments {
+      if (argument.position...argument.endPosition).contains(requestedPosition) {
+        if let trailingComma = argument.trailingComma,
+          requestedPosition >= trailingComma.endPositionBeforeTrailingTrivia
+        {
+          self.resolvedPosition = trailingComma.endPositionBeforeTrailingTrivia
+        } else {
+          self.resolvedPosition = argument.expression.positionAfterSkippingLeadingTrivia
+        }
+        return .visitChildren
+      }
+    }
+
+    return .skipChildren
+  }
+}
+
+extension SwiftLanguageService {
+  func adjustPositionToStartOfArgument(
+    _ position: Position,
+    in snapshot: DocumentSnapshot
+  ) async -> Position {
+    let tree = await self.syntaxTreeManager.syntaxTree(for: snapshot)
+    let visitor = StartOfArgumentFinder(position: snapshot.absolutePosition(of: position))
+    visitor.walk(tree)
+    if let resolvedPosition = visitor.resolvedPosition {
+      return snapshot.position(of: resolvedPosition)
+    }
+    return position
+  }
+}

Sources/SwiftLanguageService/CMakeLists.txt

@@ -44,6 +44,8 @@ add_library(SwiftLanguageService STATIC
   DocumentSymbols.swift
   SyntaxHighlightingTokens.swift
   GeneratedInterfaceManager.swift
+  SignatureHelp.swift
+  AdjustPositionToStartOfArgument.swift
 )
 set_target_properties(SwiftLanguageService PROPERTIES
   INTERFACE_INCLUDE_DIRECTORIES ${CMAKE_Swift_MODULE_DIRECTORY})