Separate signature and parameter documentation using swift-markdown

CMakeLists.txt

@@ -26,6 +26,8 @@ find_package(SwiftCollections QUIET)
 find_package(SwiftSyntax CONFIG REQUIRED)
 find_package(SwiftASN1 CONFIG REQUIRED)
 find_package(SwiftCrypto CONFIG REQUIRED)
+find_package(SwiftMarkdown CONFIG REQUIRED)
+find_package(cmark-gfm CONFIG REQUIRED)
 
 include(SwiftSupport)
 

Package.swift

@@ -613,6 +613,7 @@ var targets: [Target] = [
       "ToolchainRegistry",
       "TSCExtensions",
       .product(name: "IndexStoreDB", package: "indexstore-db"),
+      .product(name: "Markdown", package: "swift-markdown"),
       .product(name: "Crypto", package: "swift-crypto"),
       .product(name: "SwiftToolsSupport-auto", package: "swift-tools-support-core"),
     ]

Sources/SwiftLanguageService/CMakeLists.txt

@@ -46,6 +46,7 @@ add_library(SwiftLanguageService STATIC
   GeneratedInterfaceManager.swift
   SignatureHelp.swift
   AdjustPositionToStartOfArgument.swift
+  ExtractParametersDocumentation.swift
 )
 set_target_properties(SwiftLanguageService PROPERTIES
   INTERFACE_INCLUDE_DIRECTORIES ${CMAKE_Swift_MODULE_DIRECTORY})
@@ -59,6 +60,9 @@ target_link_libraries(SwiftLanguageService PUBLIC
   IndexStoreDB
   SwiftSyntax::SwiftBasicFormat
   SwiftSyntax::SwiftSyntax
+  SwiftMarkdown::Markdown
+  libcmark-gfm
+  libcmark-gfm-extensions
 )
 
 target_link_libraries(SourceKitLSP PRIVATE

Sources/SwiftLanguageService/ExtractParametersDocumentation.swift

@@ -0,0 +1,259 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 Foundation
+import Markdown
+
+private struct Parameter {
+  let name: String
+  let documentation: String
+}
+
+/// Extracts parameter documentation from a Doxygen parameter command.
+private func extractParameter(from doxygenParameter: DoxygenParameter) -> Parameter {
+  return Parameter(
+    name: doxygenParameter.name,
+    documentation: Document(doxygenParameter.blockChildren).format(),
+  )
+}
+
+/// Extracts parameter documentation from an unordered list.
+///
+/// - Returns: A new ``UnorderedList`` with the items that were not added to the parameters if any.
+private func extractParameters(
+  from unorderedList: UnorderedList
+) -> (remaining: UnorderedList?, parameters: [Parameter]) {
+  var parameters: [Parameter] = []
+  var newItems: [ListItem] = []
+
+  for item in unorderedList.listItems {
+    if let param = extractSingleParameter(from: item) {
+      parameters.append(param)
+    } else if let params = extractParametersOutline(from: item) {
+      parameters.append(contentsOf: params)
+    } else {
+      newItems.append(item)
+    }
+  }
+
+  if newItems.isEmpty {
+    return (remaining: nil, parameters: parameters)
+  }
+
+  return (remaining: UnorderedList(newItems), parameters: parameters)
+}
+
+/// Parameter documentation from a `Parameters:` outline.
+///
+/// Example:
+/// ```markdown
+/// - Parameters:
+///   - param: description
+/// ```
+///
+/// - Returns: The extracted parameters if any, nil otherwise.
+private func extractParametersOutline(from listItem: ListItem) -> [Parameter]? {
+  guard
+    let firstChild = listItem.child(at: 0) as? Paragraph,
+    let headingText = firstChild.child(at: 0) as? Text,
+    headingText.string.trimmingCharacters(in: .whitespaces).lowercased().hasPrefix("parameters:")
+  else {
+    return nil
+  }
+
+  return listItem.children.flatMap { child in
+    guard let nestedList = child as? UnorderedList else {
+      return [] as [Parameter]
+    }
+
+    return nestedList.listItems.compactMap { extractParameter(listItem: $0) }
+  }
+}
+
+/// Extracts parameter documentation from a single parameter.
+///
+/// Example:
+/// ```markdown
+/// - Parameter param: description
+/// ```
+///
+/// - Returns: The extracted parameter if any, nil otherwise.
+private func extractSingleParameter(from listItem: ListItem) -> Parameter? {
+  guard let paragraph = listItem.child(at: 0) as? Paragraph,
+    let paragraphText = paragraph.child(at: 0) as? Text
+  else {
+    return nil
+  }
+
+  let parameterPrefix = "parameter "
+  let paragraphContent = paragraphText.string
+
+  guard
+    let prefixEnd = paragraphContent.index(
+      paragraphContent.startIndex,
+      offsetBy: parameterPrefix.count,
+      limitedBy: paragraphContent.endIndex
+    )
+  else {
+    return nil
+  }
+
+  let potentialMatch = paragraphContent[..<prefixEnd].lowercased()
+
+  guard potentialMatch == parameterPrefix else {
+    return nil
+  }
+
+  let remainingContent = String(paragraphContent[prefixEnd...]).trimmingCharacters(in: .whitespaces)
+
+  // Remove the "Parameter " prefix from the list item so we can extract the parameter's documentation using `extractParameter(listItem:)`
+  var remainingParagraph = paragraph
+  if remainingContent.isEmpty {
+    // Drop the Text node if it's empty. This allows `extractParameterWithRawIdentifier` to handle both single parameters
+    // and parameter outlines uniformly.
+    remainingParagraph.replaceChildrenInRange(0..<1, with: [])
+  } else {
+    remainingParagraph.replaceChildrenInRange(0..<1, with: [Text(remainingContent)])
+  }
+
+  var remainingListItem = listItem
+  remainingListItem.replaceChildrenInRange(0..<1, with: [remainingParagraph])
+
+  return extractParameter(listItem: remainingListItem)
+}
+
+/// Extracts a parameter field from a list item.
+///
+/// - Parameters:
+///   - listItem: The list item to extract the parameter from
+///
+/// - Returns: The extracted parameter if any, nil otherwise.
+private func extractParameter(listItem: ListItem) -> Parameter? {
+  guard let paragraph = listItem.child(at: 0) as? Paragraph else {
+    return nil
+  }
+
+  guard let firstText = paragraph.child(at: 0) as? Text else {
+    return extractParameterWithRawIdentifier(from: listItem)
+  }
+
+  let components = firstText.string.split(separator: ":", maxSplits: 1, omittingEmptySubsequences: false)
+
+  guard components.count == 2 else {
+    return extractParameterWithRawIdentifier(from: listItem)
+  }
+
+  let name = String(components[0]).trimmingCharacters(in: .whitespaces)
+  guard !name.isEmpty else {
+    return nil
+  }
+
+  let remainingFirstTextContent = String(components[1]).trimmingCharacters(in: .whitespaces)
+  let remainingParagraphChildren = [Text(remainingFirstTextContent)] + paragraph.inlineChildren.dropFirst()
+  let remainingChildren = [Paragraph(remainingParagraphChildren)] + listItem.blockChildren.dropFirst()
+  let documentation = Document(remainingChildren).format()
+
+  return Parameter(name: name, documentation: documentation)
+}
+
+/// Extracts a parameter with its name as a raw identifier.
+///
+/// Example:
+/// ```markdown
+/// - Parameter `foo bar`: documentation
+/// - Parameters:
+///   - `foo bar`: documentation
+/// ```
+///
+/// - Parameters:
+///   - listItem: The list item to extract the parameter from
+private func extractParameterWithRawIdentifier(from listItem: ListItem) -> Parameter? {
+  guard let paragraph = listItem.child(at: 0) as? Paragraph,
+    let rawIdentifier = paragraph.child(at: 0) as? InlineCode,
+    let text = paragraph.child(at: 1) as? Text
+  else {
+    return nil
+  }
+
+  let textContent = text.string.trimmingCharacters(in: .whitespaces)
+
+  guard textContent.hasPrefix(":") else {
+    return nil
+  }
+
+  let remainingTextContent = String(textContent.dropFirst()).trimmingCharacters(in: .whitespaces)
+  let remainingParagraphChildren =
+    [Text(remainingTextContent)] + paragraph.inlineChildren.dropFirst(2)
+  let remainingChildren = [Paragraph(remainingParagraphChildren)] + listItem.blockChildren.dropFirst(1)
+  let documentation = Document(remainingChildren).format()
+
+  return Parameter(name: rawIdentifier.code, documentation: documentation)
+}
+
+/// Extracts parameter documentation from markdown text.
+///
+/// The parameter extraction implementation is almost ported from the implementation in the Swift compiler codebase.
+///
+/// The problem with doing that in the Swift compiler codebase is that once you parse a the comment as markdown into
+/// a `Document` you cannot easily convert it back into markdown (we'd need to write our own markdown formatter).
+/// Besides, `cmark` doesn't handle Doxygen commands.
+///
+/// We considered using `swift-docc` but we faced some problems with it:
+///
+/// 1. We would need to refactor existing use of `swift-docc` in SourceKit-LSP to reuse some of that logic here besides
+///    providing the required arguments.
+/// 2. The result returned by DocC can't be directly converted to markdown, we'd need to provide our own DocC markdown renderer.
+///
+/// Implementing this using `swift-markdown` allows us to easily parse the comment, process it, convert it back to markdown.
+/// It also provides minimal parsing for Doxygen commands (we're only interested in `\param`) allowing us to use the same
+/// implementation for Clang-based declarations.
+///
+/// Although this approach involves code duplication, it's simple enough for the initial implementation. We should consider
+/// `swift-docc` in the future.
+///
+/// - Parameter markdown: The markdown text to extract parameters from
+/// - Returns: A tuple containing the extracted parameters dictionary and the remaining markdown text
+package func extractParametersDocumentation(
+  from markdown: String
+) -> (parameters: [String: String], remaining: String) {
+  let document = Document(parsing: markdown, options: [.parseBlockDirectives, .parseMinimalDoxygen])
+
+  var parameters: [String: String] = [:]
+  var remainingBlocks: [any BlockMarkup] = []
+
+  for block in document.blockChildren {
+    switch block {
+    case let unorderedList as UnorderedList:
+      let (newUnorderedList, params) = extractParameters(from: unorderedList)
+      if let newUnorderedList {
+        remainingBlocks.append(newUnorderedList)
+      }
+
+      for param in params {
+        // If duplicate parameter documentation is found, keep the first one following swift-docc's behavior
+        parameters[param.name] = parameters[param.name] ?? param.documentation
+      }
+
+    case let doxygenParameter as DoxygenParameter:
+      let param = extractParameter(from: doxygenParameter)
+      // If duplicate parameter documentation is found, keep the first one following swift-docc's behavior
+      parameters[param.name] = parameters[param.name] ?? param.documentation
+
+    default:
+      remainingBlocks.append(block)
+    }
+  }
+
+  let remaining = Document(remainingBlocks).format()
+
+  return (parameters: parameters, remaining: remaining)
+}

Sources/SwiftLanguageService/SignatureHelp.swift

@@ -36,16 +36,24 @@ fileprivate extension String {
 }
 
 fileprivate extension ParameterInformation {
-  init?(_ parameter: SKDResponseDictionary, _ signatureLabel: String, _ keys: sourcekitd_api_keys) {
+  /// - Parameter parameterDocumentation: A dictionary with the parameter name as the key and its documentation as the value.
+  init?(
+    _ parameter: SKDResponseDictionary,
+    _ signatureLabel: String,
+    _ keys: sourcekitd_api_keys,
+    _ parameterDocumentation: [String: String]
+  ) {
     guard let nameOffset = parameter[keys.nameOffset] as Int?,
       let nameLength = parameter[keys.nameLength] as Int?
     else {
       return nil
     }
 
     let documentation: StringOrMarkupContent? =
-      if let docComment: String = parameter[keys.docComment] {
-        .markupContent(MarkupContent(kind: .markdown, value: docComment))
+      if let name = parameter[keys.name] as String?,
+        let documentation = parameterDocumentation[name]
+      {
+        .markupContent(MarkupContent(kind: .markdown, value: documentation))
       } else {
         nil
       }
@@ -68,7 +76,20 @@ fileprivate extension SignatureInformation {
       return nil
     }
 
-    let parameters = skParameters.compactMap { ParameterInformation($0, label, keys) }
+    let documentation: StringOrMarkupContent?
+    let parameterDocumentation: [String: String]
+
+    if let docComment: String = signature[keys.docComment] {
+      let (parameterComments, signatureComment) = extractParametersDocumentation(from: docComment)
+
+      documentation = .markupContent(MarkupContent(kind: .markdown, value: signatureComment))
+      parameterDocumentation = parameterComments
+    } else {
+      documentation = nil
+      parameterDocumentation = [:]
+    }
+
+    let parameters = skParameters.compactMap { ParameterInformation($0, label, keys, parameterDocumentation) }
 
     let activeParameter: Int? =
       if let activeParam: Int = signature[keys.activeParameter] {
@@ -88,13 +109,6 @@ fileprivate extension SignatureInformation {
         nil
       }
 
-    let documentation: StringOrMarkupContent? =
-      if let docComment: String = signature[keys.docComment] {
-        .markupContent(MarkupContent(kind: .markdown, value: docComment))
-      } else {
-        nil
-      }
-
     self.init(
       label: label,
       documentation: documentation,