Add a syntactic "Add Codable structs from JSON" code action

Add a syntactic action that takes JSON pasted into a Swift file or
placed in a string literal, then turns it into a set of Codable
structs that can represent the JSON. Our typical example starts like
this:

```
{
    "name": "Produce",
    "shelves": [
        {
            "name": "Discount Produce",
            "product": {
                "name": "Banana",
                "points": 200,
                "description": "A banana that's perfectly ripe."
            }
        }
    ]
}
```

and turns into this:

```swift
struct JSONValue: Codable {
    var name: String
    var shelves: [Shelves]

    struct Shelves: Codable {
        var name: String
        var product: Product

        struct Product: Codable {
            var description: String
            var name: String
            var points: Double
        }
    }
}
```

When converting to JSON, we attempt to reason about multiple JSON
objects on the same level to detect when there are optional fields,
due to either an explicit null or due to the absence of fields in some
of the JSON objects that are conceptually stored together.

The refactoring itself would live down in the swift-syntax package if
not for its dependency on Foundation. We'll move it when appropriate.

Sources/SourceKitLSP/CMakeLists.txt

@@ -23,6 +23,7 @@ target_sources(SourceKitLSP PRIVATE
   Swift/AdjustPositionToStartOfIdentifier.swift
   Swift/CodeActions/AddDocumentation.swift
   Swift/CodeActions/ConvertIntegerLiteral.swift
+  Swift/CodeActions/ConvertJSONToCodableStruct.swift
   Swift/CodeActions/PackageManifestEdits.swift
   Swift/CodeActions/SyntaxCodeActionProvider.swift
   Swift/CodeActions/SyntaxCodeActions.swift

Sources/SourceKitLSP/Swift/CodeActions/ConvertJSONToCodableStruct.swift

@@ -0,0 +1,389 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2024 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 LanguageServerProtocol
+import SwiftBasicFormat
+import SwiftRefactor
+import SwiftSyntax
+
+/// Convert JSON literals into corresponding Swift structs that conform to the
+/// `Codable` protocol.
+///
+/// ## Before
+///
+/// ```javascript
+/// {
+///   "name": "Produce",
+///   "shelves": [
+///     {
+///       "name": "Discount Produce",
+///       "product": {
+///         "name": "Banana",
+///         "points": 200,
+///         "description": "A banana that's perfectly ripe."
+///       }
+///     }
+///   ]
+/// }
+/// ```
+///
+/// ## After
+///
+/// ```swift
+/// struct JSONValue: Codable {
+///   var name: String
+///   var shelves: [Shelves]
+///
+///   struct Shelves: Codable {
+///     var name: String
+///     var product: Product
+///
+///     struct Product: Codable {
+///       var description: String
+///       var name: String
+///       var points: Double
+///     }
+///   }
+/// }
+/// ```
+@_spi(Testing)
+public struct ConvertJSONToCodableStruct: EditRefactoringProvider {
+  @_spi(Testing)
+  public static func textRefactor(
+    syntax: Syntax,
+    in context: Void
+  ) -> [SourceEdit] {
+    // Dig out a syntax node that looks like it might be JSON or have JSON
+    // in it.
+    guard let preflight = preflightRefactoring(syntax) else {
+      return []
+    }
+
+    // Dig out the text that we think might be JSON.
+    let text: String
+    switch preflight {
+    case let .closure(closure):
+      /// The outer structure of the JSON { ... } looks like a closure in the
+      /// syntax tree, albeit one with lots of ill-formed syntax in the body.
+      /// We're only going to look at the text of the closure to see if we
+      /// have JSON in there.
+      text = closure.trimmedDescription
+    case .stringLiteral(_, let literalText):
+      /// A string literal that could contain JSON within it.
+      text = literalText
+    }
+
+    // Try to process this as JSON.
+    guard
+      let object = try? JSONSerialization.jsonObject(with: text.data(using: .utf8)!),
+      let dictionary = object as? [String: Any]
+    else {
+      return []
+    }
+
+    // Create the top-level object.
+    let topLevelObject = JSONObject(dictionary: dictionary)
+
+    // Render the top-level object as a struct.
+    let indentation = BasicFormat.inferIndentation(of: syntax)
+    let format = BasicFormat(indentationWidth: indentation)
+    let decls = topLevelObject.asDeclSyntax(name: "JSONValue")
+      .formatted(using: format)
+
+    // Render the change into a set of source edits.
+    switch preflight {
+    case .closure(let closure):
+      // Closures are replaced entirely, since they were invalid code to
+      // start with.
+      return [
+        SourceEdit(range: closure.trimmedRange, replacement: decls.description)
+      ]
+    case .stringLiteral(let literal, _):
+      /// Leave the string literal in place (it might be there for testing
+      /// purposes), and put the newly-created structs afterward.
+      return [
+        SourceEdit(
+          range: literal.endPosition..<literal.endPosition,
+          replacement: "\n" + decls.description
+        )
+      ]
+    }
+  }
+}
+
+extension ConvertJSONToCodableStruct {
+  /// The result of preflighting a syntax node to try to find potential JSON
+  /// in it.
+  enum Preflight {
+    /// A closure, which is what a JSON dictionary looks like when pasted
+    /// into Swift.
+    case closure(ClosureExprSyntax)
+
+    /// A string literal that may contain JSON.
+    case stringLiteral(StringLiteralExprSyntax, String)
+  }
+
+  /// Look for either a closure or a string literal that might have JSON in it.
+  static func preflightRefactoring(_ syntax: Syntax) -> Preflight? {
+    // Preflight a closure.
+    //
+    // A blob of JSON dropped into a Swift source file will look like a
+    // closure due to the curly braces. The internals might be a syntactic
+    // disaster, but we don't actually care.
+    if let closure = syntax.as(ClosureExprSyntax.self) {
+      return .closure(closure)
+    }
+
+    // We found a string literal; its contents might be JSON.
+    if let stringLit = syntax.as(StringLiteralExprSyntax.self) {
+      // Look for an enclosing context and prefer that, because we might have
+      // a string literal that's inside a closure where the closure itself
+      // is the JSON.
+      if let parent = syntax.parent,
+        let enclosingPreflight = preflightRefactoring(parent)
+      {
+        return enclosingPreflight
+      }
+
+      guard let text = stringLit.representedLiteralValue else {
+        return nil
+      }
+
+      return .stringLiteral(stringLit, text)
+    }
+
+    // Look further up the syntax tree.
+    if let parent = syntax.parent {
+      return preflightRefactoring(parent)
+    }
+
+    return nil
+  }
+}
+
+extension ConvertJSONToCodableStruct: SyntaxRefactoringCodeActionProvider {
+  static var title = "Create Codable structs from JSON"
+}
+
+/// A JSON object, which is has a set of fields, each of which has the given
+/// type.
+fileprivate struct JSONObject {
+  /// The fields of the JSON object.
+  var fields: [String: JSONType] = [:]
+
+  /// Form a JSON object from its fields.
+  private init(fields: [String: JSONType]) {
+    self.fields = fields
+  }
+
+  /// Form a JSON object given a dictionary.
+  init(dictionary: [String: Any]) {
+    fields = dictionary.mapValues { JSONType(value: $0) }
+  }
+
+  /// Merge the fields of this JSON object with another JSON object to produce
+  /// a JSON object
+  func merging(with other: JSONObject) -> JSONObject {
+    // Collect the set of all keys from both JSON objects.
+    var allKeys: Set<String> = []
+    allKeys.formUnion(fields.keys)
+    allKeys.formUnion(other.fields.keys)
+
+    // Form a new JSON object containing the union of the fields
+    let newFields = allKeys.map { key in
+      let myValue = fields[key] ?? .null
+      let otherValue = other.fields[key] ?? .null
+      return (key, myValue.merging(with: otherValue))
+    }
+    return JSONObject(fields: [String: JSONType](uniqueKeysWithValues: newFields))
+  }
+
+  /// Render this JSON object into a struct.
+  func asDeclSyntax(name: String) -> DeclSyntax {
+    /// The list of fields in this object, sorted alphabetically.
+    let sortedFields = fields.sorted(by: { $0.key < $1.key })
+
+    // Collect the nested types
+    let nestedTypes: [(String, JSONObject)] = sortedFields.compactMap { (name, type) in
+      guard let object = type.innerObject else {
+        return nil
+      }
+
+      return (name.capitalized, object)
+    }
+
+    let members = MemberBlockItemListSyntax {
+      // Print the fields of this type.
+      for (fieldName, fieldType) in sortedFields {
+        MemberBlockItemSyntax(
+          leadingTrivia: .newline,
+          decl: "var \(raw: fieldName): \(fieldType.asTypeSyntax(name: fieldName))" as DeclSyntax
+        )
+      }
+
+      // Print any nested types.
+      for (typeName, object) in nestedTypes {
+        MemberBlockItemSyntax(
+          leadingTrivia: (typeName == nestedTypes.first?.0) ? .newlines(2) : .newline,
+          decl: object.asDeclSyntax(name: typeName)
+        )
+      }
+    }
+
+    return """
+      struct \(raw: name): Codable {
+        \(members.trimmed)
+      }
+      """
+  }
+}
+
+/// Describes the type of JSON data.
+fileprivate enum JSONType {
+  /// String data
+  case string
+
+  /// Numeric data
+  case number
+
+  /// Boolean data
+  case boolean
+
+  /// A "null", which implies optionality but without any underlying type
+  /// information.
+  case null
+
+  /// An array.
+  indirect case array(JSONType)
+
+  /// An object.
+  indirect case object(JSONObject)
+
+  /// A value that is optional, for example because it is missing or null in
+  /// other cases.
+  indirect case optional(JSONType)
+
+  /// Determine the type of a JSON value.
+  init(value: Any) {
+    switch value {
+    case let string as String:
+      if string == "true" || string == "false" {
+        self = .boolean
+      } else {
+        self = .string
+      }
+    case is NSNumber:
+      self = .number
+    case is NSArray:
+      let array = value as! [Any]
+
+      // Use null as a fallback for an empty array.
+      guard let firstValue = array.first else {
+        self = .array(.null)
+        return
+      }
+
+      // Merge the array elements.
+      let elementType: JSONType = array[1...].reduce(
+        JSONType(value: firstValue)
+      ) { (result, value) in
+        result.merging(with: JSONType(value: value))
+      }
+      self = .array(elementType)
+
+    case is NSNull:
+      self = .null
+    case is NSDictionary:
+      self = .object(JSONObject(dictionary: value as! [String: Any]))
+    default:
+      self = .string
+    }
+  }
+
+  /// Merge this JSON type with another JSON type, producing a new JSON type
+  /// that abstracts over the two.
+  func merging(with other: JSONType) -> JSONType {
+    switch (self, other) {
+    // Exact matches are easy.
+    case (.string, .string): return .string
+    case (.number, .number): return .number
+    case (.boolean, .boolean): return .boolean
+    case (.null, .null): return .null
+
+    case (.array(let inner), .array(.null)), (.array(.null), .array(let inner)):
+      // Merging an array with an array of null leaves the array.
+      return .array(inner)
+
+    case (.array(let inner), .null), (.null, .array(let inner)):
+      // Merging an array with a null just leaves an array.
+      return .array(inner)
+
+    case (.array(let left), .array(let right)):
+      // Merging two arrays merges the element types
+      return .array(left.merging(with: right))
+
+    case (.object(let left), .object(let right)):
+      // Merging two arrays merges the element types
+      return .object(left.merging(with: right))
+
+    // Merging a string with a Boolean means we misinterpreted "true" or
+    // "false" as Boolean when it was meant as a string.
+    case (.string, .boolean), (.boolean, .string): return .string
+
+    // Merging 'null' with an optional returns the optional.
+    case (.optional(let inner), .null), (.null, .optional(let inner)):
+      return .optional(inner)
+
+    // Merging 'null' with anything else makes it an optional.
+    case (let inner, .null), (.null, let inner):
+      return .optional(inner)
+
+    // Merging two optionals merges the underlying types and makes the
+    // result optional.
+    case (.optional(let left), .optional(let right)):
+      return .optional(left.merging(with: right))
+
+    // Merging an optional with anything else merges the underlying bits and
+    // makes them optional.
+    case (let outer, .optional(let inner)), (.optional(let inner), let outer):
+      return .optional(inner.merging(with: outer))
+
+    // Fall back to the null case when we don't know.
+    default:
+      return .null
+    }
+  }
+
+  /// Dig out the JSON inner object referenced by this type.
+  var innerObject: JSONObject? {
+    switch self {
+    case .string, .null, .number, .boolean: nil
+    case .optional(let inner): inner.innerObject
+    case .array(let inner): inner.innerObject
+    case .object(let object): object
+    }
+  }
+
+  /// Render this JSON type into type syntax.
+  func asTypeSyntax(name: String) -> TypeSyntax {
+    switch self {
+    case .string: "String"
+    case .number: "Double"
+    case .boolean: "Bool"
+    case .null: "Void"
+    case .optional(let inner): "\(inner.asTypeSyntax(name: name))?"
+    case .array(let inner): "[\(inner.asTypeSyntax(name: name))]"
+    case .object(_): "\(raw: name.capitalized)"
+    }
+  }
+}