Add appendInterpolation with optional syntax nodes

CodeGeneration/Sources/generate-swiftsyntax/templates/swiftsyntax/RawSyntaxNodesFile.swift

@@ -217,8 +217,9 @@ let rawSyntaxNodesFile = SourceFileSyntax(leadingTrivia: copyrightHeader) {
             let list = ExprListSyntax {
               ExprSyntax("layout.initialize(repeating: nil)")
               for (index, child) in node.children.enumerated() {
-                let optionalMark = child.isOptional ? "?" : ""
-                ExprSyntax("layout[\(raw: index)] = \(child.varOrCaseName.backtickedIfNeeded)\(raw: optionalMark).raw")
+                let optionalMark = child.isOptional ? TokenSyntax.postfixQuestionMarkToken() : nil
+
+                ExprSyntax("layout[\(raw: index)] = \(child.varOrCaseName.backtickedIfNeeded)\(optionalMark).raw")
                   .with(\.leadingTrivia, .newline)
               }
             }
@@ -239,12 +240,12 @@ let rawSyntaxNodesFile = SourceFileSyntax(leadingTrivia: copyrightHeader) {
 
         for (index, child) in node.children.enumerated() {
           try VariableDeclSyntax("public var \(child.varOrCaseName.backtickedIfNeeded): Raw\(child.buildableType.buildable)") {
-            let iuoMark = child.isOptional ? "" : "!"
+            let exclamationMark = child.isOptional ? nil : TokenSyntax.exclamationMarkToken()
 
             if child.syntaxNodeKind == .syntax {
-              ExprSyntax("layoutView.children[\(raw: index)]\(raw: iuoMark)")
+              ExprSyntax("layoutView.children[\(raw: index)]\(exclamationMark)")
             } else {
-              ExprSyntax("layoutView.children[\(raw: index)].map(\(child.syntaxNodeKind.rawType).init(raw:))\(raw: iuoMark)")
+              ExprSyntax("layoutView.children[\(raw: index)].map(\(child.syntaxNodeKind.rawType).init(raw:))\(exclamationMark)")
             }
           }
         }

CodeGeneration/Sources/generate-swiftsyntax/templates/swiftsyntax/SyntaxNodesFile.swift

@@ -177,10 +177,12 @@ func syntaxNode(emitKind: SyntaxNodeKind) -> SourceFileSyntax {
               }
             }
 
+            let questionMark = child.isOptional ? TokenSyntax.postfixQuestionMarkToken() : nil
+
             AccessorDeclSyntax(
               """
               set(value) {
-                self = \(node.kind.syntaxType)(data.replacingChild(at: \(raw: index), with: value\(raw: child.isOptional ? "?" : "").data, arena: SyntaxArena()))
+                self = \(node.kind.syntaxType)(data.replacingChild(at: \(raw: index), with: value\(questionMark).data, arena: SyntaxArena()))
               }
               """
             )

CodeGeneration/Sources/generate-swiftsyntax/templates/swiftsyntax/SyntaxTraitsFile.swift

@@ -26,10 +26,12 @@ let syntaxTraitsFile = SourceFileSyntax(leadingTrivia: copyrightHeader) {
       """
     ) {
       for child in trait.children {
+        let questionMark = child.isOptional ? TokenSyntax.postfixQuestionMarkToken() : nil
+
         DeclSyntax(
           """
           \(child.documentation)
-          var \(child.varOrCaseName): \(child.syntaxNodeKind.syntaxType)\(raw: child.isOptional ? "?" : "") { get set }
+          var \(child.varOrCaseName): \(child.syntaxNodeKind.syntaxType)\(questionMark) { get set }
           """
         )
       }

Sources/SwiftSyntaxBuilder/Syntax+StringInterpolation.swift

@@ -63,6 +63,12 @@ extension SyntaxStringInterpolation: StringInterpolationProtocol {
   }
 
   /// Append a syntax node to the interpolation.
+  ///
+  /// This method accepts a syntax node and appends it to the interpolation.
+  /// If there was a previous indentation value, the method will indent the
+  /// syntax node with that value. If not, it will use the syntax node as-is.
+  ///
+  /// - Parameter node: A syntax node that conforms to `SyntaxProtocol`.
   public mutating func appendInterpolation<Node: SyntaxProtocol>(
     _ node: Node
   ) {
@@ -84,6 +90,20 @@ extension SyntaxStringInterpolation: StringInterpolationProtocol {
     self.lastIndentation = nil
   }
 
+  /// Append an optional syntax node to the interpolation.
+  ///
+  /// This method accepts an optional syntax node and appends it to the interpolation
+  /// if it exists. If the syntax node is nil, this method does nothing.
+  ///
+  /// - Parameter node: An optional syntax node that conforms to `SyntaxProtocol`.
+  public mutating func appendInterpolation<Node: SyntaxProtocol>(
+    _ node: Node?
+  ) {
+    if let node {
+      appendInterpolation(node)
+    }
+  }
+
   public mutating func appendInterpolation<T>(raw value: T) {
     sourceText.append(contentsOf: String(describing: value).utf8)
     self.lastIndentation = nil

Tests/SwiftSyntaxBuilderTest/StringInterpolationTests.swift

@@ -72,6 +72,22 @@ final class StringInterpolationTests: XCTestCase {
     )
   }
 
+  func testOptionalInterpolationWithNil() {
+    let tokenSyntax: TokenSyntax? = nil
+
+    let funcSyntax: DeclSyntax = "func foo\(tokenSyntax)()"
+    XCTAssertTrue(funcSyntax.is(FunctionDeclSyntax.self))
+    XCTAssertEqual(funcSyntax.description, "func foo()")
+  }
+
+  func testOptionalInterpolationWithValue() {
+    let tokenSyntax: TokenSyntax? = .identifier("Bar")
+
+    let funcSyntax: DeclSyntax = "func foo\(tokenSyntax)()"
+    XCTAssertTrue(funcSyntax.is(FunctionDeclSyntax.self))
+    XCTAssertEqual(funcSyntax.description, "func fooBar()")
+  }
+
   func testPatternInterpolation() {
     let letPattern: PatternSyntax = "let x"
     XCTAssertTrue(letPattern.is(ValueBindingPatternSyntax.self))