Support for comments modifiers

Sources/SwiftSyntaxBuilder/BuildableNodes.swift.gyb

@@ -31,6 +31,7 @@ public struct ${type.buildable()}: ${base_type.buildable()}, ${type.expressible_
 %   for child in children:
   let ${child.name()}: ${child.type().buildable()}
 %   end
+  public var itemLeadingTriviaPieces: [TriviaPiece] = []
 
   /// Creates a `${type.buildable()}` using the provided parameters.
   /// - Parameters:
@@ -106,7 +107,32 @@ public struct ${type.buildable()}: ${base_type.buildable()}, ${type.expressible_
 
   /// Conformance to `${base_type.buildable()}`.
   public func build${base_type.base_name()}(format: Format, leadingTrivia: Trivia? = nil) -> ${base_type.syntax()} {
+    %   if type.buildable() != 'SourceFile':
     let result = build${type.base_name()}(format: format, leadingTrivia: leadingTrivia)
+    % end
+    %   if type.buildable() == 'SourceFile':
+    let indentedLines = itemLeadingTriviaPieces
+      .map { Trivia(pieces: [$0, .newlines(1)]) }
+      .reduce(.zero, +)
+
+    let combinedTrivia = [format._makeIndent(),
+                          leadingTrivia,
+                          indentedLines]
+      .compactMap { $0 }
+      .reduce(.zero, +)
+    %   if type.buildable() == 'SourceFile':
+    let result = buildSourceFile(format: format, leadingTrivia: combinedTrivia)
+    % end
+    % else:
+
+      if !itemLeadingTriviaPieces.isEmpty {
+        let indentedLeadingTrivia = itemLeadingTriviaPieces
+          .compactMap { Trivia(pieces:[ $0, .newlines(1)]) }
+          .map { $0 + format._makeIndent() }
+          .reduce(.zero, +)
+        return ${base_type.syntax()}(result).withLeadingTrivia(indentedLeadingTrivia)
+      }
+    %   end
     return ${base_type.syntax()}(result)
   }
 
@@ -130,5 +156,59 @@ public struct ${type.buildable()}: ${base_type.buildable()}, ${type.expressible_
     return self
   }
 % end
+
+  public func lineComment(_ text: String) -> Self {
+
+    let textWithSlashesPieces = prefixLines(of: text, with: "//")
+      .map { TriviaPiece.lineComment("\($0)") }
+    return addCommentPiece(commentPieces: textWithSlashesPieces)
+  }
+
+  /// A documentation block comment, starting with '/**' and ending with '*/'.
+  /// - Parameter text:  inserted between /*  and  */.
+  /// - Returns: a `${type.buildable()}` with the added line comment.
+  public func blockComment(_ text: String) -> Self {
+    let piece = TriviaPiece.blockComment("/* \(text) */")
+    return addCommentPiece(commentPieces: [piece])
+  }
+
+  /// A documentation line comment, starting with '/**' and ending with '*/'.
+  /// - Parameter text: comment to be inserted after ///
+  /// - Returns: a `${type.buildable()}` with the added documentation line comment.
+  public func docLineComment(_ text: String) -> Self {
+    let textWithSlashesPieces = prefixLines(of: text, with: "//")
+      .map { TriviaPiece.docLineComment("\($0)") }
+    return addCommentPiece(commentPieces: textWithSlashesPieces)
+  }
+
+
+  /// A documentation block comment, starting with '/**' and ending with '*/'.
+  /// - Parameter text: comment to be inserted between /** and '*/.
+  /// - Returns: a `${type.buildable()}` with the added  documentation block comment.
+  public func docBlockComment(_ text: String) -> Self {
+    let piece = TriviaPiece.docBlockComment("/** \(text) */")
+    return addCommentPiece(commentPieces: [piece])
+  }
+
+  private func addCommentPiece(commentPieces: [TriviaPiece]) -> Self {
+    var newSelf = self
+    newSelf.itemLeadingTriviaPieces.append(contentsOf: commentPieces)
+    return newSelf
+  }
+
+  /// Extracts Lines from a given text, and add a prefix to it
+  /// - Parameters:
+  ///   - text: Text which its lines will be prefixed
+  ///   - prefix: prefix for each line
+  /// - Returns: an array of new string, which has prefixed lines
+  private func prefixLines(of text: String, with prefix: String) -> [String] {
+
+    let prefixedLines = text
+      .split(whereSeparator: \.isNewline)
+      .map {"\(prefix) \($0)"}
+
+    return prefixedLines
+  }
+
 }
 % end