Fix indentation when there is a leading new line

Author: kimdv

Sources/SwiftSyntax/Trivia.swift.gyb

@@ -77,10 +77,24 @@ extension TriviaPiece: CustomDebugStringConvertible {
   }
 }
 
+extension TriviaPiece {
+  /// Returns true if the trivia is `.newlines`, `.carriageReturns` or `.carriageReturnLineFeeds`
+  public var isNewline: Bool {
+    switch self {
+    case .newlines,
+         .carriageReturns,
+         .carriageReturnLineFeeds:
+      return true
+    default:
+      return false
+    }
+  }
+}
+
 /// A collection of leading or trailing trivia. This is the main data structure
 /// for thinking about trivia.
 public struct Trivia {
-  let pieces: [TriviaPiece]
+  public let pieces: [TriviaPiece]
 
   /// Creates Trivia with the provided underlying pieces.
   public init(pieces: [TriviaPiece]) {

Sources/SwiftSyntax/gyb_generated/Trivia.swift

@@ -123,10 +123,24 @@ extension TriviaPiece: CustomDebugStringConvertible {
   }
 }
 
+extension TriviaPiece {
+  /// Returns true if the trivia is `.newlines`, `.carriageReturns` or `.carriageReturnLineFeeds`
+  public var isNewline: Bool {
+    switch self {
+    case .newlines,
+         .carriageReturns,
+         .carriageReturnLineFeeds:
+      return true
+    default:
+      return false
+    }
+  }
+}
+
 /// A collection of leading or trailing trivia. This is the main data structure
 /// for thinking about trivia.
 public struct Trivia {
-  let pieces: [TriviaPiece]
+  public let pieces: [TriviaPiece]
 
   /// Creates Trivia with the provided underlying pieces.
   public init(pieces: [TriviaPiece]) {

Sources/SwiftSyntaxBuilder/BuildableCollectionNodes.swift.gyb

@@ -65,7 +65,7 @@ public struct ${type.buildable()}: ExpressibleByArrayLiteral, SyntaxBuildable, $
     })
 %   end
     if let leadingTrivia = leadingTrivia {
-      return result.withLeadingTrivia(leadingTrivia + (result.leadingTrivia ?? []))
+      return result.withLeadingTrivia((leadingTrivia + (result.leadingTrivia ?? [])).addingSpacingAfterNewlinesIfNeeded())
     } else {
       return result
     }

Sources/SwiftSyntaxBuilder/BuildableNodes.swift.gyb

@@ -120,7 +120,7 @@ public struct ${type.buildable()}${conformance_clause(conformances)} {
       ${',\n      '.join(['%s: %s' % (child.name(), child.generate_expr_build_syntax_node(child.name(), 'format')) for child in children])}
     )
     let combinedLeadingTrivia = leadingTrivia + (additionalLeadingTrivia ?? []) + (result.leadingTrivia ?? [])
-    return result.withLeadingTrivia(combinedLeadingTrivia)
+    return result.withLeadingTrivia(combinedLeadingTrivia.addingSpacingAfterNewlinesIfNeeded())
   }
 
   /// Conformance to `${base_type.buildable()}`.

Sources/SwiftSyntaxBuilder/Trivia+Extension.swift

@@ -0,0 +1,52 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2022 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 SwiftSyntax
+
+extension Trivia {
+  /// Re-indents trivia that form a block comment to have consistent indentation.
+  ///
+  /// If the trivia start with a newline (i.e. the first non-whitespace trivia is on a fresh line) and that newline starts with spacing, the same spacing is applied to all the following lines. For example.
+  /// ```
+  ///   // first line
+  /// // second line
+  /// // third line
+  /// ```
+  /// picks up the indentation from the first line and transforms the trivia to
+  /// ```
+  ///   // first line
+  ///   // second line
+  ///   // third line
+  /// ```
+  func addingSpacingAfterNewlinesIfNeeded() -> Trivia {
+    var updatedTriviaPieces = self.pieces
+    if updatedTriviaPieces.count > 2,
+       updatedTriviaPieces[0].isNewline,
+       let indentation = self.makeIndentation(trivia: updatedTriviaPieces[1]) {
+      
+      for i in (2..<updatedTriviaPieces.count).reversed() where updatedTriviaPieces[i].isNewline {
+        updatedTriviaPieces.insert(indentation, at: i + 1)
+      }
+    }
+    
+    return Trivia(pieces: updatedTriviaPieces)
+  }
+  
+  private func makeIndentation(trivia: TriviaPiece) -> TriviaPiece? {
+    switch trivia {
+    case .spaces,
+         .tabs: return trivia
+    default:
+      return nil
+    }
+  }
+}