Add a configuration to preserve whitespace on lines containing only whitespace

Documentation/Configuration.md

@@ -94,6 +94,11 @@ top-level keys and values:
 
 *  `multiElementCollectionTrailingCommas` _(boolean)_: Determines whether multi-element collection literals should have trailing commas.
     Defaults to `true`.
+
+*  `indentBlankLines` _(boolean)_: Determines whether blank lines should be modified 
+    to match the current indentation. When this setting is true, blank lines will be modified 
+    to match the indentation level, adding indentation whether or not there is existing whitespace. 
+    When false (the default), all whitespace in blank lines will be completely removed.
 
 > TODO: Add support for enabling/disabling specific syntax transformations in
 > the pipeline.

Sources/SwiftFormat/API/Configuration+Default.swift

@@ -41,5 +41,6 @@ extension Configuration {
     self.noAssignmentInExpressions = NoAssignmentInExpressionsConfiguration()
     self.multiElementCollectionTrailingCommas = true
     self.reflowMultilineStringLiterals = .never
+    self.indentBlankLines = false
   }
 }

Sources/SwiftFormat/API/Configuration.swift

@@ -46,6 +46,7 @@ public struct Configuration: Codable, Equatable {
     case noAssignmentInExpressions
     case multiElementCollectionTrailingCommas
     case reflowMultilineStringLiterals
+    case indentBlankLines
   }
 
   /// A dictionary containing the default enabled/disabled states of rules, keyed by the rules'
@@ -260,6 +261,13 @@ public struct Configuration: Codable, Equatable {
 
   public var reflowMultilineStringLiterals: MultilineStringReflowBehavior
 
+  /// Determines whether to add indentation whitespace to blank lines or remove it entirely.
+  ///
+  /// If true, blank lines will be modified to match the current indentation level:
+  /// if they contain whitespace, the existing whitespace will be adjusted, and if they are empty, spaces will be added to match the indentation.
+  /// If false (the default), the whitespace in blank lines will be removed entirely.
+  public var indentBlankLines: Bool
+
   /// Creates a new `Configuration` by loading it from a configuration file.
   public init(contentsOf url: URL) throws {
     let data = try Data(contentsOf: url)
@@ -368,6 +376,12 @@ public struct Configuration: Codable, Equatable {
     self.reflowMultilineStringLiterals =
       try container.decodeIfPresent(MultilineStringReflowBehavior.self, forKey: .reflowMultilineStringLiterals)
       ?? defaults.reflowMultilineStringLiterals
+    self.indentBlankLines =
+      try container.decodeIfPresent(
+        Bool.self,
+        forKey: .indentBlankLines
+      )
+      ?? defaults.indentBlankLines
 
     // If the `rules` key is not present at all, default it to the built-in set
     // so that the behavior is the same as if the configuration had been

Sources/SwiftFormat/PrettyPrint/PrettyPrint.swift

@@ -441,7 +441,7 @@ public class PrettyPrinter {
           outputBuffer.enqueueSpaces(size)
           outputBuffer.write("\\")
         }
-        outputBuffer.writeNewlines(newline)
+        outputBuffer.writeNewlines(newline, shouldIndentBlankLines: configuration.indentBlankLines)
         lastBreak = true
       } else {
         if outputBuffer.isAtStartOfLine {
@@ -458,6 +458,10 @@ public class PrettyPrinter {
 
     // Print out the number of spaces according to the size, and adjust spaceRemaining.
     case .space(let size, _):
+      if configuration.indentBlankLines, outputBuffer.isAtStartOfLine {
+        // An empty string write is needed to add line-leading indentation that matches the current indentation on a line that contains only whitespaces.
+        outputBuffer.write("")
+      }
       outputBuffer.enqueueSpaces(size)
 
     // Print any indentation required, followed by the text content of the syntax token.

Sources/SwiftFormat/PrettyPrint/PrettyPrintBuffer.swift

@@ -70,8 +70,11 @@ struct PrettyPrintBuffer {
   /// subtract the previously written newlines during the second call so that we end up with the
   /// correct number overall.
   ///
-  /// - Parameter newlines: The number and type of newlines to write.
-  mutating func writeNewlines(_ newlines: NewlineBehavior) {
+  /// - Parameters:
+  ///   - newlines: The number and type of newlines to write.
+  ///   - shouldIndentBlankLines: A Boolean value indicating whether to insert spaces
+  ///     for blank lines based on the current indentation level.
+  mutating func writeNewlines(_ newlines: NewlineBehavior, shouldIndentBlankLines: Bool) {
     let numberToPrint: Int
     switch newlines {
     case .elective:
@@ -86,7 +89,13 @@ struct PrettyPrintBuffer {
     }
 
     guard numberToPrint > 0 else { return }
-    writeRaw(String(repeating: "\n", count: numberToPrint))
+    for number in 0..<numberToPrint {
+      if shouldIndentBlankLines, number >= 1 {
+        writeRaw(currentIndentation.indentation())
+      }
+      writeRaw("\n")
+    }
+
     lineNumber += numberToPrint
     isAtStartOfLine = true
     consecutiveNewlineCount += numberToPrint

Sources/SwiftFormat/PrettyPrint/TokenStreamCreator.swift

@@ -3521,6 +3521,12 @@ fileprivate final class TokenStreamCreator: SyntaxVisitor {
         leadingIndent = nil
 
       case .newlines(let count), .carriageReturns(let count), .carriageReturnLineFeeds(let count):
+        if config.indentBlankLines,
+          let leadingIndent, leadingIndent.count > 0
+        {
+          requiresNextNewline = true
+        }
+
         leadingIndent = .spaces(0)
         guard !isStartOfFile else { break }
 
@@ -3557,6 +3563,7 @@ fileprivate final class TokenStreamCreator: SyntaxVisitor {
       case .spaces(let n):
         guard leadingIndent == .spaces(0) else { break }
         leadingIndent = .spaces(n)
+
       case .tabs(let n):
         guard leadingIndent == .spaces(0) else { break }
         leadingIndent = .tabs(n)

Sources/_SwiftFormatTestSupport/Configuration+Testing.swift

@@ -41,6 +41,7 @@ extension Configuration {
     config.spacesAroundRangeFormationOperators = false
     config.noAssignmentInExpressions = NoAssignmentInExpressionsConfiguration()
     config.multiElementCollectionTrailingCommas = true
+    config.indentBlankLines = false
     return config
   }
 }