Add AllowWhitespaceOnlyLines configuration

Documentation/Configuration.md

@@ -94,6 +94,9 @@ top-level keys and values:
 
 *  `multiElementCollectionTrailingCommas` _(boolean)_: Determines whether multi-element collection literals should have trailing commas.
     Defaults to `true`.
+
+*  `allowWhitespaceOnlyLines` _(boolean)_: Determines whether lines containing only whitespace should be preserved. When this setting is true, lines that consist solely of whitespace will not have the whitespace removed but will be modified to match the indentation. 
+    Defaults to `false`
 
 > 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.allowWhitespaceOnlyLines = false
   }
 }

Sources/SwiftFormat/API/Configuration.swift

@@ -46,6 +46,7 @@ public struct Configuration: Codable, Equatable {
     case noAssignmentInExpressions
     case multiElementCollectionTrailingCommas
     case reflowMultilineStringLiterals
+    case allowWhitespaceOnlyLines
   }
 
   /// A dictionary containing the default enabled/disabled states of rules, keyed by the rules'
@@ -259,6 +260,12 @@ public struct Configuration: Codable, Equatable {
   }
 
   public var reflowMultilineStringLiterals: MultilineStringReflowBehavior
+
+  /// Determines whether lines containing only whitespace should be preserved or removed.
+  ///
+  /// If true, lines that consist solely of whitespace will be modified to match the current indentation
+  /// without removing the whitespace. If false (the default), the whitespace on such lines will be completely removed.
+  public var allowWhitespaceOnlyLines: Bool
 
   /// Creates a new `Configuration` by loading it from a configuration file.
   public init(contentsOf url: URL) throws {
@@ -352,10 +359,13 @@ public struct Configuration: Codable, Equatable {
       try container.decodeIfPresent(
         Bool.self, forKey: .multiElementCollectionTrailingCommas)
     ?? defaults.multiElementCollectionTrailingCommas
-
     self.reflowMultilineStringLiterals =
       try container.decodeIfPresent(MultilineStringReflowBehavior.self, forKey: .reflowMultilineStringLiterals)
       ?? defaults.reflowMultilineStringLiterals
+    self.allowWhitespaceOnlyLines =
+      try container.decodeIfPresent(
+        Bool.self, forKey: .allowWhitespaceOnlyLines)
+    ?? defaults.allowWhitespaceOnlyLines
 
     // 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

@@ -449,7 +449,11 @@ public class PrettyPrinter {
 
     // Print out the number of spaces according to the size, and adjust spaceRemaining.
     case .space(let size, _):
-      outputBuffer.enqueueSpaces(size)
+      if configuration.allowWhitespaceOnlyLines, outputBuffer.isAtStartOfLine {
+        outputBuffer.write(String())
+      } else {
+        outputBuffer.enqueueSpaces(size)
+      }
 
     // Print any indentation required, followed by the text content of the syntax token.
     case .syntax(let text):

Sources/SwiftFormat/PrettyPrint/TokenStreamCreator.swift

@@ -3465,7 +3465,12 @@ fileprivate final class TokenStreamCreator: SyntaxVisitor {
 
       case .spaces(let n):
         guard leadingIndent == .spaces(0) else { break }
-        leadingIndent = .spaces(n)
+        if config.allowWhitespaceOnlyLines, trivia.count > index + 1, trivia[index + 1].isNewline {
+          appendToken(.space(size: n))
+          requiresNextNewline = true
+        } else {
+          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.allowWhitespaceOnlyLines = false
     return config
   }
 }

Tests/SwiftFormatTests/PrettyPrint/AllowWhitespaceOnlyLinesTests.swift

@@ -0,0 +1,163 @@
+import SwiftFormat
+
+final class AllowWhitespaceOnlyLinesTests: PrettyPrintTestCase {
+  func testAllowWhitespaceOnlyLinesEnabled() {
+    let input =
+      """
+      class A {
+        func foo() -> Int {
+          return 1
+        }
+      \u{0020}\u{0020}
+        func bar() -> Int {
+          return 2
+        }
+      }
+      """
+
+    let expected =
+      """
+      class A {
+        func foo() -> Int {
+          return 1
+        }
+      \u{0020}\u{0020}
+        func bar() -> Int {
+          return 2
+        }
+      }
+      
+      """
+    var config = Configuration.forTesting
+    config.allowWhitespaceOnlyLines = true
+    assertPrettyPrintEqual(input: input, expected: expected, linelength: 80, configuration: config)
+  }
+
+  func testAllowWhitespaceOnlyLinesDisabled() {
+    let input =
+      """
+      class A {
+        func foo() -> Int {
+          return 1
+        }
+      \u{0020}\u{0020}
+        func bar() -> Int {
+          return 2
+        }
+      }
+      """
+
+    let expected =
+      """
+      class A {
+        func foo() -> Int {
+          return 1
+        }
+      
+        func bar() -> Int {
+          return 2
+        }
+      }
+      
+      """
+    var config = Configuration.forTesting
+    config.allowWhitespaceOnlyLines = false
+    assertPrettyPrintEqual(input: input, expected: expected, linelength: 80, configuration: config)
+  }
+
+  func testLineWithMoreWhitespacesThanIndentation() {
+    let input =
+      """
+      class A {
+        func foo() -> Int {
+          return 1
+        }
+      \u{0020}\u{0020}\u{0020}\u{0020}\u{0020}
+        func bar() -> Int {
+          return 2
+        }
+      }
+      """
+
+    let expected =
+      """
+      class A {
+        func foo() -> Int {
+          return 1
+        }
+      \u{0020}\u{0020}
+        func bar() -> Int {
+          return 2
+        }
+      }
+      
+      """
+    var config = Configuration.forTesting
+    config.allowWhitespaceOnlyLines = true
+    assertPrettyPrintEqual(input: input, expected: expected, linelength: 80, configuration: config)
+  }
+
+  func testLineWithFewerWhitespacesThanIndentation() {
+    let input =
+      """
+      class A {
+        func foo() -> Int {
+          return 1
+        }
+      \u{0020}
+        func bar() -> Int {
+          return 2
+        }
+      }
+      """
+
+    let expected =
+      """
+      class A {
+        func foo() -> Int {
+          return 1
+        }
+      \u{0020}\u{0020}
+        func bar() -> Int {
+          return 2
+        }
+      }
+      
+      """
+    var config = Configuration.forTesting
+    config.allowWhitespaceOnlyLines = true
+    assertPrettyPrintEqual(input: input, expected: expected, linelength: 80, configuration: config)
+  }
+
+  func testExpressionsWithUnnecessaryWhitespaces() {
+    let input =
+      """
+          class A {
+        func   foo() -> Int {
+          return 1
+        }
+      \u{0020}\u{0020}
+        func bar()    -> Int {
+          return 2
+        }
+      }
+      """
+
+    let expected =
+      """
+      class A {
+        func foo() -> Int {
+          return 1
+        }
+      \u{0020}\u{0020}
+        func bar() -> Int {
+          return 2
+        }
+      }
+      
+      """
+    var config = Configuration.forTesting
+    config.allowWhitespaceOnlyLines = true
+    assertPrettyPrintEqual(input: input, expected: expected, linelength: 80, configuration: config)
+  }
+}