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. 
+    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 preserved as-is 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 +449,12 @@ 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 {
+        let currentIndentationSpaceSize = outputBuffer.currentIndentation.indentation().count
+        outputBuffer.write(String(repeating: " ", count: size - currentIndentationSpaceSize))
+      } 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 +1,99 @@
+import SwiftFormat
+
+final class AllowWhitespaceOnlyLinesTests: PrettyPrintTestCase {
+  func testAllowWhitespaceOnlyLinesEnabled() {
+    let input =
+      """
+      class A {
+        func foo() -> Int {
+          return 1
+        }
+        
+        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 = true
+    assertPrettyPrintEqual(input: input, expected: expected, linelength: 80, configuration: config)
+  }
+
+  func testAllowWhitespaceOnlyLinesDisabled() {
+    let input =
+      """
+      class A {
+        func foo() -> Int {
+          return 1
+        }
+        
+        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 testExpressionsWithUnnecessaryWhitespaces() {
+    let input =
+      """
+          class A {
+        func   foo() -> Int {
+          return 1
+        }
+        
+        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 = true
+    assertPrettyPrintEqual(input: input, expected: expected, linelength: 80, configuration: config)
+  }
+}