Add option to disable trailing commas on multi-line collections

Documentation/Configuration.md

@@ -82,6 +82,9 @@ top-level keys and values:
 *  `spacesAroundRangeFormationOperators` _(boolean)_: Determines whether whitespace should be forced
    before and after the range formation operators `...` and `..<`.
 
+*  `multilineCollectionTrailingCommas` _(boolean)_: Determines whether multi-line collections should have trailing commas.
+    Defaults to `true`.
+
 > TODO: Add support for enabling/disabling specific syntax transformations in
 > the pipeline.
 

Sources/SwiftFormat/API/Configuration+Default.swift

@@ -37,5 +37,6 @@ extension Configuration {
     self.indentSwitchCaseLabels = false
     self.spacesAroundRangeFormationOperators = false
     self.noAssignmentInExpressions = NoAssignmentInExpressionsConfiguration()
+    self.multilineCollectionTrailingCommas = true
   }
 }

Sources/SwiftFormat/API/Configuration.swift

@@ -42,6 +42,7 @@ public struct Configuration: Codable, Equatable {
     case rules
     case spacesAroundRangeFormationOperators
     case noAssignmentInExpressions
+    case multilineCollectionTrailingCommas
   }
 
   /// A dictionary containing the default enabled/disabled states of rules, keyed by the rules'
@@ -162,6 +163,9 @@ public struct Configuration: Codable, Equatable {
   /// Contains exceptions for the `NoAssignmentInExpressions` rule.
   public var noAssignmentInExpressions: NoAssignmentInExpressionsConfiguration
 
+  /// Determines whether multi-line list initializers should have trailing commas
+  public var multilineCollectionTrailingCommas: Bool
+
   /// Constructs a Configuration by loading it from a configuration file.
   public init(contentsOf url: URL) throws {
     let data = try Data(contentsOf: url)
@@ -239,6 +243,10 @@ public struct Configuration: Codable, Equatable {
       try container.decodeIfPresent(
         NoAssignmentInExpressionsConfiguration.self, forKey: .noAssignmentInExpressions)
       ?? defaults.noAssignmentInExpressions
+    self.multilineCollectionTrailingCommas =
+      try container.decodeIfPresent(
+        Bool.self, forKey: .multilineCollectionTrailingCommas)
+    ?? defaults.multilineCollectionTrailingCommas
 
     // 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
@@ -271,6 +279,7 @@ public struct Configuration: Codable, Equatable {
     try container.encode(fileScopedDeclarationPrivacy, forKey: .fileScopedDeclarationPrivacy)
     try container.encode(indentSwitchCaseLabels, forKey: .indentSwitchCaseLabels)
     try container.encode(noAssignmentInExpressions, forKey: .noAssignmentInExpressions)
+    try container.encode(multilineCollectionTrailingCommas, forKey: .multilineCollectionTrailingCommas)
     try container.encode(rules, forKey: .rules)
   }
 

Sources/SwiftFormat/PrettyPrint/PrettyPrint.swift

@@ -557,7 +557,7 @@ public class PrettyPrinter {
       // We never want to add a trailing comma in an initializer so we disable trailing commas on
       // single element collections.
       let shouldHaveTrailingComma =
-        startLineNumber != openCloseBreakCompensatingLineNumber && !isSingleElement
+        startLineNumber != openCloseBreakCompensatingLineNumber && !isSingleElement && configuration.multilineCollectionTrailingCommas
       if shouldHaveTrailingComma && !hasTrailingComma {
         diagnose(.addTrailingComma, category: .trailingComma)
       } else if !shouldHaveTrailingComma && hasTrailingComma {

Sources/_SwiftFormatTestSupport/Configuration+Testing.swift

@@ -40,6 +40,7 @@ extension Configuration {
     config.indentSwitchCaseLabels = false
     config.spacesAroundRangeFormationOperators = false
     config.noAssignmentInExpressions = NoAssignmentInExpressionsConfiguration()
+    config.multilineCollectionTrailingCommas = true
     return config
   }
 }

Tests/SwiftFormatTests/PrettyPrint/CommaTests.swift

@@ -0,0 +1,145 @@
+import SwiftFormat
+
+final class CommaTests: PrettyPrintTestCase {
+  func testCommasAbsentEnabled() {
+    let input =
+      """
+      let MyList = [
+        1,
+        2,
+        3
+      ]
+
+      """
+
+    let expected =
+      """
+      let MyList = [
+        1,
+        2,
+        3,
+      ]
+
+      """
+
+    var configuration = Configuration.forTesting
+    configuration.multilineCollectionTrailingCommas = true
+    assertPrettyPrintEqual(input: input, expected: expected, linelength: 20, configuration: configuration)
+  }
+
+  func testCommasAbsentDisabled() {
+    let input =
+      """
+      let MyList = [
+        1,
+        2,
+        3
+      ]
+
+      """
+
+    let expected =
+      """
+      let MyList = [
+        1,
+        2,
+        3
+      ]
+
+      """
+
+    var configuration = Configuration.forTesting
+    configuration.multilineCollectionTrailingCommas = false
+    assertPrettyPrintEqual(input: input, expected: expected, linelength: 20, configuration: configuration)
+  }
+
+  func testCommasPresentEnabled() {
+    let input =
+      """
+      let MyList = [
+        1,
+        2,
+        3,
+      ]
+
+      """
+
+    let expected =
+      """
+      let MyList = [
+        1,
+        2,
+        3,
+      ]
+
+      """
+
+    var configuration = Configuration.forTesting
+    configuration.multilineCollectionTrailingCommas = true
+    assertPrettyPrintEqual(input: input, expected: expected, linelength: 20, configuration: configuration)
+  }
+
+  func testCommasPresentDisabled() {
+    let input =
+      """
+      let MyList = [
+        1,
+        2,
+        3,
+      ]
+
+      """
+
+    let expected =
+      """
+      let MyList = [
+        1,
+        2,
+        3
+      ]
+
+      """
+
+    var configuration = Configuration.forTesting
+    configuration.multilineCollectionTrailingCommas = false
+    assertPrettyPrintEqual(input: input, expected: expected, linelength: 20, configuration: configuration)
+  }
+
+  func testCommasPresentSingleLineDisabled() {
+    let input =
+      """
+      let MyList = [1, 2, 3,]
+
+      """
+
+    // no effect expected
+    let expected =
+      """
+      let MyList = [1, 2, 3]
+
+      """
+
+    var configuration = Configuration.forTesting
+    configuration.multilineCollectionTrailingCommas = true
+    assertPrettyPrintEqual(input: input, expected: expected, linelength: 40, configuration: configuration)
+  }
+
+  func testCommasPresentSingleLineEnabled() {
+    let input =
+      """
+      let MyList = [1, 2, 3,]
+
+      """
+
+    // no effect expected
+    let expected =
+      """
+      let MyList = [1, 2, 3]
+
+      """
+
+    var configuration = Configuration.forTesting
+    configuration.multilineCollectionTrailingCommas = false
+    assertPrettyPrintEqual(input: input, expected: expected, linelength: 40, configuration: configuration)
+  }
+}