Replaces the MultiLineTrailingCommas phase 1 rule with Pretty Printer tokens.

Sources/SwiftFormat/Pipelines+Generated.swift

@@ -18,11 +18,6 @@ import SwiftSyntax
 
 extension LintPipeline {
 
-  func visit(_ node: ArrayExprSyntax) -> SyntaxVisitorContinueKind {
-    visitIfEnabled(MultiLineTrailingCommas.visit, in: context, for: node)
-    return .visitChildren
-  }
-
   func visit(_ node: AsExprSyntax) -> SyntaxVisitorContinueKind {
     visitIfEnabled(NeverForceUnwrap.visit, in: context, for: node)
     return .visitChildren
@@ -71,11 +66,6 @@ extension LintPipeline {
     return .visitChildren
   }
 
-  func visit(_ node: DictionaryExprSyntax) -> SyntaxVisitorContinueKind {
-    visitIfEnabled(MultiLineTrailingCommas.visit, in: context, for: node)
-    return .visitChildren
-  }
-
   func visit(_ node: EnumCaseElementSyntax) -> SyntaxVisitorContinueKind {
     visitIfEnabled(AlwaysUseLowerCamelCase.visit, in: context, for: node)
     visitIfEnabled(NoLeadingUnderscores.visit, in: context, for: node)
@@ -243,8 +233,12 @@ extension LintPipeline {
     return .visitChildren
   }
 
-  func visit(_ node: SwitchStmtSyntax) -> SyntaxVisitorContinueKind {
+  func visit(_ node: SwitchCaseListSyntax) -> SyntaxVisitorContinueKind {
     visitIfEnabled(NoCasesWithOnlyFallthrough.visit, in: context, for: node)
+    return .visitChildren
+  }
+
+  func visit(_ node: SwitchStmtSyntax) -> SyntaxVisitorContinueKind {
     visitIfEnabled(NoParensAroundConditions.visit, in: context, for: node)
     return .visitChildren
   }
@@ -285,7 +279,6 @@ extension FormatPipeline {
     node = DoNotUseSemicolons(context: context).visit(node)
     node = FullyIndirectEnum(context: context).visit(node)
     node = GroupNumericLiterals(context: context).visit(node)
-    node = MultiLineTrailingCommas(context: context).visit(node)
     node = NoAccessLevelOnExtensionDeclaration(context: context).visit(node)
     node = NoBlockComments(context: context).visit(node)
     node = NoCasesWithOnlyFallthrough(context: context).visit(node)

Sources/SwiftFormatConfiguration/RuleRegistry+Generated.swift

@@ -24,7 +24,6 @@ enum RuleRegistry {
     "FullyIndirectEnum": true,
     "GroupNumericLiterals": true,
     "IdentifiersMustBeASCII": true,
-    "MultiLineTrailingCommas": true,
     "NeverForceUnwrap": true,
     "NeverUseForceTry": true,
     "NeverUseImplicitlyUnwrappedOptionals": true,

Sources/SwiftFormatPrettyPrint/PrettyPrint.swift

@@ -79,6 +79,10 @@ public class PrettyPrinter {
   /// Keeps track of the continuation line state as you go into and out of open-close break groups.
   private var continuationStack: [Bool] = []
 
+  /// Keeps track of the line number where comma regions started. Line numbers are removed as their
+  /// corresponding end token are encountered.
+  private var commaDelimitedRegionStack: [Int] = []
+
   /// Keeps track of the most recent number of consecutive newlines that have been printed.
   ///
   /// This value is reset to zero whenever non-newline content is printed.
@@ -443,6 +447,18 @@ public class PrettyPrinter {
       case .enableBreaking:
         activeBreakSuppressionCount -= 1
       }
+
+    case .commaDelimitedRegionStart:
+      commaDelimitedRegionStack.append(openCloseBreakCompensatingLineNumber)
+
+    case .commaDelimitedRegionEnd:
+      guard let startLineNumber = commaDelimitedRegionStack.popLast() else {
+        fatalError("Found trailing comma end with no corresponding start.")
+      }
+
+      if startLineNumber != openCloseBreakCompensatingLineNumber {
+        write(",")
+      }
     }
   }
 
@@ -535,6 +551,21 @@ public class PrettyPrinter {
       case .printerControl:
         // Control tokens have no length. They aren't printed.
         lengths.append(0)
+
+      case .commaDelimitedRegionStart:
+        lengths.append(0)
+
+      case .commaDelimitedRegionEnd:
+        // The trailing comma needs to be included in the length of the preceding break, but is not
+        // included in the length of the enclosing group. A trailing comma cannot cause the group
+        // to break onto multiple lines, because the comma isn't printed for a single line group.
+        if let index = delimIndexStack.last, case .break = tokens[index] {
+          lengths[index] += 1
+        }
+        // If the closest delimiter token is an open, instead of a break, then adding the comma's
+        // length isn't necessary. In that case, the comma is printed if the preceding break fires.
+
+        lengths.append(1)
       }
     }
 
@@ -621,6 +652,14 @@ public class PrettyPrinter {
     case .printerControl(let kind):
       printDebugIndent()
       print("[PRINTER CONTROL Kind: \(kind) Idx: \(idx)]")
+
+    case .commaDelimitedRegionStart:
+      printDebugIndent()
+      print("[COMMA DELIMITED START Idx: \(idx)]")
+
+      case .commaDelimitedRegionEnd:
+        printDebugIndent()
+        print("[COMMA DELIMITED END Idx: \(idx)]")
     }
   }
 

Sources/SwiftFormatPrettyPrint/Token.swift

@@ -171,6 +171,14 @@ enum Token {
   case verbatim(Verbatim)
   case printerControl(kind: PrinterControlKind)
 
+  /// Marks the beginning of a comma delimited collection, where a trailing comma should be inserted
+  /// at `commaDelimitedRegionEnd` if and only if the collection spans multiple lines.
+  case commaDelimitedRegionStart
+
+  /// Marks the end of a comma delimited collection, where a trailing comma should be inserted
+  /// if and only if the collection spans multiple lines.
+  case commaDelimitedRegionEnd
+
   // Convenience overloads for the enum types
   static let open = Token.open(.inconsistent, 0)
 

Sources/SwiftFormatPrettyPrint/TokenStreamCreator.swift

@@ -47,6 +47,10 @@ private final class TokenStreamCreator: SyntaxVisitor {
   /// stream.
   private var pendingMultilineStringSegmentPrefixLengths = [TokenSyntax: Int]()
 
+  /// Lists tokens that shouldn't be appended to the token stream as `syntax` tokens. They will be
+  /// printed conditionally using a different type of token.
+  private var ignoredTokens = Set<TokenSyntax>()
+
   init(configuration: Configuration, operatorContext: OperatorContext) {
     self.config = configuration
     self.operatorContext = operatorContext
@@ -698,6 +702,13 @@ private final class TokenStreamCreator: SyntaxVisitor {
 
   func visit(_ node: ArrayElementListSyntax) -> SyntaxVisitorContinueKind {
     insertTokens(.break(.same), betweenElementsOf: node)
+    if let firstElement = node.first, let lastElement = node.last {
+      before(firstElement.firstToken, tokens: .commaDelimitedRegionStart)
+      after(lastElement.lastToken, tokens: .commaDelimitedRegionEnd)
+      if let existingTrailingComma = lastElement.trailingComma {
+        ignoredTokens.insert(existingTrailingComma)
+      }
+    }
     return .visitChildren
   }
 
@@ -715,6 +726,13 @@ private final class TokenStreamCreator: SyntaxVisitor {
 
   func visit(_ node: DictionaryElementListSyntax) -> SyntaxVisitorContinueKind {
     insertTokens(.break(.same), betweenElementsOf: node)
+    if let firstElement = node.first, let lastElement = node.last {
+      before(firstElement.firstToken, tokens: .commaDelimitedRegionStart)
+      after(lastElement.lastToken, tokens: .commaDelimitedRegionEnd)
+      if let existingTrailingComma = lastElement.trailingComma {
+        ignoredTokens.insert(existingTrailingComma)
+      }
+    }
     return .visitChildren
   }
 
@@ -1936,7 +1954,7 @@ private final class TokenStreamCreator: SyntaxVisitor {
 
     if let pendingSegmentIndex = pendingMultilineStringSegmentPrefixLengths.index(forKey: token) {
       appendMultilineStringSegments(at: pendingSegmentIndex)
-    } else {
+    } else if !ignoredTokens.contains(token) {
       // Otherwise, it's just a regular token, so add the text as-is.
       appendToken(.syntax(token.text))
     }

Tests/SwiftFormatPrettyPrintTests/ArrayDeclTests.swift

@@ -2,32 +2,42 @@ public class ArrayDeclTests: PrettyPrintTestCase {
   public func testBasicArrays() {
     let input =
       """
-      let a = [1, 2, 3]
+      let a = [1, 2, 3,]
       let a: [Bool] = [false, true, true, false]
+      let a = [11111111, 2222222, 33333333, 444444]
       let a = [11111111, 2222222, 33333333, 4444444]
       let a: [String] = ["One", "Two", "Three", "Four"]
       let a: [String] = ["One", "Two", "Three", "Four", "Five", "Six", "Seven"]
       let a: [String] = ["One", "Two", "Three", "Four", "Five", "Six", "Seven",]
+      let a: [String] = [
+        "One", "Two", "Three", "Four", "Five",
+        "Six", "Seven", "Eight",
+      ]
       """
 
     let expected =
       """
       let a = [1, 2, 3]
       let a: [Bool] = [false, true, true, false]
+      let a = [11111111, 2222222, 33333333, 444444]
       let a = [
-        11111111, 2222222, 33333333, 4444444
+        11111111, 2222222, 33333333, 4444444,
       ]
       let a: [String] = [
-        "One", "Two", "Three", "Four"
+        "One", "Two", "Three", "Four",
       ]
       let a: [String] = [
         "One", "Two", "Three", "Four", "Five",
-        "Six", "Seven"
+        "Six", "Seven",
       ]
       let a: [String] = [
         "One", "Two", "Three", "Four", "Five",
         "Six", "Seven",
       ]
+      let a: [String] = [
+        "One", "Two", "Three", "Four", "Five",
+        "Six", "Seven", "Eight",
+      ]
 
       """
 

Tests/SwiftFormatPrettyPrintTests/CommentTests.swift

@@ -220,30 +220,30 @@ public class CommentTests: PrettyPrintTestCase {
       // Array comment
       let a = [
         456,  // small comment
-        789
+        789,
       ]
 
       // Dictionary comment
       let b = [
         "abc": 456,  // small comment
-        "def": 789
+        "def": 789,
       ]
 
       // Trailing comment
       let c = [
-        123, 456  // small comment
+        123, 456,  // small comment
       ]
 
       /* Array comment */
       let a = [
         456, /* small comment */
-        789
+        789,
       ]
 
       /* Dictionary comment */
       let b = [
         "abc": 456, /* small comment */
-        "def": 789
+        "def": 789,
       ]
 
       """