Merge pull request #2864 from roopekv/code-gen-keep-whitespace-after-comma-if-it-precedes-other-trivia

Fix `CodeGenerationFormat` to correctly handle trailing trivia when formatting list elements

Author: ahoppen

CodeGeneration/Sources/Utils/CodeGenerationFormat.swift

@@ -158,21 +158,30 @@ public class CodeGenerationFormat: BasicFormat {
     }
     formattedChildren = formattedChildren.map { child in
       var child = child
-      child.trailingTrivia = Trivia(pieces: child.trailingTrivia.drop(while: \.isSpaceOrTab))
+
+      if let firstNonSpaceOrTabIndex = child.trailingTrivia.firstIndex(where: { !$0.isSpaceOrTab }) {
+        if child.trailingTrivia[firstNonSpaceOrTabIndex].isNewline {
+          child.trailingTrivia = Trivia(pieces: child.trailingTrivia.suffix(from: firstNonSpaceOrTabIndex))
+        }
+      } else {
+        child.trailingTrivia = Trivia()
+      }
 
       if !child.startsOnNewline {
         child.leadingTrivia = indentedNewline + child.leadingTrivia
       }
       return child
     }
     decreaseIndentationLevel()
-    if let lastChild = formattedChildren.last {
+    if let lastChild = formattedChildren.last,
+      !lastChild.trailingTrivia.contains(where: \.isNewline)
+    {
       let nextTokenStartsWithNewline =
         lastChild.nextToken(viewMode: .sourceAccurate)?.leadingTrivia.first?.isNewline ?? false
       if !nextTokenStartsWithNewline {
         formattedChildren[formattedChildren.count - 1] = lastChild.with(
           \.trailingTrivia,
-          indentedNewline
+          lastChild.trailingTrivia + indentedNewline
         )
       }
     }