Don’t add an implicit leading newline in CodeBlockItemList result builder if previous element already has a trailing newline

Sources/SwiftSyntaxBuilder/ResultBuilderExtensions.swift

@@ -42,9 +42,15 @@ extension CodeBlockItemListBuilder {
   }
 
   public static func buildFinalResult(_ component: Component) -> CodeBlockItemListSyntax {
-    .init(
-      component.enumerated().map { (index, expression) in
-        if index > component.startIndex, !expression.leadingTrivia.contains(where: \.isNewline) {
+    // Treat the first element as being on a new line. It doesn't need a leading newline
+    var previousEndedInNewline = true
+
+    return CodeBlockItemListSyntax(
+      component.map { expression in
+        defer {
+          previousEndedInNewline = expression.trailingTrivia.pieces.last?.isNewline ?? false
+        }
+        if !previousEndedInNewline, !expression.leadingTrivia.contains(where: \.isNewline) {
           return expression.with(\.leadingTrivia, .newline.merging(expression.leadingTrivia))
         } else {
           return expression

Sources/_SwiftSyntaxGenericTestSupport/AssertEqualWithDiff.swift

@@ -83,8 +83,8 @@ public func failStringsEqualWithDiff(
 
   // Use `CollectionDifference` on supported platforms to get `diff`-like line-based output. On
   // older platforms, fall back to simple string comparison.
-  let actualLines = actual.split(separator: "\n")
-  let expectedLines = expected.split(separator: "\n")
+  let actualLines = actual.split(separator: "\n", omittingEmptySubsequences: false)
+  let expectedLines = expected.split(separator: "\n", omittingEmptySubsequences: false)
 
   let difference = actualLines.difference(from: expectedLines)
 

Tests/SwiftSyntaxBuilderTest/CollectionNodeFlatteningTests.swift

@@ -15,7 +15,7 @@ import SwiftSyntaxBuilder
 import XCTest
 
 final class CollectionNodeFlatteningTests: XCTestCase {
-  func test_FlattenCodeBlockItemListWithBuilder() {
+  func testFlattenCodeBlockItemListWithBuilder() {
     @CodeBlockItemListBuilder
     func buildInnerCodeBlockItemList() -> CodeBlockItemListSyntax {
       [ExprSyntax("innerBuilder1"), ExprSyntax("innerBuilder2")].lazy.map {
@@ -47,7 +47,7 @@ final class CollectionNodeFlatteningTests: XCTestCase {
     )
   }
 
-  func test_FlattenCodeBlockItemListWithCodeBlockItemStrings() {
+  func testFlattenCodeBlockItemListWithCodeBlockItemStrings() {
     let buildable = CodeBlockItemListSyntax {
       "let one = object.methodOne()"
       "let two = object.methodTwo()"
@@ -62,7 +62,7 @@ final class CollectionNodeFlatteningTests: XCTestCase {
     )
   }
 
-  func test_FlattenCodeBlockItemListWithCodeBlockItemStringArray() {
+  func testFlattenCodeBlockItemListWithCodeBlockItemStringArray() {
     let buildable = CodeBlockItemListSyntax {
       ["let one = object.methodOne()", "let two = object.methodTwo()"]
     }
@@ -76,7 +76,7 @@ final class CollectionNodeFlatteningTests: XCTestCase {
     )
   }
 
-  func test_FlattenCodeBlockItemListWithCodeBlockInterpolated() {
+  func testFlattenCodeBlockItemListWithCodeBlockInterpolated() {
     let block = CodeBlockItemListSyntax {
       "let a = 1"
       "let b = 2"
@@ -102,4 +102,21 @@ final class CollectionNodeFlatteningTests: XCTestCase {
       """
     )
   }
+
+  func testFlattenCodeBlockItemListWithTrailingNewline() {
+    let buildable = CodeBlockItemListSyntax {
+      DeclSyntax("let a = 1").with(\.trailingTrivia, .newline)
+      DeclSyntax("let b = 2").with(\.trailingTrivia, .newline)
+    }
+
+    assertBuildResult(
+      buildable,
+      """
+      let a = 1
+      let b = 2
+
+      """
+    )
+  }
+
 }