Update SA1513 codefix to use the existing newline character sequence

#3360

StyleCop.Analyzers/StyleCop.Analyzers.CodeFixes/LayoutRules/SA1513CodeFixProvider.cs

@@ -65,7 +65,8 @@ private static async Task<SyntaxNode> GetTransformedDocumentAsync(Document docum
                 diagnostics.Select(diagnostic => root.FindToken(diagnostic.Location.SourceSpan.End)),
                 (originalToken, rewrittenToken) =>
                 {
-                    var newTrivia = rewrittenToken.LeadingTrivia.Insert(0, SyntaxFactory.CarriageReturnLineFeed);
+                    var endOfLineTrivia = rewrittenToken.GetPrecedingEndOfLineTrivia();
+                    var newTrivia = rewrittenToken.LeadingTrivia.Insert(0, endOfLineTrivia);
                     return rewrittenToken.WithLeadingTrivia(newTrivia);
                 });
         }

StyleCop.Analyzers/StyleCop.Analyzers.Test/Helpers/StringExtensions.cs

@@ -0,0 +1,23 @@
+// Copyright (c) Tunnel Vision Laboratories, LLC. All Rights Reserved.
+// Licensed under the MIT License. See LICENSE in the project root for license information.
+
+namespace StyleCop.Analyzers.Test.Helpers
+{
+    public static class StringExtensions
+    {
+        public static string ReplaceLineEndings(this string input, string replacementText)
+        {
+            // First normalize to LF
+            var lineFeedInput = input
+                .Replace("\r\n", "\n")
+                .Replace("\r", "\n")
+                .Replace("\f", "\n")
+                .Replace("\x0085", "\n")
+                .Replace("\x2028", "\n")
+                .Replace("\x2029", "\n");
+
+            // Then normalize to the replacement text
+            return lineFeedInput.Replace("\n", replacementText);
+        }
+    }
+}

StyleCop.Analyzers/StyleCop.Analyzers.Test/LayoutRules/SA1513UnitTests.cs

@@ -9,6 +9,7 @@ namespace StyleCop.Analyzers.Test.LayoutRules
     using System.Threading.Tasks;
     using Microsoft.CodeAnalysis.Testing;
     using StyleCop.Analyzers.LayoutRules;
+    using StyleCop.Analyzers.Test.Helpers;
     using Xunit;
     using static StyleCop.Analyzers.Test.Verifiers.StyleCopCodeFixVerifier<
         StyleCop.Analyzers.LayoutRules.SA1513ClosingBraceMustBeFollowedByBlankLine,
@@ -1013,5 +1014,25 @@ public void TestMethod(string extraSupport)
 
             await VerifyCSharpDiagnosticAsync(testCode, DiagnosticResult.EmptyDiagnosticResults, CancellationToken.None).ConfigureAwait(false);
         }
+
+        [Fact]
+        [WorkItem(3360, "https://github.com/DotNetAnalyzers/StyleCopAnalyzers/issues/3360")]
+        public async Task TestLineFeedEndOfLinesAsync()
+        {
+            var testCode = @"
+public class TestClass
+{
+}[|
+|]// Hello".ReplaceLineEndings("\n");
+
+            var fixedCode = @"
+public class TestClass
+{
+}
+
+// Hello".ReplaceLineEndings("\n");
+
+            await VerifyCSharpFixAsync(testCode, DiagnosticResult.EmptyDiagnosticResults, fixedCode, CancellationToken.None).ConfigureAwait(false);
+        }
     }
 }

StyleCop.Analyzers/StyleCop.Analyzers/Helpers/TokenHelper.cs

@@ -175,5 +175,35 @@ internal static bool IsFollowedByWhitespace(this SyntaxToken token)
             triviaList = token.GetNextToken().LeadingTrivia;
             return triviaList.Count > 0 && triviaList.First().IsKind(SyntaxKind.WhitespaceTrivia);
         }
+
+        /// <summary>
+        /// Returns the closest end of line trivia preceding the <paramref name="token"/>.
+        /// This currently only looks immediately before the specified token.
+        /// </summary>
+        /// <param name="token">The token to process.</param>
+        /// <returns>The closest preceding end of line trivia, or <see cref="SyntaxFactory.CarriageReturnLineFeed"/> if none is found.</returns>
+        internal static SyntaxTrivia GetPrecedingEndOfLineTrivia(this SyntaxToken token)
+        {
+            var leadingTrivia = token.LeadingTrivia;
+            for (var i = leadingTrivia.Count - 1; i >= 0; i--)
+            {
+                if (leadingTrivia[i].IsKind(SyntaxKind.EndOfLineTrivia))
+                {
+                    return leadingTrivia[i];
+                }
+            }
+
+            var prevToken = token.GetPreviousToken();
+            var prevTrailingTrivia = prevToken.TrailingTrivia;
+            for (var i = prevTrailingTrivia.Count - 1; i >= 0; i--)
+            {
+                if (prevTrailingTrivia[i].IsKind(SyntaxKind.EndOfLineTrivia))
+                {
+                    return prevTrailingTrivia[i];
+                }
+            }
+
+            return SyntaxFactory.CarriageReturnLineFeed;
+        }
     }
 }