Fixed issues with SA1120 code fix

Author: vweijsters

StyleCop.Analyzers/StyleCop.Analyzers.CodeFixes/ReadabilityRules/SA1120CodeFixProvider.cs

@@ -88,6 +88,12 @@ private static async Task<Document> GetTransformedDocumentAsync(Document documen
 
         private static bool TriviaHasLeadingContentOnLine(SyntaxNode root, SyntaxTrivia commentTrivia)
         {
+            if (commentTrivia.SpanStart == 0)
+            {
+                // It is impossible to have leading content at the start of the file.
+                return false;
+            }
+
             var nodeBeforeStart = commentTrivia.SpanStart - 1;
             var nodeBefore = root.FindNode(new Microsoft.CodeAnalysis.Text.TextSpan(nodeBeforeStart, 1));
 
@@ -96,6 +102,12 @@ private static bool TriviaHasLeadingContentOnLine(SyntaxNode root, SyntaxTrivia
 
         private static bool TriviaHasTrailingContentOnLine(SyntaxNode root, SyntaxTrivia commentTrivia)
         {
+            if (commentTrivia.Span.End == root.Span.End)
+            {
+                // It is impossible to have trailing content at the end of the file.
+                return false;
+            }
+
             var nodeAfterTriviaStart = commentTrivia.Span.End + 1;
             var nodeAfterTrivia = root.FindNode(new Microsoft.CodeAnalysis.Text.TextSpan(nodeAfterTriviaStart, 1));
 

StyleCop.Analyzers/StyleCop.Analyzers.Test/ReadabilityRules/SA1120UnitTests.cs

@@ -354,6 +354,56 @@ public SomeException()
             await this.VerifyCSharpFixAsync(testCode, fixedCode).ConfigureAwait(false);
         }
 
+        /// <summary>
+        /// Verifies that an empty comment at the start of a source file will be handled correctly.
+        /// This is a regression test for #1708
+        /// </summary>
+        /// <returns>A <see cref="Task"/> representing the asynchronous unit test.</returns>
+        [Fact]
+        public async Task VerifyThatEmptyCommentAtFileStartWillBeHandledProperlyAsync()
+        {
+            var testCode = @"//
+public class TestClass
+{
+}
+";
+
+            var fixedTestCode = @"public class TestClass
+{
+}
+";
+
+            var expected = this.CSharpDiagnostic().WithLocation(1, 1);
+
+            await this.VerifyCSharpDiagnosticAsync(testCode, expected, CancellationToken.None).ConfigureAwait(false);
+            await this.VerifyCSharpDiagnosticAsync(fixedTestCode, EmptyDiagnosticResults, CancellationToken.None).ConfigureAwait(false);
+            await this.VerifyCSharpFixAsync(testCode, fixedTestCode).ConfigureAwait(false);
+        }
+
+        /// <summary>
+        /// Verifies that an empty comment at the end of a source file will be handled correctly.
+        /// </summary>
+        /// <returns>A <see cref="Task"/> representing the asynchronous unit test.</returns>
+        [Fact]
+        public async Task VerifyThatEmptyCommentAtFileEndWillBeHandledProperlyAsync()
+        {
+            var testCode = @"public class TestClass
+{
+}
+//";
+
+            var fixedTestCode = @"public class TestClass
+{
+}
+";
+
+            var expected = this.CSharpDiagnostic().WithLocation(4, 1);
+
+            await this.VerifyCSharpDiagnosticAsync(testCode, expected, CancellationToken.None).ConfigureAwait(false);
+            await this.VerifyCSharpDiagnosticAsync(fixedTestCode, EmptyDiagnosticResults, CancellationToken.None).ConfigureAwait(false);
+            await this.VerifyCSharpFixAsync(testCode, fixedTestCode).ConfigureAwait(false);
+        }
+
         protected override IEnumerable<DiagnosticAnalyzer> GetCSharpDiagnosticAnalyzers()
         {
             yield return new SA1120CommentsMustContainText();