Preserve trivia

src/EditorFeatures/CSharpTest/UsePatternMatching/CSharpAsAndNullCheckTests.cs

@@ -431,5 +431,39 @@ void M()
     }
 }");
         }
+
+        [WorkItem(15957, "https://github.com/dotnet/roslyn/issues/15957")]
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsInlineTypeCheck)]
+        public async Task TestTrivia1()
+        {
+            await TestAsync(
+@"class C
+{
+    void M(object y)
+    {
+        if (y != null)
+        {
+        }
+
+        [|var|] x = o as string;
+        if (x != null)
+        {
+        }
     }
+}",
+@"class C
+{
+    void M(object y)
+    {
+        if (y != null)
+        {
+        }
+
+        if (o is string x)
+        {
+        }
+    }
+}", compareTokens: false);
+        }
+        }
 }
\ No newline at end of file

src/Features/CSharp/Portable/UsePatternMatching/CSharpAsAndNullCheckCodeFixProvider.cs

@@ -57,26 +57,29 @@ public override Task RegisterCodeFixesAsync(CodeFixContext context)
 
             var localDeclaration = (LocalDeclarationStatementSyntax)localDeclarationLocation.FindNode(cancellationToken);
             var ifStatement = (IfStatementSyntax)ifStatementLocation.FindNode(cancellationToken);
-            var condition = (BinaryExpressionSyntax)conditionLocation.FindNode(cancellationToken);
+            var conditionPart = (BinaryExpressionSyntax)conditionLocation.FindNode(cancellationToken);
             var asExpression = (BinaryExpressionSyntax)asExpressionLocation.FindNode(cancellationToken);
 
-            var updatedCondition = SyntaxFactory.IsPatternExpression(
+            var updatedConditionPart = SyntaxFactory.IsPatternExpression(
                 asExpression.Left, SyntaxFactory.DeclarationPattern(
                     ((TypeSyntax)asExpression.Right).WithoutTrivia(),
                     SyntaxFactory.SingleVariableDesignation(
                         localDeclaration.Declaration.Variables[0].Identifier.WithoutTrivia())));
 
-            var trivia = localDeclaration.GetLeadingTrivia().Concat(localDeclaration.GetTrailingTrivia())
-                                         .Where(t => t.IsSingleOrMultiLineComment())
-                                         .SelectMany(t => ImmutableArray.Create(t, SyntaxFactory.ElasticCarriageReturnLineFeed))
-                                         .ToImmutableArray();
+            var finalCondition = ifStatement.Condition.ReplaceNode(conditionPart, updatedConditionPart);
 
-            var updatedIfStatement = ifStatement.ReplaceNode(condition, updatedCondition)
-                                                .WithPrependedLeadingTrivia(trivia)
-                                                .WithAdditionalAnnotations(Formatter.Annotation);
-
-            editor.RemoveNode(localDeclaration);
-            editor.ReplaceNode(ifStatement, updatedIfStatement);
+            // Keep the trivia on the node we're removing.  But format the next statement so 
+            // they look ok when they move to it.
+            var removeOptions = localDeclaration.GetTrailingTrivia().Any(t => t.IsRegularOrDocComment())
+                ? SyntaxRemoveOptions.KeepLeadingTrivia | SyntaxRemoveOptions.KeepTrailingTrivia
+                : SyntaxRemoveOptions.KeepLeadingTrivia;
+            editor.RemoveNode(localDeclaration, removeOptions);
+            editor.ReplaceNode(ifStatement, (i, g) =>
+            {
+                var currentIf = (IfStatementSyntax)i;
+                var updatedIf = currentIf.ReplaceNode(currentIf.Condition, finalCondition);
+                return updatedIf.WithAdditionalAnnotations(Formatter.Annotation);
+            });
         }
 
         private class MyCodeAction : CodeAction.DocumentChangeAction