Fix trivia handling while removing this.

Author: jaredpar

src/Microsoft.DotNet.CodeFormatting.Tests/Rules/ExplicitThisRuleTests.cs

@@ -94,6 +94,66 @@ void M()
         this.field3 = null;
     }
 }
+";
+            Verify(text, expected, runFormatter: false);
+        }
+
+        [Fact]
+        public void TestFieldAssignmentWithTrivia()
+        {
+            var text = @"
+class C1
+{
+    int _field;
+
+    void M()
+    {
+        this. /* comment1 */ _field /* comment 2 */ = 0;
+    }
+}
+";
+
+            var expected = @"
+class C1
+{
+    int _field;
+
+    void M()
+    {
+         /* comment1 */ _field /* comment 2 */ = 0;
+    }
+}
+";
+            Verify(text, expected, runFormatter: false);
+        }
+
+        [Fact]
+        public void TestFieldBadName()
+        {
+            var text = @"
+class C1
+{
+    int _field;
+
+    void M()
+    {
+        // Not a valid field access, can't reliably remove this.
+        this.field1 = 0;
+    }
+}
+";
+
+            var expected = @"
+class C1
+{
+    int _field;
+
+    void M()
+    {
+        // Not a valid field access, can't reliably remove this.
+        this.field1 = 0;
+    }
+}
 ";
             Verify(text, expected, runFormatter: false);
         }

src/Microsoft.DotNet.CodeFormatting/Rules/ExplicitThisRule.cs

@@ -37,19 +37,24 @@ public override SyntaxNode VisitMemberAccessExpression(MemberAccessExpressionSyn
                         var field = (IFieldSymbol)symbolInfo.Symbol;
                         if (field.DeclaredAccessibility == Accessibility.Private)
                         {
-                            return node.Name
-                                .WithLeadingTrivia(node.GetLeadingTrivia())
-                                .WithTrailingTrivia(node.GetTrailingTrivia());
+                            return RemoveQualification(node);
                         }
                     }
                 }
 
                 return node;
             }
 
-            public override SyntaxNode VisitAssignmentExpression(AssignmentExpressionSyntax node)
+            private static NameSyntax RemoveQualification(MemberAccessExpressionSyntax memberSyntax)
             {
-                return base.VisitAssignmentExpression(node);
+                var thisSyntax = memberSyntax.Expression;
+                var nameSyntax = memberSyntax.Name;
+                var triviaList = thisSyntax
+                    .GetLeadingTrivia()
+                    .AddRange(thisSyntax.GetTrailingTrivia())
+                    .AddRange(memberSyntax.OperatorToken.GetAllTrivia())
+                    .AddRange(nameSyntax.GetLeadingTrivia());
+                return nameSyntax.WithLeadingTrivia(triviaList);
             }
         }