Do not treat unrecognized switches as formatting targets

Author: davkean

src/CodeFormatter/CommandLineParser.cs

@@ -245,6 +245,10 @@ public static CommandLineParseResult Parse(string[] args)
                 {
                     return CommandLineParseResult.CreateSuccess(CommandLineOptions.ListRules);
                 }
+                else if (arg.StartsWith("/", comparison))
+                {
+                    return CommandLineParseResult.CreateError($"Unrecognized option \"{arg}\"");
+                }
                 else
                 {
                     formatTargets.Add(arg);

src/Microsoft.DotNet.CodeFormatting.Tests/CommandLineParserTests.cs

@@ -17,6 +17,15 @@ private CommandLineOptions Parse(params string[] args)
             return options;
         }
 
+        private CommandLineOptions FailToParse(params string[] args)
+        {
+            CommandLineOptions options;
+            Assert.False(CommandLineParser.TryParse(args, out options));
+            Assert.Null(options);
+
+            return options;
+        }
+
         [Fact]
         public void Rules()
         {
@@ -114,5 +123,23 @@ public void CopyrightEnable2()
             Assert.True(options.RuleMap[FormattingDefaults.CopyrightRuleName]);
             Assert.Equal(new[] { "test.csproj" }, options.FormatTargets);
         }
+
+        [Fact]
+        public void SingleUnrecognizedOption()
+        {
+            FailToParse("/unrecognized");
+        }
+
+        [Fact]
+        public void UnrecognizedOptionWithFormatTarget()
+        {
+            FailToParse("test.csproj", "/unrecognized");
+        }
+
+        [Fact]
+        public void UnrecognizedOptionWithOtherwiseValidArguments()
+        {
+            FailToParse("test.csproj", "/nocopyright", "/unrecognized");
+        }
     }
 }