Use stderr for watch output (#50820)

Author: tmat

src/BuiltInTools/DotNetDeltaApplier/StartupHook.cs

@@ -89,7 +89,7 @@ private static void Log(string message)
         if (!string.IsNullOrEmpty(prefix))
         {
             Console.ForegroundColor = ConsoleColor.DarkGray;
-            Console.WriteLine($"{prefix} {message}");
+            Console.Error.WriteLine($"{prefix} {message}");
             Console.ResetColor();
         }
     }

src/BuiltInTools/dotnet-watch/UI/ConsoleReporter.cs

@@ -56,24 +56,24 @@ public void Report(EventId id, Emoji emoji, MessageSeverity severity, string mes
             {
                 case MessageSeverity.Error:
                     // Use stdout for error messages to preserve ordering with respect to other output.
-                    WriteLine(console.Out, message, ConsoleColor.Red, emoji);
+                    WriteLine(console.Error, message, ConsoleColor.Red, emoji);
                     break;
 
                 case MessageSeverity.Warning:
-                    WriteLine(console.Out, message, ConsoleColor.Yellow, emoji);
+                    WriteLine(console.Error, message, ConsoleColor.Yellow, emoji);
                     break;
 
                 case MessageSeverity.Output:
                     if (!IsQuiet)
                     {
-                        WriteLine(console.Out, message, color: null, emoji);
+                        WriteLine(console.Error, message, color: null, emoji);
                     }
                     break;
 
                 case MessageSeverity.Verbose:
                     if (IsVerbose)
                     {
-                        WriteLine(console.Out, message, ConsoleColor.DarkGray, emoji);
+                        WriteLine(console.Error, message, ConsoleColor.DarkGray, emoji);
                     }
                     break;
             }

test/dotnet-watch.Tests/ConsoleReporterTests.cs

@@ -18,19 +18,19 @@ public void WritesToStandardStreams(bool suppressEmojis)
             var reporter = new ConsoleReporter(testConsole, verbose: true, quiet: false, suppressEmojis: suppressEmojis);
 
             reporter.Report(id: default, Emoji.Watch, MessageSeverity.Verbose, "verbose {0}");
-            Assert.Equal($"dotnet watch {(suppressEmojis ? ":" : "⌚")} verbose {{0}}" + EOL, testConsole.GetOutput());
+            Assert.Equal($"dotnet watch {(suppressEmojis ? ":" : "⌚")} verbose {{0}}" + EOL, testConsole.GetError());
             testConsole.Clear();
 
             reporter.Report(id: default, Emoji.Watch, MessageSeverity.Output, "out");
-            Assert.Equal($"dotnet watch {(suppressEmojis ? ":" : "⌚")} out" + EOL, testConsole.GetOutput());
+            Assert.Equal($"dotnet watch {(suppressEmojis ? ":" : "⌚")} out" + EOL, testConsole.GetError());
             testConsole.Clear();
 
             reporter.Report(id: default, Emoji.Warning, MessageSeverity.Warning, "warn");
-            Assert.Equal($"dotnet watch {(suppressEmojis ? ":" : "⚠")} warn" + EOL, testConsole.GetOutput());
+            Assert.Equal($"dotnet watch {(suppressEmojis ? ":" : "⚠")} warn" + EOL, testConsole.GetError());
             testConsole.Clear();
 
             reporter.Report(id: default, Emoji.Error, MessageSeverity.Error, "error");
-            Assert.Equal($"dotnet watch {(suppressEmojis ? ":" : "❌")} error" + EOL, testConsole.GetOutput());
+            Assert.Equal($"dotnet watch {(suppressEmojis ? ":" : "❌")} error" + EOL, testConsole.GetError());
             testConsole.Clear();
         }
 

test/dotnet.Tests/CommandTests/Run/RunFileTests.cs

@@ -332,10 +332,10 @@ public void FilePath_AsProjectArgument()
     /// </summary>
     [Theory]
     // error MSB1003: Specify a project or solution file. The current working directory does not contain a project or solution file.
-    [InlineData("build", "MSB1003")]
+    [InlineData("build", "MSB1003", false)]
     // dotnet watch: Could not find a MSBuild project file in '...'. Specify which project to use with the --project option.
-    [InlineData("watch", "--project")]
-    public void Precedence_BuiltInCommand(string cmd, string error)
+    [InlineData("watch", "--project", true)]
+    public void Precedence_BuiltInCommand(string cmd, string error, bool errorInStdErr)
     {
         var testInstance = _testAssetsManager.CreateTestDirectory();
         File.WriteAllText(Path.Join(testInstance.Path, cmd), """
@@ -348,18 +348,26 @@ public void Precedence_BuiltInCommand(string cmd, string error)
             """);
 
         // dotnet build -> built-in command
-        new DotnetCommand(Log, cmd)
+        var failure = new DotnetCommand(Log, cmd)
             .WithWorkingDirectory(testInstance.Path)
             .Execute()
-            .Should().Fail()
-            .And.HaveStdOutContaining(error);
+            .Should().Fail();
+
+        if (errorInStdErr)
+        {
+            failure.And.HaveStdErrContaining(error);
+        }
+        else
+        {
+            failure.And.HaveStdOutContaining(error);
+        }
 
         // dotnet ./build -> file-based app
         new DotnetCommand(Log, $"./{cmd}")
-            .WithWorkingDirectory(testInstance.Path)
-            .Execute()
-            .Should().Pass()
-            .And.HaveStdOut("hello 1");
+                .WithWorkingDirectory(testInstance.Path)
+                .Execute()
+                .Should().Pass()
+                .And.HaveStdOut("hello 1");
 
         // dotnet run build -> file-based app
         new DotnetCommand(Log, "run", cmd)