Log warnings and errors to the standard error stream

src/Logging/SimpleConsoleLogger.cs

@@ -15,7 +15,8 @@ internal class SimpleConsoleLogger : ILogger
 
         private readonly IConsole _console;
         private readonly ITerminal _terminal;
-        private readonly LogLevel _logLevel;
+        private readonly LogLevel _minimalLogLevel;
+        private readonly LogLevel _minimalErrorLevel;
 
         private static ImmutableDictionary<LogLevel, ConsoleColor> LogLevelColorMap => new Dictionary<LogLevel, ConsoleColor>
         {
@@ -28,11 +29,12 @@ internal class SimpleConsoleLogger : ILogger
             [LogLevel.None] = ConsoleColor.White,
         }.ToImmutableDictionary();
 
-        public SimpleConsoleLogger(IConsole console, LogLevel logLevel)
+        public SimpleConsoleLogger(IConsole console, LogLevel minimalLogLevel, LogLevel minimalErrorLevel)
         {
             _terminal = console.GetTerminal();
             _console = console;
-            _logLevel = logLevel;
+            _minimalLogLevel = minimalLogLevel;
+            _minimalErrorLevel = minimalErrorLevel;
         }
 
         public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Exception exception, Func<TState, Exception, string> formatter)
@@ -45,38 +47,48 @@ public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Except
             lock (_gate)
             {
                 var message = formatter(state, exception);
+                var logToErrorStream = logLevel >= _minimalErrorLevel;
                 if (_terminal is null)
                 {
-                    LogToConsole(message);
+                    LogToConsole(_console, message, logToErrorStream);
                 }
                 else
                 {
-                    LogToTerminal(message, logLevel);
+                    LogToTerminal(message, logLevel, logToErrorStream);
                 }
             }
         }
 
         public bool IsEnabled(LogLevel logLevel)
         {
-            return (int)logLevel >= (int)_logLevel;
+            return (int)logLevel >= (int)_minimalLogLevel;
         }
 
         public IDisposable BeginScope<TState>(TState state)
         {
             return NullScope.Instance;
         }
 
-        private void LogToTerminal(string message, LogLevel logLevel)
+        private void LogToTerminal(string message, LogLevel logLevel, bool logToErrorStream)
         {
             var messageColor = LogLevelColorMap[logLevel];
             _terminal.ForegroundColor = messageColor;
-            _terminal.Out.Write($"  {message}{Environment.NewLine}");
+
+            LogToConsole(_terminal, message, logToErrorStream);
+
             _terminal.ResetColor();
         }
 
-        private void LogToConsole(string message)
+        private void LogToConsole(IConsole console, string message, bool logToErrorStream)
         {
-            _console.Out.Write($"  {message}{Environment.NewLine}");
+            if (logToErrorStream)
+            {
+                console.Error.Write($"  {message}{Environment.NewLine}");
+            }
+            else
+            {
+                console.Out.Write($"  {message}{Environment.NewLine}");
+            }
         }
     }
 }

src/Logging/SimpleConsoleLoggerFactoryExtensions.cs

@@ -7,9 +7,9 @@ namespace Microsoft.CodeAnalysis.Tools.Logging
 {
     internal static class SimpleConsoleLoggerFactoryExtensions
     {
-        public static ILoggerFactory AddSimpleConsole(this ILoggerFactory factory, IConsole console, LogLevel logLevel)
+        public static ILoggerFactory AddSimpleConsole(this ILoggerFactory factory, IConsole console, LogLevel minimalLogLevel, LogLevel minimalErrorLevel)
         {
-            factory.AddProvider(new SimpleConsoleLoggerProvider(console, logLevel));
+            factory.AddProvider(new SimpleConsoleLoggerProvider(console, minimalLogLevel, minimalErrorLevel));
             return factory;
         }
     }

src/Logging/SimpleConsoleLoggerProvider.cs

@@ -8,17 +8,19 @@ namespace Microsoft.CodeAnalysis.Tools.Logging
     internal class SimpleConsoleLoggerProvider : ILoggerProvider
     {
         private readonly IConsole _console;
-        private readonly LogLevel _logLevel;
+        private readonly LogLevel _minimalLogLevel;
+        private readonly LogLevel _minimalErrorLevel;
 
-        public SimpleConsoleLoggerProvider(IConsole console, LogLevel logLevel)
+        public SimpleConsoleLoggerProvider(IConsole console, LogLevel minimalLogLevel, LogLevel minimalErrorLevel)
         {
             _console = console;
-            _logLevel = logLevel;
+            _minimalLogLevel = minimalLogLevel;
+            _minimalErrorLevel = minimalErrorLevel;
         }
 
         public ILogger CreateLogger(string name)
         {
-            return new SimpleConsoleLogger(_console, _logLevel);
+            return new SimpleConsoleLogger(_console, _minimalLogLevel, _minimalErrorLevel);
         }
 
         public void Dispose()

src/Program.cs

@@ -62,7 +62,7 @@ private static async Task<int> Main(string[] args)
 
             // Setup logging.
             var logLevel = GetLogLevel(verbosity);
-            var logger = SetupLogging(console, logLevel);
+            var logger = SetupLogging(console, minimalLogLevel: logLevel, minimalErrorLevel: LogLevel.Warning);
 
             // Hook so we can cancel and exit when ctrl+c is pressed.
             var cancellationTokenSource = new CancellationTokenSource();
@@ -288,10 +288,10 @@ internal static DiagnosticSeverity GetSeverity(string? severity)
             };
         }
 
-        private static ILogger<Program> SetupLogging(IConsole console, LogLevel logLevel)
+        private static ILogger<Program> SetupLogging(IConsole console, LogLevel minimalLogLevel, LogLevel minimalErrorLevel)
         {
             var serviceCollection = new ServiceCollection();
-            serviceCollection.AddSingleton(new LoggerFactory().AddSimpleConsole(console, logLevel));
+            serviceCollection.AddSingleton(new LoggerFactory().AddSimpleConsole(console, minimalLogLevel, minimalErrorLevel));
             serviceCollection.AddLogging();
 
             var serviceProvider = serviceCollection.BuildServiceProvider();