[tools] Ignore a few warnings by default. Fixes #20670.

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -613,6 +613,7 @@
 				TargetFramework=$(_ComputedTargetFrameworkMoniker)
 				UseLlvm=$(MtouchUseLlvm)
 				Verbosity=$(_BundlerVerbosity)
+				Warn=$(_BundlerWarn)
 				WarnAsError=$(_BundlerWarnAsError)
 				XamarinNativeLibraryDirectory=$(_XamarinNativeLibraryDirectory)
 				XamarinRuntime=$(_XamarinRuntime)

msbuild/Xamarin.MacDev.Tasks/Tasks/ParseBundlerArguments.cs

@@ -67,6 +67,9 @@ public class ParseBundlerArguments : XamarinTask {
 		[Output]
 		public int Verbosity { get; set; }
 
+		[Output]
+		public string Warn { get; set; }
+
 		[Output]
 		public string WarnAsError { get; set; }
 
@@ -233,6 +236,15 @@ public override bool Execute ()
 							NoWarn += "," + value;
 						}
 						break;
+					case "warn":
+						if (!hasValue)
+							value = "-1"; // all warnings
+						if (string.IsNullOrEmpty (Warn)) {
+							Warn = value;
+						} else {
+							Warn += "," + value;
+						}
+						break;
 					default:
 						// Handle arguments like -vvv and -qqqq
 						if (value.Length == 0 && name.Length > 1 && name.All (ch => ch == name [0])) {

msbuild/Xamarin.Shared/Xamarin.Shared.targets

@@ -2039,6 +2039,7 @@ Copyright (C) 2018 Microsoft. All rights reserved.
 			<Output TaskParameter="SkipMarkingNSObjectsInUserAssemblies" PropertyName="_SkipMarkingNSObjectsInUserAssemblies" />
 			<Output TaskParameter="Verbosity" PropertyName="_BundlerVerbosity" />
 			<Output TaskParameter="WarnAsError" PropertyName="_BundlerWarnAsError" />
+			<Output TaskParameter="Warn" PropertyName="_BundlerWarn" />
 			<Output TaskParameter="XmlDefinitions" ItemName="_BundlerXmlDefinitions" />
 			<Output TaskParameter="NoStrip" PropertyName="EnableAssemblyILStripping" />
 		</ParseBundlerArguments>

tools/common/Application.cs

@@ -1797,5 +1797,18 @@ public bool VerifyDynamicFramework (string framework_path)
 
 			return dynamic;
 		}
+
+		static Application ()
+		{
+			SetDefaultHiddenWarnings ();
+		}
+
+		public static void SetDefaultHiddenWarnings ()
+		{
+			// People don't like these warnings (#20670), and they also complicate our tests, so ignore them.
+			ErrorHelper.ParseWarningLevel (ErrorHelper.WarningLevel.Disable, "4178"); // The class '{0}' will not be registered because the {1} framework has been removed from the {2} SDK.
+			ErrorHelper.ParseWarningLevel (ErrorHelper.WarningLevel.Disable, "4189"); // The class '{0}' will not be registered because it has been removed from the {1} SDK.
+			ErrorHelper.ParseWarningLevel (ErrorHelper.WarningLevel.Disable, "4190"); // The class '{0}' will not be registered because the {1} framework has been deprecated from the {2} SDK.
+		}
 	}
 }

tools/common/Driver.cs

@@ -129,6 +129,13 @@ static bool ParseOptions (Application app, Mono.Options.OptionSet options, strin
 					throw ErrorHelper.CreateError (26, ex, Errors.MX0026, "--nowarn", ex.Message);
 				}
 			});
+			options.Add ("warn:", "An optional comma-separated list of warning codes to report as warnings (if no warnings are specified all warnings reported).", v => {
+				try {
+					ErrorHelper.ParseWarningLevel (ErrorHelper.WarningLevel.Warning, v);
+				} catch (Exception ex) {
+					throw ErrorHelper.CreateError (26, ex, Errors.MX0026, "--warn", ex.Message);
+				}
+			});
 			options.Add ("coop:", "If the GC should run in cooperative mode.", v => { app.EnableCoopGC = ParseBool (v, "coop"); }, hidden: true);
 			options.Add ("sgen-conc", "Enable the *experimental* concurrent garbage collector.", v => { app.EnableSGenConc = true; });
 			options.Add ("marshal-objectivec-exceptions:", "Specify how Objective-C exceptions should be marshalled. Valid values: default, unwindmanagedcode, throwmanagedexception, abort and disable. The default depends on the target platform (on watchOS the default is 'throwmanagedexception', while on all other platforms it's 'disable').", v => {

tools/dotnet-linker/LinkerConfiguration.cs

@@ -351,6 +351,13 @@ public static LinkerConfiguration GetInstance (LinkContext context)
 						throw new InvalidOperationException ($"Invalid Verbosity '{value}' in {linker_file}");
 					Driver.Verbosity += verbosity;
 					break;
+				case "Warn":
+					try {
+						ErrorHelper.ParseWarningLevel (ErrorHelper.WarningLevel.Warning, value);
+					} catch (Exception ex) {
+						throw new InvalidOperationException ($"Invalid Warn '{value}' in {linker_file}", ex);
+					}
+					break;
 				case "WarnAsError":
 					try {
 						ErrorHelper.ParseWarningLevel (ErrorHelper.WarningLevel.Error, value);