[dotnet] Make sure to not run the linker when we're on a disconnected Windows build. (#15076)

This also means that we shouldn't load the linker's output. Note that we need
to check _LoadLinkerOutput even if we've already disabled the linker, because
there may be linker output from a previous (connected) build, and we don't
want to load that.

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1542438.

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -267,6 +267,11 @@
 			<PublishTrimmed Condition="'$(PublishTrimmed)' == '' And '$(_MustTrim)' == 'true'">true</PublishTrimmed>
 		</PropertyGroup>
 		<Error Condition="'$(_MustTrim)' == 'true' And '$(PublishTrimmed)' != 'true'" Text="$(_PlatformName) projects must build with PublishTrimmed=true. Current value: $(PublishTrimmed)." />
+		<PropertyGroup Condition="'$(PublishTrimmed)' != '' And '$(IsMacEnabled)' != 'true'">
+			<_PreviousPublishTrimmedValue>$(PublishTrimmed)</_PreviousPublishTrimmedValue>
+			<PublishTrimmed />
+		</PropertyGroup>
+		<Warning Condition="'$(_PreviousPublishTrimmedValue)' != ''" Text="The linker has been disabled because there's no connection to a Mac." />
 	</Target>
 
 	<Target Name="_WarnRuntimeIdentifiersClash" Condition="'$(_RuntimeIdentifiersClashMessage)' != ''">
@@ -756,7 +761,7 @@
 	</Target>
 
 
-	<Target Name="_LoadLinkerOutput" DependsOnTargets="ComputeFilesToPublish">
+	<Target Name="_LoadLinkerOutput" DependsOnTargets="ComputeFilesToPublish" Condition="'$(IsMacEnabled)' == 'true'">
 		<!-- Load _MainFile -->
 		<ReadItemsFromFile SessionId="$(BuildSessionId)" File="$(_LinkerItemsDirectory)/_MainFile.items" Condition="Exists('$(_LinkerItemsDirectory)/_MainFile.items')">
 			<Output TaskParameter="Items" ItemName="_MainFile" />

tests/dotnet/UnitTests/ProjectTest.cs

@@ -418,7 +418,8 @@ namespace Xamarin.Tests {
 		[Test]
 		[TestCase ("iossimulator-x64", false)]
 		[TestCase ("ios-arm64", true)]
-		public void IsNotMacBuild (string runtimeIdentifier, bool isDeviceBuild)
+		[TestCase ("ios-arm64", true, "PublishTrimmed=true;UseInterpreter=true")]
+		public void IsNotMacBuild (string runtimeIdentifier, bool isDeviceBuild, string extraProperties = null)
 		{
 			if (isDeviceBuild)
 				Configuration.AssertDeviceAvailable ();
@@ -429,6 +430,12 @@ namespace Xamarin.Tests {
 			Clean (project_path);
 			var properties = GetDefaultProperties (runtimeIdentifier);
 			properties ["IsMacEnabled"] = "false";
+			if (extraProperties is not null) {
+				foreach (var assignment in extraProperties.Split (';')) {
+					var split = assignment.Split ('=');
+					properties [split [0]] = split [1];
+				}
+			}
 			var result = DotNet.AssertBuild (project_path, properties);
 			AssertThatLinkerDidNotExecute (result);
 			var appExecutable = Path.Combine (appPath, Path.GetFileName (project_path));