[dotnet] Work around an expectation mismatch between ILLink and library projects. Fixes #19037. (#19049)

ILLink doesn't handle library projects the way we need: the library is
automatically treated as a root assembly, with the entry point as the root.
This doesn't work, because library projects don't have an entry point.

In earlier versions of .NET (and Xamarin), we'd solved this by a custom linker
step that would manually root everything that needed to be rooted, but that
doesn't work anymore because we hit this sanity check in ILLink:

> ILLink : error IL1034: Root assembly 'MyExtension, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' does not have entry point.

Technically this happens because the library project is configured as a root
assembly, where the entry point is the root point (and in that regards the
sanity check is rather sane).

The best solution would be if we could just treat the library assembly as any
other assembly, and manually root it in our custom linker steps - but the
custom linker step we have for this kind of rooting will only iterate over
types in assemblies that are already marked somehow, and that's not
necessarily the case for app extension projects - the end result is that the
entire app extension assembly is linked away.

A close runner-up as the second best solution is to provide the API that needs
to be rooted as an xml file to the linker. This works, but we currently don't
have the infrastructure in the code to generate this xml file before running
the linker (it would be a rather major undertaking). This work is tentatively
scheduled for .NET 9 (#17693).

So I went for the third option: set RootMode="Library" for the library
assembly. I'm not sure exactly what that means ILLink will mark, but as long
as _anything_ is marked, our custom linker step will run. This seems like an
acceptable workaround until we can implement the previous solution.

Fixes #19037.

Author: rolfbjarne

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -1164,6 +1164,15 @@
 		</ItemGroup>
 	</Target>
 
+	<!-- https://github.com/xamarin/xamarin-macios/issues/19037 -->
+	<!-- The trimmer will set the RootMode for the current assembly to 'EntryPoint', but that doesn't work for app extension projects, because those are library projects that don't have entry points.
+		 So here we set RootMode=Library for these cases. -->
+	<Target Name="_FixRootAssemblyForAppExtensions" AfterTargets="PrepareForILLink" Condition="'$(IsAppExtension)' == 'true'">
+		<ItemGroup>
+			<TrimmerRootAssembly Update="@(TrimmerRootAssembly)" Condition=" '%(TrimmerRootAssembly.RootMode)' == 'EntryPoint' " RootMode="Library" />
+		</ItemGroup>
+	</Target>
+
 	<Target Name="_CreateAOTDedupAssembly"
 			Condition="'$(_RunAotCompiler)' == 'true' And '$(IsMacEnabled)' == 'true'"
 			DependsOnTargets="_ComputeManagedAssemblyToLink"

tests/dotnet/UnitTests/ProjectTest.cs

@@ -1009,6 +1009,24 @@ public void BuildProjectsWithExtensionsAndFrameworks (ApplePlatform platform)
 			ExecuteWithMagicWordAndAssert (platform, runtimeIdentifiers, appExecutable);
 		}
 
+
+		[TestCase (ApplePlatform.iOS)]
+		[TestCase (ApplePlatform.TVOS)]
+		[TestCase (ApplePlatform.MacOSX)]
+		// [TestCase ("MacCatalyst", "")] - No extension support yet
+		public void BuildTrimmedExtensionProject (ApplePlatform platform)
+		{
+			Configuration.IgnoreIfIgnoredPlatform (platform);
+			var project_path = GetProjectPath ("ExtensionProject", platform: platform);
+
+			Clean (project_path);
+
+			var properties = GetDefaultProperties ();
+			properties ["MtouchLink"] = "Full";
+			properties ["LinkMode"] = "Full";
+			DotNet.AssertBuild (project_path, properties);
+		}
+
 		[TestCase (ApplePlatform.iOS, "iossimulator-x64;iossimulator-arm64")]
 		[TestCase (ApplePlatform.TVOS, "tvossimulator-x64")]
 		[TestCase (ApplePlatform.MacCatalyst, "maccatalyst-x64")]