Linking an empty .NET MAUI app produces inconsistent binaries

[filipnavara]

Repro:

• Create a MAUI app with dotnet new maui
• Build it for MacCatalyst or iOS, eg. dotnet build -f net8.0-maccatalyst
• Check the linked assemblies in obj/Debug/net8.0-maccatalyst/maccatalyst-x64/linked/.

Observations:

• In Microsoft.Maui.Controls.dll the INavigation.PopToRootAsync() method is trimmed from the interface.
• In Microsoft.Maui.Compatibility.dll the Microsoft.Maui.Controls.Compatibility.Platform.iOS.Platform type keeps an explicit implementation of the INavigation.PopToRootAsync() method.

Expectations:

• Either the method is not trimmed from the interface, or all explicit implementations of the method are trimmed too.

[ghost]

Tagging subscribers to this area: @agocke, @sbomer, @vitek-karas
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Repro:

• Create a MAUI app with dotnet new maui
• Build it for MacCatalyst or iOS, eg. dotnet build -f net8.0-maccatalyst
• Check the linked assemblies in obj/Debug/net8.0-maccatalyst/maccatalyst-x64/linked/.

Observations:

• In Microsoft.Maui.Controls.dll the INavigation.PopToRootAsync() method is trimmed from the interface.
• In Microsoft.Maui.Compatibility.dll the Microsoft.Maui.Controls.Compatibility.Platform.iOS.Platform type keeps an explicit implementation of the INavigation.PopToRootAsync() method.

Expectations:

• Either the method is not trimmed from the interface, or all explicit implementation of the method are trimmed too.

Author:	filipnavara
Assignees:	-
Labels:	area-Tools-ILLink
Milestone:	-

[sbomer]

I was able to reproduce the issue with the repro that you shared with @vitek-karas. Here's what's happening:

• The custom step ManagedRegistrarStep sets the assembly action for Microsoft.Maui.Controls.Compatibility.dll to save (it calls AppBundleRewriter.SaveCurrentAssembly). On its own, this might have been enough to repro the issue because the fix Mark .overrides from marked methods in a copy assembly #82197 doesn't take into account save assemblies - we should probably fix that.
• On top of that, MarkStep never marks the Platform implementation of INavigation.PopToRootAsync, because of the custom data DisableMarkingOfCopyAssemblies:

  runtime/src/tools/illink/src/linker/Linker.Steps/MarkStep.cs

  Line 1414 in f73356c

  if (!Context.TryGetCustomData ("DisableMarkingOfCopyAssemblies", out string? disableMarkingOfCopyAssembliesValue) ||

  So even if we fix the above, it doesn't help in this case.

I don't have full context, but it seems like the action is set to save because the registrar is injecting UnmanagedCallersOnly trampoline methods for interop. Maybe we should change the behavior of DisableMarkingOfCopyAssemblies so that it doesn't also skip marking of save assemblies. @vitek-karas thoughts?

[filipnavara]

Thanks for analyzing it!

Before I reported the issue I made sure that it reproduces on stock runtime/linker/workloads. The repro I sent, however, used a new feature introduced in xamarin/xamarin-macios#18268. In that case the ManagedRegistrarStep is an additional step that takes place in the linker pipeline. It does generate new code and as a consequence changes the action from Copy to Save. Unfortunately that opened a small can of worms, where the behavior of the Save action is sometimes unintuitive or broken.

I suppose that on the stock workload the issue is the use of DisableMarkingOfCopyAssemblies you described. I'll try to selectively disable that and check the output.

[filipnavara]

I suppose that on the stock workload the issue is the use of DisableMarkingOfCopyAssemblies you described. I'll try to selectively disable that and check the output.

Removing DisableMarkingOfCopyAssemblies (by setting <MarkCopiedAssemblies>true</MarkCopiedAssemblies> in the .csproj file) seems to have solved the issue even for the case with ManagedRegistrarStep.

I was not able reproduce the issue on stock workloads this time. I'm not sure if I accidentally changed something or if it's a result of Tuesday's updates (unlikely). I'll try again tomorrow.