MSB4096 with user-defined TrimmerRootAssembly

[sbomer]

Summary

The _FixRootAssembly target in Microsoft.Android.Sdk.ILLink.targets uses %(RootMode) without fully qualifying it as %(TrimmerRootAssembly.RootMode). This causes MSB4096 when user-defined TrimmerRootAssembly items don't have the RootMode metadata.

Repro

Create a MAUI app with TrimmerRootAssembly items that don't specify RootMode:

<ItemGroup>
  <TrimmerRootAssembly Include="Microsoft.Maui" />
  <TrimmerRootAssembly Include="Microsoft.Maui.Controls" />
</ItemGroup>

Error

error MSB4096: The item "Microsoft.Maui" in item list "TrimmerRootAssembly" does not define a value for metadata "RootMode". In order to use this metadata, either qualify it by specifying %(TrimmerRootAssembly.RootMode), or ensure that all items in this list define a value for this metadata.

In Microsoft.Android.Sdk.ILLink.targets:

<Target Name="_FixRootAssembly" AfterTargets="PrepareForILLink">
  <ItemGroup>
    <TrimmerRootAssembly Update="@(TrimmerRootAssembly)" Condition=" '%(RootMode)' == 'EntryPoint' " RootMode="All" />
  </ItemGroup>
</Target>

The condition uses %(RootMode) which triggers MSBuild item batching. When batching, MSBuild requires all items to have the metadata defined.

Related: dotnet/runtime#123972 (comment)
Discovered in dotnet/maui#33756