Marking the application's assembly as trimmable passes completely wrong input to linker

[vitek-karas]

Create a console app named "TrimApp"
Modify .csproj like this:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net5.0</TargetFramework>
  </PropertyGroup>

  <Target Name="ConfigureTrimming"
          BeforeTargets="PrepareForILLink">
    <ItemGroup>
      <ManagedAssemblyToLink Condition="'%(Filename)' == 'TrimApp'">
        <IsTrimmable>true</IsTrimmable>
      </ManagedAssemblyToLink>
    </ItemGroup>
  </Target>

</Project>

Publishing this app with:

dotnet publish -r win-x64 /p:PublishTrimmed=true /p:TrimMode=link

Actually fails:

  TrimApp -> F:\ILLinker\repro\TrimApp\bin\Debug\net5.0\win-x64\TrimApp.dll
ILLink : error IL1020: No files to link were specified. Use one of '-a|-r|-x' options [F:\ILLinker\repro\TrimApp\TrimApp.csproj]

If I add a project reference to a simple classlib, then linker will actually not fail, but will be given command line where the classlib is treated as the "application assembly" - which leads to deleting the actual app's assembly from the output - producing completely broken app (without failing the build).

[eerhardt]

In the runtime tests, we work around this problem with the following:

https://github.com/dotnet/runtime/blob/1beb1c635a2961d96d56d52025ee0fa7322933bb/eng/testing/linker/SupportFiles/Directory.Build.targets#L30

<TrimmerRootAssembly Include="@(IntermediateAssembly)" />

[sbomer]

A short-term fix is just to add IntermediateAssembly to TrimmerRootAssembly, but that won't do the right thing for IsTrimmable libraries (it would keep everything in the library). I actually think that TrimmerRootAssembly is a bit broken, and the longer-term fix is to clean up the linker options for rooting assemblies - I filed dotnet/linker#1541 for this.

Long-term, I think the SDK behavior should be:

• !IsTrimmable assemblies -> root entire assembly (entry point or not)
• IsTrimmable IntermediateAssembly -> root entry point or public surface (for app or library, respectively)
• other IsTrimmable -> don't root

[sbomer]

I believe this was fixed a while ago by #16094.