.NET 6: Trimmer does not follow async flows

[yaakov-h]

This code:

using System.Diagnostics.CodeAnalysis;

namespace Trimmable
{
    public class DoSomething
    {
        public static async Task WithAsync<[DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.All)] T>()
        {
            await Task.Delay(100);
            With<T>();
        }

        public static void With<[DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.All)] T>()
        {
        }
    }
}

looks like it should be safe, as both methods share the same annotation and one simply calls the other, yet it yields this warning:

Trim analysis warning IL2091: Trimmable.DoSomething.d__0.MoveNext(): 'T' generic argument does not satisfy 'DynamicallyAccessedMemberTypes.All' in 'Trimmable.DoSomething.With()'. The generic parameter 'T' of 'Trimmable.DoSomething.d__0' does not have matching annotations. The source value must declare at least the same requirements as those declared on the target location it is assigned to.

I am assuming the linker got lost in the compiler-generated async state machine code.

[marek-safar]

/cc @sbomer @vitek-karas

[vitek-karas]

This is a known limitation in .NET 6. We implemented only handling of RequiresUnreferencedCodeAttribute and warning suppression for async state machines. The data flow for annotations (DynamicallyAccessedMembersAttribute and friends) was not implemented in .NET 6.

Master issue: #2001

Document describing the desired behavior and some solutions is here: https://github.com/dotnet/linker/blob/main/docs/design/compiler-generated-code-handling.md

[sbomer]

Closing because we don't plan to fix this for the .NET 6 illink. Please use a later version of .NET to get the fixes from #2001.