[UnsafeAccessor] throws MissingFieldException on .NET 9 when used on unspeakable fields

[Sergio0694]

Description

Spotted while porting my ComputeSharp samples to .NET 9 (see here). It seems that [UnsafeAccessor] is broken in .NET 9 when used to access unspeakable fields (captured primary constructor parameters being a common example).

Reproduction Steps

using System.Runtime.CompilerServices;

Blah blah = new("Bob");

string name = Blah.GetName(in blah);

Console.WriteLine(name);

public readonly struct Blah(string name)
{
    public string EnsureCaptured => name;

    [UnsafeAccessor(UnsafeAccessorKind.Field, Name = "<name>P")]
    public static extern ref readonly string GetName(ref readonly Blah value);
}

Expected behavior

Should work as expected.

Actual behavior

https://github.com/Sergio0694/ComputeSharp/pull/871

Regression?

Yes. Works fine on .NET 8.

Known Workarounds

None that I can think of.

Configuration

• .NET SDK 9.0.100

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[Sergio0694]

@AaronRobinsonMSFT it seems the problem is the ref readonly return. If I change it to just ref, then it works on .NET 9 as well. I suppose I can use that as a workaround for now, but shouldn't that work too though? It was working fine on .NET 8, and it feels like it should? 🤔

[AaronRobinsonMSFT]

It is possible this was fixed in https://github.com/dotnet/runtime/pull/109694. Can you check the nightly build?