LDM Notes for May 13th, 2024

[333fred]

https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-05-13.md

Agenda

• First-class span types questions
  • Delegate conversions
  • Variant conversion existence
• Overload resolution priority questions
  • Attribute shape and inheritance
  • Extension overload resolution

[hamarb123]

We could generate a <PrivateImplementationDetails> method for converting the span. On platforms with built-in ReadOnlySpan<T> support, and in particular .NET Core 2.2 and later, this seems like it would work fine. However, we're concerned about other platforms; users can bring their own ReadOnlySpan<T> implementations, and there's no guarantee that whatever bit-blit strategy the compiler choses would be safe for that implementation. We also aren't prepared to make strong statements on whether this would always be safe on the .NET Framework implementation of ReadOnlySpan<T>.

Just a note (for people who aren't aware) that bit-blit is not required for .NET Core 2.1+ and .NET Standard 2.1 (specifically, whenever MemoryMarshal.CreateReadOnlySpan is available).
The following could be used, which is the same implementation as in the runtime effectively (but using MemoryMarshal instead of the internal APIs, since that's how they're exposed publicly):

class <PrivateImplementationDetails>
{
    public static ReadOnlySpan<T> CastUp<TDerived>(ReadOnlySpan<TDerived> items) where TDerived : class?, T
    {
        return MemoryMarshal.CreateReadOnlySpan(ref Unsafe.As<TDerived, T>(ref MemoryMarshal.GetReference(items)), items.Length);
    }
}

Runtime implementation:
https://github.com/dotnet/runtime/blob/main/src/libraries/System.Private.CoreLib/src/System/ReadOnlySpan.cs#L226-L229

[CyrusNajmabadi]

@stephentoub ?

[333fred]

Yes, that's correct, it's what Stephen said during the meeting, and is in fact what I meant when I said it would be fine on .NET Core 2.2 and higher. It's the other platforms where we are concerned 🙂.