Keep ref struct interfaces behind preview flag

[jaredpar]

Keep ref struct interfaces behind preview flag

[hamarb123]

I have some code like this in some scenarios that require very low overhead to be useful:

interface IAction
{
    void Run(/*extra parameters here for some variants of it*/);
}

And I was hoping to update it to allow ref struct IActions so that I didn't have to do the pointer trick in these cases. (Note that I pass the generic TAction by reference to allow state changes to it when it runs)

I will post more use cases when I have a chance to go through more of the code where I ran into similar issues as I find or think of them.

[AlekseyTs]

@hamarb123 Note, the plan is to keep ability for ref structs to implement interface behind a preview flag, the allows ref struct constraint on type parameters is not affected and will be fully supported.

[jaredpar]

Related .NET issues

• dotnet/runtime#103270
• dotnet/runtime#103605
• dotnet/runtime#103604

Related Customer Issues

• Need UnscopefRef on interfaces
• csharplang discussion on the topic
• https://github.com/AssetRipper/AssetRipper.Text.Html

[tannergooding]

While ref structs implementing interfaces isn't something terribly important to me for .NET 9/C# 13, there is the slightly related feature of being able to mark interface members with [UnscopedRef] that is important and makes a difference in how the .NET Libraries team will ship ITensor<T>

We want to expose ROSpan<nint> Lengths { get; } which for structs (regular -or- ref struct) requires it be annotated as [UnscopedRef] so that you can return an InlineArray or equivalent to allow allocation free tracking of the multidimensional lengths buffer.