Unable to consume C# 7.2 stack-only types in VB 15.5

[Nukepayload2]

If I use stack only type in my VB code, error BC30668 will block compilation. I'm using VS2017 15.6.7.

Steps to reproduce:

1. Create a .NET Core VB Console project.
2. Edit the .vbproj file, add <LangVersion>Latest</LangVersion> in <PropertyGroup>.
3. Create a .NET Standard 2.0 C# class library.
4. Create a new C# code file, and add public ref struct StackOnlyType {} in that .cs file.
5. Upgrade the C# project to 7.2 .
6. In the VB console app, add reference to that C# project.
7. Add Dim errorType As StackOnlyType to Sub Main.
8. Build the whole solution.

Expected behavior:

The code should compile, because I'm not trying to put a stack-only variable on the heap.

Actual behavior:

Error: BC30668 "Types with embedded references are not supported in this version of your compiler"

[Nukepayload2]

They have confirmed that this behavior is by design. I don't understand why they are not intend to let stack-only structures consumable in VB when c# is able to define them. Maybe they thought ref structs are similar to unsafe types. So, in most cases, VB developers may not hurry to consume these types. But this assumption does not make sense, because ref structs are not unsafe codes. They can be defined or be used in C# 7.2 outside of the unsafe context, why can't we just consume them in VB?

Both System.Span(Of T) and System.ReadOnlySpan(Of T) are important new APIs for .NET developers who care about both performance and type safety. We should feel natural when using them, rather than seeing the annoying BC30668 compilation error.

[AdamSpeight2008]

Just throwing out a possible syntax.

Sub M(ReadOnly ByRef parameter As T)
  Dim x As Readonly ByRef T = arg
End Sub

The first Readonly is new Parameter modifier.
The second ReadOnly and ByRef as extending the type signature, akin to nullables.

[Berrysoft]

This code won't cause error:

Module Program
    Sub Main(args As String())
        Dim arr As Integer() = {1, 2, 3, 4, 5}
        Dim span = arr.AsSpan()
        Console.WriteLine(span(1))
    End Sub
End Module

It seems that the compiler only doesn't ignore the ObsoleteAttribute.

[Nukepayload2]

@Berrysoft the VB compiler should verify that all ref structs are not stored on the heap, rather than simply ignore the ObsoleteAttribute. The following code should produce compile-time error, but the current behavior is compilation succeed without any warning.

' Depends on nuget package "System.Memory 4.5.0"
Module Program
    Dim span1 As Object
    Sub Main(args As String())
        Dim arr = New Byte() {1, 2, 3, 4, 5}
        span1 = arr.AsSpan ' No compilation error here. The expected error code is BC31394.
        Console.WriteLine("Hello World!")
    End Sub
End Module

[Berrysoft]

@Nukepayload2 Surely it should, though your code will cause a runtime exception: System.InvalidProgramException:“Cannot create boxed ByRef-like values.” It may be thrown by JIT.

And I find that this code will cause InvalidProgramException too:

Module Program
    Sub Main(args As String())
        Dim rls As RefLikeStruct
        Console.WriteLine(rls.ToString())
    End Sub
End Module

<System.Runtime.CompilerServices.IsByRefLike()>
Structure RefLikeStruct
    Public A As Integer
End Structure

Seems that RefLikeStruct is a real stack-only type!

[gafter]

Aside from the bugs recorded in dotnet/roslyn#28558, this is by design. We'll take this as a feature request to support the use of ref structs in VB.