[Compatibility issue] VB compiler generates tons of BC36954 errors when I upgrade .NET 6 projects to .NET 7

Version Used: 4.4.0-3.22518.13+7856a68c109b22de70e07580018fec601e944f46

Steps to Reproduce:

The following code reports BC36954 error when I upgrade a .NET 6 project to .NET 7.

Dim s = "123".AsSpan()

https://sharplab.io/#v2:EYLgxg9gTgpgtADwGwBYA0AbEA3YaAuIUArgHZoAmIA1AD4CSAtgA7T4DOABAMoCe7+GIwCwAKAAKxYBgCWYTgFkIFYhhidxUCAHMoAQxGjOxjVNnzuUxXpmkAFAEoxJl5wAiMxpy4BeTgCIARgAmAGZ/ADoAQXZuZj17JyMTAFFSCh4pMTSMpRU1IA=

Out commercial .NET 6 VB projects have a lot of “Ref Struct” consuming code like this, because many 3rd-party dependencies require it.

The migration to other languages is unacceptable. If we can’t migrate our .NET 6 projects to newer .NET versions smoothly, we have no choice but stop upgrading to newer .NET versions and begin porting these incompatible products to non-.NET platforms.

Diagnostic Id:

BC36954

Expected Behavior: No compilation errors.

Actual Behavior:

error BC36954: Overload resolution failed because no accessible 'AsSpan' can be called:
    Extension method 'Public Function AsSpan() As ReadOnlySpan(Of Char)' defined in 'MemoryExtensions': 'ReadOnlySpan(Of T)' requires compiler feature 'RefStructs', which is not supported by this version of the Visual Basic compiler.
    Extension method 'Public Function AsSpan(start As Integer) As ReadOnlySpan(Of Char)' defined in 'MemoryExtensions': 'ReadOnlySpan(Of T)' requires compiler feature 'RefStructs', which is not supported by this version of the Visual Basic compiler.
    Extension method 'Public Function AsSpan(start As Integer, length As Integer) As ReadOnlySpan(Of Char)' defined in 'MemoryExtensions': 'ReadOnlySpan(Of T)' requires compiler feature 'RefStructs', which is not supported by this version of the Visual Basic compiler.

[jcouv]

@Nukepayload2 From discussion with the team, this API pattern (passing ref structs through) was meant to be supported. I’ll get started to fix this regression. Sorry about that.

[jcouv]

@Nukepayload2 These Span APIs should have been inaccessible from VB in .NET 6 already. A mechanism was in place (attribute [Obsolete("Types with embedded references are not supported in this version of your compiler.")] emitted on ref structs) to prevent compilers without ref sruct support from using types they didn’t understand/support.

This mechanism was imperfect as the following example shows (compiled against .NET 6 TFM):

In .NET 7, this mechanism was strengthened, resulting in the message you’re seing: “… requires compiler feature ‘RefStructs’, which is not supported by this version of the Visual Basic compiler.”.

Tagging @333fred @jaredpar @AlekseyTs in case anything to add or if they can think of a possible mitigation.