Add support for the new System.Runtime.InteropServices.ExtendedLayoutAttribute

[jkoritzinsky]

The interop team has decided to add a new mechanism to specify type layouts. In particular, the team has decided to use the last remaining value in the "Layout Kind" mask of a type def's attributes to represent "extended layout". When this value it set, the runtime will look for the System.Runtime.InteropServices.ExtendedLayoutAttribute to provide all layout information.

As part of the design process, we noticed that Roslyn blocks emitting a type with the last remaining value of the System.Runtime.InteropServices.LayoutKind enumeration in the StructLayout pseudo-attribute as the compiler can't map it to a valid TypeAttributes value. I do not know if F# does the same today.

The System.Runtime.InteropServices.ExtendedLayoutAttribute type was approved in dotnet/runtime#100896 and has the first part of runtime implementation (the TypeAttributes bit and the LayoutKind value) in dotnet/runtime#116082.

The support in F# should be either:

• Allow the user to specify the ExtendedLayoutAttribute on a type. When they do so, the correct layout bit will be set. The user cannot manually specify the new "extended layout" value. The only way in F# to specify that value is to put this attribute on the type.
  • Disallow the FieldOffset attribute for extended layout types.
• Block user usage of ExtendedLayoutAttribute.

There is a PR to add support to Roslyn for C# and VB.NET at dotnet/roslyn#78741

[T-Gro]

@jkoritzinsky :

Could you please help with crafting prompts for testing this properly?
Also w.r.t to what it should be doing at runtime in an interop-relevant scenario.

[jkoritzinsky]

We've only implemented CStruct so far in .NET 11, which has identical behavior to LayoutKind.Sequential.

It looks like you've got a good set of tests already in the PR. Left one comment.