Allow a ref field to refer to a ref struct type

[cston]

Currently, ref fields cannot refer to ref struct types.

ref struct R1<T>
{
}

ref struct R2<T>
{
    public ref R1<T> F; // error CS9050: A ref field cannot refer to a ref struct.
}

See #62186

See also #62098

[SK-Genius]

And Allow it recursive:

ref struct ParentIds
{
    int Id;
    readonly ref ParentIds Parent;
} 

without

error CS9050: A ref field cannot refer to a ref struct

and

error CS0523: Struct member 'ParentIds.Parent' of type 'ParentIds' cause a cycle in the struct layout

[SK-Genius]

Maybe also introduce something like

ref struct ReadOnlyRefStack<T>
{
    T Item;
    readonly ref readonly ReadOnlyRefStack<T> Prev;

    //[...]
}

and

static string Join(ReadOnlySpan<char> seperator, params ReadOnlyRefStack<string> values) => // ...

[jaredpar]

Closing as this is less a compiler issue and more a language design issue.