Semantics of $fvis

In introduction.md @nickm writes:

There is a similar similar $fvis that expands to the visibility of the current field.

(Since enums variants are always visible, there is no $vvis.) TODO: Document ${if is_enum { $tvis } else { $fvis }}.

Or maybe change it so $fvis is pub for enums? -nickm

The difficulty is that one must use $fvis when deriving a new type definition. There, $fvis must vanish for enums. So we have two possible semantics, for which we might provide a template keyword:

In enum, expands to	(nothing)	pub
Expansion is	same as written in the driver	synthetic
Suitable for, e.g.	defining a derived type	defining an accessor function
Option 1 (current)	$fvis	${if ...}
Option 1a	$fvis	$f_effective_vis*
Option 2	${if ...}	$fvis
Option 3	$fdefvis*	$f_effective_vis

• names TBD obviously.

I think having the expansion mirror what's written in the driver is fairly important. Perhaps important enough to give the more-usually-useful keyword a less-convenient name. So I think I'm leaning towards Option 1a. But I'm quite unsure.