How is field visibility/unsafe fields handled?

[ComputerDruid]

The demo for facet shows using facet_poke to build a struct with reflection. How would that work with private fields, especially ones which are intended to be "unsafe fields" (fields with safety invariants)?

I poked around the rustdocs but didn't find anything mentioning field visibility at all.

I also tried to look to see how that was handled for Vec.len, but Vec is handled as a list instead of a struct, so it gets special treatment there.

[fasterthanlime]

The crates haven't been published in a day or so — there's an invariants attribute for the derive macro — I encourage you to look at the sources for now.

Some structs are almost POD and it makes sense for them to have an invariant checking function like this, some are really special and they should be treated as scalars, because they're very opaque.

[ComputerDruid]

I did some reading through it, and I think I get invariants now; basically a -> bool function deciding whether to allow construction.

I think that's excellent for partial-POD types, but probably not very useful for unsafe fields (like Vec.len), since I think usually it's ~impossible to check correctness at runtime. Unless you can handle it like Vec and construct it from vtable methods. It doesn't look like there's a shape that lets you do that for struct-like things, but I can totally see how there could be.

You could certainly disable construction by having an invariants function that just returns false, but I don't yet understand the tradeoffs between that and treating it like a scalar. If it was common to allow readonly reflection like that, you might end up splitting out a marker trait from Facet to be able to check at compile time whether something allows construction or not.

Thanks for the pointers, I feel like I'm starting to get it now. I think eventually you'll want to add something to the README explaining some of the nuances of "what kinds of types facet is for", which would probably have helped it click for me faster, but you may want to hold off on that until we know more about what kinds of types facet is for!