Automatically validate deserialized structs

[ggutoski]

Serde does not automatically validate deserialized structs. This is annoying because a struct instance that is otherwise guaranteed to be valid by construction can be made invalid via serde round trip. Examples in tofn include GroupPublicInfo and SharePublicInfo.

I spent a long time digging for ergonomic ways to validate serde-deserialized data. 😞 Seems like the best solution is to use deserialize_with as described here serde-rs/serde#1858 (review). Documentation on deserialize_with is sparse---this is all I could find: Field attributes · Serde

If we do go this route then presumably the clean way to do it is for each struct would have a validate() method that is called in two places:

1. at initial construction (eg. in new())
2. in deserialize_with

That's a pretty generic pattern---might even be worth a Validator trait or something. Note that this crate exists https://github.com/Keats/validator but at first glance it doesn't seem to meet our needs.

Not sure yet whether we have a strong need for this fix but I can envision a time when we do.

[tarcieri]

A pattern I'd recommend for this is having a separate "serde type" where you derive Serialize/Deserialize, and then impl TryFrom to convert from the "serde type" to a validated struct. You can then have deserialize_with decode to the "serde type" and then call TryFrom

[ggutoski]

A pattern I'd recommend for this is having a separate "serde type" ...

Thanks for the suggestion! This pattern is described in detail at serde-rs/serde#642 (comment)

[tarcieri]

TIL:

#[serde(try_from = "MyTypeShadow")]