Figure out something better than an `_extra` struct member

[iliana]

tough::schema struct types have an _extra member:

tough/tough/src/schema/mod.rs

Lines 83 to 95 in 533d8db

	#[derive(Debug, Clone, Deserialize, Serialize, PartialEq)]
	pub struct RoleKeys {
	pub keyids: Vec<Decoded<Hex>>,
	pub threshold: NonZeroU64,
	/// Extra arguments found during deserialization.
	///
	/// We must store these to correctly verify signatures for this object.
	///
	/// If you're instantiating this struct, you should make this `HashMap::empty()`.
	#[serde(flatten)]
	pub _extra: HashMap<String, Value>,
	}

This is to handle keys that tough doesn't know about (yet), while still correctly validating signatures during serialization.

It'd be nice if consumers can create these structs without having to specify _extra: HashMap::new() or ..Default::default(). Not sure if there's any type magick we can perform.

[iliana]

Had a sudden idea here:

#[derive(Deserialize, Serialize)]
struct Verbatim<T> {
    #[serde(flatten)]
    pub inner: T,
    #[serde(flatten)]
    pub extra: HashMap<String, Value>,
}

impl<T> From<T> for Verbatim<T> {
    ...
}

No idea if it's possible to have two items with #[serde(flatten)] but this feels pretty ideal to me for a lot of reasons.

[iliana]

Apparently the Verbatim<T> struct idea works. https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=05c006e27cd6f4a577c7fca408477c4a

serde_derive frightens me.

[ecpullen]

Apparently the Verbatim<T> struct idea works. https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=05c006e27cd6f4a577c7fca408477c4a

serde_derive frightens me.

Working towards implementing this feature.