feat(x509): add PkiPath type

[npmccallum]

No description provided.

[tarcieri]

Seems like this could use some sort of accessor for the inner Certificates, such as AsRef<[Certificate<'a>]> and/or something that exposes an Iterator<Item=&Certificate<'a>>

[npmccallum]

@tarcieri Like #[derive(Newtype)]? :)

[npmccallum]

https://github.com/RustCrypto/formats/blob/master/der/derive/src/newtype.rs#L78-L92

[tarcieri]

Oh right, I forgot it was adding Deref/DerefMut impls.

Thinking about that again, doing that unilaterally somewhat defeats the point of having a newtype, since it effectively leaks the inner type and precludes adding your own inherent methods (at least if you want it to be an idiomatic use of Deref).

But that's something we can circle back on another day, I suppose.

[npmccallum]

The intent of newtype is to expose the inner type, but allow the outer type to pick up implementations of traits relevant to specific contexts. Of note here is this new type: #479

This is particularly useful where an identical inner type is used in multiple contexts. A primary example of this is Vec<Extension> which has different OIDs in different contexts. The outer type determines which Identifiable instance to give to the inner type.

[tarcieri]

The intent of newtype is to expose the inner type

This description is antithetical to the typical notion of "newtype", which is to encapsulate the inner type

[npmccallum]

@tarcieri I don't think the invariants of "newtype" are that well defined. They are commonly used to allow the implementation of a trait from one crate on the type from a second crate within a third crate, which is roughly how we're using them here.

[tarcieri]

Implicit deref coercion has somewhat magical and often surprising powers which allow you to transparently treat one type as another, which go well beyond what's typically meant by "newtype". It means you can't encapsulate the inner type, and it's a mandatory part of the public API.

A very common usage of the newtype pattern is to encapsulate another type to keep it out of the public API, so you can upgrade the dependency that provides the inner type without it resulting in a breaking change to the public API of the consumer providing the outer newtype.