Improve decoder error type

[dvic]

I wanted to open up a discussion around the error type of the decoder:

/// Represents usual errors that can happen in a nif. This enables you
/// to return an error from anywhere, even places where you don't have
/// an Env available.
pub enum Error {
    /// Returned when the NIF has been called with the wrong number or type of
    /// arguments.
    BadArg,
    /// Encodes the string into an atom and returns it from the NIF.
    Atom(&'static str),
    RaiseAtom(&'static str),
    RaiseTerm(Box<dyn Encoder>),
    /// Encodes an arbitrary Boxed Encoder and returns `{:error, term}` from
    /// the NIF. Very useful for returning descriptive, context-full errors.
    Term(Box<dyn Encoder>),
}

I notice that the decode errors are quite cryptic when working with collections and it can be quite hard to figure out what's going on. For example, using Vec<T> where T is decoded using a derived NifMap implementation produces the following error: ** (ErlangError) Erlang error: "Could not decode field :attributes on %{}". There is no indication what went wrong or in which position this occurred.

Now I can improve the situation by changing the implementation of Decoder for Vec<_> but before I do that: is this something that has been discussed before?

Would it be possible to change the error type of decode to include nested path information without introducing breaking changes? Something like

pub enum Path {
    Root,
    Field(String),
    Index(u8)
}

pub type DecodeError = (Vec<Path>, Box<dyn Encoder>);

[dvic]

Also to add: it's quite difficult to compose errors currently because I believe that Encode is not implemented for Error itself, probably because it contains variants to raise instead of "return" a value.