Serialization errors can only be created from the `Encoder`/`Decoder` methods

[huonw]

There is no way for a custom generic Decodable/Encodable instance (compatible with #[deriving] other) to return a new error since it's completely generic, that is,

#[deriving(Decodable)]
struct Foo {
    x: Bar
}

struct Bar { y: int }

// E has to be that generic to work with the deriving above,
// but this means this impl cannot manually create new errors
// (e.g. if y is supposed to be an even number, there's no way
// to emit "error: found odd number".)
impl<E, D: Decoder<E>> Decodable<D, E> for Bar { ... }

It might be nice if there was a E: Error trait (or some such) that deriving used, which has some basic constructors.

[huonw]

This might be somewhat addressable by having an method like fn error(&self, msg: &str) -> E on Decoder/Encoder (allowing a decoder to e.g. insert line/column information), although it may need to be more detailed.

[huonw]

cc @erickt

[aturon]

cc rust-lang/rfcs#201