serde Error traits change super trait bounds based on cargo features

[KodrAus]

Hi! :wave:

The ser::Error and de::Error traits conditionally require a std::error::Error bound when serde is compiled with the std feature (enabled by default).

This can be problematic for crates offering Serializers or Deserializers that conditionally work in no-std environments, because they can't tell internally whether serde is pulled in with its std feature or not (this can also happen when including a library like serde-test as a dev dependency).

In a future with a unified std this might just be a non-issue, but is a bit of a head-scratcher at the moment. Some work-arounds I've thought of that might help crates in this position are:

---

Offer some newtype structs in serde that always implement the appropriate error trait, but require a consistent bound from the type they wrap:

pub struct ErrorMsg<E>(E);

impl<E> ser::Error for ErrorMsg<E>
where
    E: fmt::Display,
{
    ..
}

That internal bound could be something like fmt::Display, or some serde::ser::IntoError trait.

---

Offer a way to derive ser::Error or de::Error on caller structs that also handles std::error::Error if it's needed.

---

Offer a macro in serde like serde_if_std, so that crates could produce a nicer user-facing error message if their expectations of the error bounds don't align with serde's.

Note this one could be enough for an external crate to look at building derive support.

---

What do you think?

[dtolnay]

I believe this is fixed with the reexports in 1.0.100:
https://github.com/serde-rs/serde/releases/tag/v1.0.100

Data formats can provide the right Error impl whether or not Serde is built with "std" by writing:

impl serde::ser::StdError for MySerError {}

impl serde::de::StdError for MyDeError {}