Derive RustcDecodable not working when rustc-serialize crate is imported inside a module about rustc-serialize

[gbersac]

I think it is best practice to move all the extern crate statement in the lib.rs/main.rs file. That's what I always do.

[moll]

Is the crate root requirement a limitation of derive and worked on somewhere? I don't particularly think coupling submodules with the crate root this way is a good practice.

[rking788]

I should have checked the github issues first. Thanks for the feedback @gbersac.

[darxriggs]

I am new to Rust and this issue bit me within minutes and I took me some time to find it here.

So the question is, can this be fixed in the crate itself or is this a fundamental issue?

The current usage statement didn't click for me:

Add this to your crate root: extern crate rustc_serialize;

So in the meantime it would be a good idea to update the usage section in README.md to mention main.rs and lib.rs explicitly. And to state if used anywhere else this issue applies.

[dtolnay]

This is not a limitation of derive. In particular, Serde does not have this problem.

The code generated by RustcEncodable and RustcDecodable looks like this:

impl ::rustc_serialize::Encodable for Test { /* ... */ }
impl ::rustc_serialize::Decodable for Test { /* ... */ }

which requires rustc_serialize to be in the crate root.

In Serde we avoid this problem by generating the following code instead:

const IMPL_SERIALIZE_FOR_Test: () = {
    extern crate serde as _serde;
    impl _serde::Serialize for Test { /* ... */ }
};
const IMPL_DESERIALIZE_FOR_Test: () = {
    extern crate serde as _serde;
    impl _serde::Deserialize for Test { /* ... */ }
};

This works whether or not serde is in the crate root, in fact whether or not you even import serde anywhere yourself.

[alexcrichton]

I'm going to close this now that this crate is deprecated in favor of serde. We're discontinuing feature development in rustc-serialize but will still continue to merge bug fixes if they arise.