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

[mhristache]

When importing rustc-serialize inside a module, [derive(RustcDecodable, RustcEncodable)] fails.

The code inside the module file:

extern crate "rustc-serialize" as rustc_serialize;
use self::rustc_serialize::json::{self, ToJson, Json, DecoderError};

#[derive(RustcDecodable, RustcEncodable)]
struct Test {
    test: String
    }

fn test(s: String) -> Test {
    Test {test: s}
    }

When compiling I get the following errors:

$ cargo build
   Compiling serialize_issue v0.0.1 (file://rust-playground/serialize_issue)
src/mod1/mod.rs:4:26: 4:41 error: failed to resolve. Did you mean     `self::rustc_serialize`?
src/mod1/mod.rs:4 #[derive(RustcDecodable, RustcEncodable)]
                                           ^~~~~~~~~~~~~~~
note: in expansion of #[derive]
src/mod1/mod.rs:4:1: 4:42 note: expansion site
src/mod1/mod.rs:4:26: 4:40 error: attempt to implement a nonexistent trait    `rustc_serialize::Encodable`
src/mod1/mod.rs:4 #[derive(RustcDecodable, RustcEncodable)]
                                           ^~~~~~~~~~~~~~
 note: in expansion of #[derive]
 src/mod1/mod.rs:4:1: 4:42 note: expansion site
 src/mod1/mod.rs:4:26: 4:41 error: failed to resolve. Did you mean     `self::rustc_serialize`?
 src/mod1/mod.rs:4 #[derive(RustcDecodable, RustcEncodable)]
                                           ^~~~~~~~~~~~~~~
 note: in expansion of #[derive]
 src/mod1/mod.rs:4:1: 4:42 note: expansion site
 src/mod1/mod.rs:4:26: 4:40 error: attempt to bound type parameter with a nonexistent    trait `rustc_serialize::Encoder`
 src/mod1/mod.rs:4 #[derive(RustcDecodable, RustcEncodable)]
                                           ^~~~~~~~~~~~~~
 note: in expansion of #[derive]
 src/mod1/mod.rs:4:1: 4:42 note: expansion site
 src/mod1/mod.rs:4:10: 4:25 error: failed to resolve. Did you mean     `self::rustc_serialize`?
 src/mod1/mod.rs:4 #[derive(RustcDecodable, RustcEncodable)]
                           ^~~~~~~~~~~~~~~
 note: in expansion of #[derive]
 src/mod1/mod.rs:4:1: 4:42 note: expansion site
 src/mod1/mod.rs:4:10: 4:24 error: attempt to implement a nonexistent trait    `rustc_serialize::Decodable`
src/mod1/mod.rs:4 #[derive(RustcDecodable, RustcEncodable)]
                           ^~~~~~~~~~~~~~
note: in expansion of #[derive]
src/mod1/mod.rs:4:1: 4:42 note: expansion site
src/mod1/mod.rs:4:10: 4:25 error: failed to resolve. Did you mean     `self::rustc_serialize`?
src/mod1/mod.rs:4 #[derive(RustcDecodable, RustcEncodable)]
                           ^~~~~~~~~~~~~~~
note: in expansion of #[derive]
src/mod1/mod.rs:4:1: 4:42 note: expansion site
src/mod1/mod.rs:4:10: 4:24 error: attempt to bound type parameter with a nonexistent    trait `rustc_serialize::Decoder`
 src/mod1/mod.rs:4 #[derive(RustcDecodable, RustcEncodable)]
                           ^~~~~~~~~~~~~~
 note: in expansion of #[derive]
 src/mod1/mod.rs:4:1: 4:42 note: expansion site
 src/mod1/mod.rs:4:10: 4:25 error: failed to resolve. Did you mean     `self::rustc_serialize::Decodable`?
 src/mod1/mod.rs:4 #[derive(RustcDecodable, RustcEncodable)]
                           ^~~~~~~~~~~~~~~
 note: in expansion of #[derive]
 src/mod1/mod.rs:4:1: 4:42 note: expansion site
 src/mod1/mod.rs:4:10: 4:24 error: unresolved name `rustc_serialize::Decodable::decode`
 src/mod1/mod.rs:4 #[derive(RustcDecodable, RustcEncodable)]
                           ^~~~~~~~~~~~~~
 note: in expansion of #[derive]
 src/mod1/mod.rs:4:1: 4:42 note: expansion site
 error: aborting due to 10 previous errors
 Could not compile `serialize_issue`. 

The issue can be workaround by moving the imports to the lib.rs file.

$ cat src/mod1/mod.rs 
use super::rustc_serialize::json::{self, ToJson, Json, DecoderError};

#[derive(RustcDecodable, RustcEncodable)]
struct Test {
    test: String
    }

fn test(s: String) -> Test {
    Test {test: s}
    }

$ cat src/lib.rs 
extern crate "rustc-serialize" as rustc_serialize;
mod mod1;

[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.

[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.

[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.

[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.