Conditionally using Serde (cargo feature)

[hgrecco]

I am trying to use serde as optional feature defined in cargo. Most of the time is quite easy because I can just use cfg to conditional compile certain functions or modules. But I am having trouble to conditionally compile Serialize, Deserialize within a derive call. The only way I have found is repeating the struct twice but is not convenient. Is there any convenient way to achieve it?

[dtolnay]

Does cfg_attr do what you want?

#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
struct X { ... }

Currently there is a bug (#148) where this does not work if you use a build.rs to run the code generator, but it should work if you use #![plugin(serde_macros)].

[hgrecco]

That should do it. What would be the right way if I want to derive Debug in all cases?

[hgrecco]

Additionally, have you thought about creating a no op crate? In that way I could just change the extern part. It would make thing much cleaner

[dtolnay]

#[derive(Debug)]
#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
struct X { ... }

Do you mean a no-op crate specifically for derive(Serialize, Deserialize) which marks the attribute as used but ignores it? We could make one, but it would not replace cfg_attr for people who use it to (1) work with stable Rust which does not support compiler plugins, or (2) avoid a dependency.

[hgrecco]

Thanks for taking the time to answer. Indeed I was thinking about a crate that allows you to do:

#[cfg(feature="serde")]
extern crate serde;
#[cfg(not(feature="serde"))]
extern crate serde_noop;

#[derive(Debug,Serialize, Deserialize)]
struct Point {
    x: i32,
    y: i32,
}

Might not seem like a big win. But when you have lots of structs, it is certainly helpful and keeps the code cleaner. Not sure if you can do something like that works in stable rust.

Regarding the the dependency, I think that an extra light weight dependency is not such a big deal. In particular with Cargo that makes things really simple.

[dtolnay]

I filed #289 to track your idea.