Add JsValue::{from_serde, into_serde}

[alexcrichton]

These functions are activated with the serde-serialization serde-serialize feature of the
wasm-bindgen crate. When activated they will allow passing any arbitrary value
into JS that implements the Serialize trait and receiving any value from JS
using the Deserialize trait. The interchange between JS and Rust is JSON.

Closes #96

[udoprog]

Oh. This looks really good. So we serialize incoming objects and pass the string in, vice-versa for passing them out?

I think this effectively minimises the number of context switches and is the most pragmatic solution right now.

A potential future improvement could be to encode objects in a way which has a more efficient representation and supports skipping properties when decoding. But it will be hard to compete with the efficiency of the built-in JSON support.

[alexcrichton]

@udoprog precisely! You basically got all the intentions there :)

[David-OConnor]

Can someone provide an example of how to enable this?

wasm-bindgen = {version = "^0.2", features = [“serde-serialization”,]}

in Cargo.toml, and

#[cfg(feature = "serde-serialization")]
 extern crate serde;

both lead to this:

error[E0433]: failed to resolve. Maybe a missing extern crate wasm_bindgen;?

[alexcrichton]

@David-OConnor oh so after you enable the features in Cargo.toml you don't need the #[cfg] in your own code for serde, you should just be able to use wasm-bindgen as usual. Do you have an example I could poke around though?

[jsheard]

The feature flag is actually serde-serialize, not serde-serialization.

@alexcrichton made a typo in the OP here 😛

[alexcrichton]

Oops, thanks @jsheard!

[David-OConnor]

Thank you for the help. I think I'm just not understanding the syntax of how to enable the feature; I've attempted the above two with the correct spelling, plus two additional attempts:

[dependencies.wasm-bindgen]
version = "^0.2"
features = ["serde-serialize"],

in Cargo.toml: invalid TOML error.
and
#![feature(proc_macro, wasm_custom_section, wasm_import_module, wasm_serde_serialize)]
at the top of my lib.rs; doesn't break anything like the other attempts, but doesn't expose from_serde.

[alexcrichton]

@David-OConnor oh the trailing comma there is invalid syntax, but if you remove that you should be good!

[David-OConnor]

Thank you! That worked.