Document how to support Serde as an optional library feature

[dhardy]

This is mentioned under feature flags and in the source.

It would be nice to have an example or at least mention under Using derive.

As I understand it, doing so involves this:

serde = { version = "1.0", optional = true, features = ["derive"] }

and this:

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

and this:

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

[dhardy]

Related to this is a question about dependency versions; e.g. if Serde 2.0 is ever released, should a library provide serde_1 and serde_2 optional features? Or must all libraries in the dependency chain upgrade their Serde versions at once?

[dtolnay]

Crates that want to support multiple Serde versions at once can use https://github.com/serde-rs/legacy.

[WiSaGaN]

In https://serde.rs/derive.html , it is still suggested to declare serde_derive as a dependency directly if you want to use #[derive(Deserialize, Serialize)]. Should an optional feature derive declaration of serde be prefered?

[dtolnay]

I typically still prefer depending on serde_derive. The re-export from serde is intended for crates that optionally depend on serde and need derive along with it, which is uncommon. Other than that, I don't have a compelling argument in either direction.

[dhardy]

which is uncommon

and will remain that way without a clear guide.

Serde Legacy doesn't give much usage guide either. Is it possible to do something like #[derive(serde09::Deserialize)?

Can a library seamlessly migrate from using a single version of Serde to using Serde Legacy to support multiple versions without breaking dependant crates?

Is it likely that Serde 1 will be superseded in the future, or is there a promise never to release another breaking change?