Is it possible to conditionally enable an attribute like `derive`?

Question

I have added a feature in my crate which adds serde support. However, I don't quite understand how to use it properly:

// #[derive(Debug, Serialize, Deserialize, Clone)] // goes to:

#[derive(Debug, Clone)]
#[cfg(feature = "serde_support")]
#[derive(Serialize, Deserialize)]
pub struct MyStruct;

This code treats everything below cfg(feature) as conditionally compiled, so without my serde_support feature my crate does not have MyStruct also.

I have tried to wrap it with braces but it gives another error:

Code:

#[derive(Debug, Clone)]
#[cfg(feature = "serde_support")] {
#[derive(Serialize, Deserialize)]
}
pub struct MyStruct;

Error:

error: expected item after attributes
  --> mycrate/src/lib.rs:65:33
   |
65 | #[cfg(feature = "serde_support")] {
   |                                 ^

So how to do this?

Answer 1

You can use the cfg_attr(a, b) attribute:

#[derive(Debug, Clone)]
#[cfg_attr(feature = "serde_support", derive(Serialize, Deserialize))]
pub struct MyStruct;

It's described in the Rust reference about "conditional compilation":

#[cfg_attr(a, b)]
item

Will be the same as #[b] item if a is set by cfg, and item otherwise.