Rust conditional derive trick

dummy-macro_lib.rs

// make sure to put the attributes thing there, so
// #[serde()] attributes are allowed for the dummy macros
#[proc_macro_derive(Deserialize, attributes(serde))]
pub fn dummy_deserialize(_input: TokenStream) -> TokenStream {
    TokenStream::new()
}

// could maybe just export the same macro twice under the same name,
// here or in the re-export module, as an alternative
#[proc_macro_derive(Serialize, attributes(serde))]
pub fn dummy_deserialize(_input: TokenStream) -> TokenStream {
    TokenStream::new()
}

my-lib_lib.rs

mod _hack {
    pub use serde::*;

    // shadow serde's macros (but not the traits) conditionally
    #[cfg(not(feature = "serde")]
    pub use dummy_macro::{Deserialize, Serialize};
}

pub use _hack::{Deserialize, Serialize};

#[derive(Deserialize, Serialize)]
#[serde(whatever)]
struct Foo {
    #[serde(rename = "type")]
    type_: FooType,

    #[serde(default = "default_fn")]
    bar: i64,
}

// because the dummy macro doesn't use default_fn,
// have to cfg it to avoid dead_code warnings
#[cfg(feature = "serde")]
fn default_fn() -> i64 { 5 }