Macro cargo_emit::rustc_cfg

macro_rules! rustc_cfg {
    (to : $stream : expr, $feature : expr $(, $($args : tt) *) ?) => { ... };
    ($feature : expr $(, $($args : tt) *) ?) => { ... };
}

Tells Cargo to enable a $feature.

This is equivalent to:

println!("cargo:rustc-cfg=$feature");

Examples

Useful for conditionally enabling certain code to run.

if cargo.can_bench() {
    cargo_emit::rustc_cfg!("bench");
}

or, in case you want it to emit to a custom stream:

let mut stdout = std::io::stdout();    
if cargo.can_bench() {
    cargo_emit::rustc_cfg!(
        to: stdout,
        "bench"
    );
}

Then outside of build.rs:

#[cfg(bench)]
mod benches {
    // ...
}