Macro pair

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

Emits a $key/$value pair to Cargo based on build script outputs.

This is equivalent to:

println!("cargo:$key=$value");

This is the base macro upon which the other macros are built.

Examples

This can be used to emit arbitrary user-defined metadata.

cargo_emit::pair!("root", "/path/to/root");

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

let mut stdout = std::io::stdout();
cargo_emit::pair!(
    to: stdout,
    "root", "/path/to/root"
);

The $key and $value parameters get concatenated into a single formatting string. Formatting runtime values can be done by passing subsequent values.

let name = "foo";
cargo_emit::pair!("{lib}dir", "/path/to/{lib}", lib = name);