conditional compilation

Choosing which code to compile for each target, with #[cfg], cfg! and cfg_select!.

#[cfg(pred)] and cfg_select! remove code whose predicate is false before type checking; cfg!(pred) is a bool expression, so every branch must compile on every target.

Examples

#[cfg(unix)] fn home() and #[cfg(windows)] fn home() give one version per target; if cfg!(windows) { .. } keeps both branches, so both must compile everywhere.