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Rust 1.37.0
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Conditional Compilation

There is a new edition of the book and this is an old link.

Sometimes one wants to have different compiler outputs from the same code, depending on build target, such as targeted operating system, or to enable release builds. Configuration options are either provided by the compiler or passed in on the command line using. Rust code then checks for their presence using the #[cfg(...)] attribute

// The function is only included in the build when compiling for macOS
#[cfg(target_os = "macos")]
fn macos_only() {
  // ...
}Run

This particular chapter does not exist in the second edition. The best place to learn about it is the Rust Reference.