`dead_code`: disable the unused lint

The compiler provides a dead_code lint that will warn about unused functions.

An attribute can be used to disable the lint.

fn used_function() {}

// `#[allow(dead_code)]` is an attribute that disables the `dead_code` lint
#[allow(dead_code)]
fn unused_function() {}

fn noisy_unused_function() {}
// FIXME ^ Add an attribute to suppress the warning

fn main() {
    used_function();
}

Note that in real programs, you should eliminate dead code.
In these examples we’ll allow dead code in some places because of the interactive nature of the examples.