Macro trace::init_depth_var

init_depth_var!() { /* proc-macro */ }

A convenience macro for declaring the DEPTH variable used for indenting the output

Calling this macro is equivalent to:

use std::cell::Cell;

thread_local! {
    static DEPTH: Cell<usize> = Cell::new(0);
}

It is required to declare a DEPTH variable unless using #[trace] on a mod, in which case the variable is declared for you.