Macro flex_error::define_error_with_tracer

macro_rules! define_error_with_tracer {
    ( $tracer:ty; $name:ident; $(
      $suberror:ident
      $( { $( $arg_name:ident : $arg_type:ty ),* $(,)? } )?
      $( [ $source:ty ] )?
      | $formatter_arg:pat | $formatter:expr
    ),* $(,)?
  ) => { ... };
}

This macro allows error types to be defined with custom error tracer types other than DefaultTracer. Behind the scene, a macro call to define_error!{ ... } really expands to define_error_with_tracer!{ flex_error::DefaultTracer; … }