Macro trace 

macro_rules! trace {
    ($($args:tt)+) => { ... };
}

Emit a note of kind NoteKind::Trace with a formatted message.

Trace notes are meant to provide information about the execution history that led to the emission of the previous diagnostic. This is usally done by matching on a Result and emit the trace note in the Err case.

Example:

 match parse_int(s) {
    Ok(value) => Ok(something(value)),
    Err(_) => {
        trace!(span, "while parsing a message");
        Err(DiagnosticEmitted)
    }
 }