Macro debug 

macro_rules! debug {
    (tag: $tag:expr, $($arg:tt)*) => { ... };
    ($($arg:tt)*) => { ... };
}

Logs a debug message using Level::DEBUG.

Compiled out entirely in release builds (when debug_assertions is disabled), so there is no runtime cost in production. Accepts the same syntax as format!. An optional tag: argument places a context label at the start of the line.

Examples

debug!("parsed {} tokens", count);
debug!(tag: "parser", "state = {:?}", state);