Macro debug_once 

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

Logs a message once at the debug level.

The log event will only be emitted once for each combinaison of target/arguments.

Logging at this level is disabled if any of the following features are present: max_level_off, max_level_error, max_level_warn, or max_level_info.

When building in release mode (i.e., without the debug_assertions option), logging at this level is also disabled if any of the following features are present: release_max_level_off, release_max_level_error, release_max_level_warn, or release_max_level_info.