Add once and per-level tracing macros that are called only once

[mnmaita]

Feature Request

Crates

tracing

Motivation

While working on extracting some tracing code in bevy repo, a suggestion arose to include part of it in the tracing crate. The code has to do with per-level tracing macros that should only fire once. These are useful in bevy's context because of the "called-every-frame" nature of game systems, so we are wondering if this kind of functionality would be useful for the wider audience.

Proposal

Add the following macros to tracing crate:

/// Call some expression only once per call site.
#[macro_export]
macro_rules! once {
    ($expression:expr) => {{
        use ::std::sync::atomic::{AtomicBool, Ordering};

        static SHOULD_FIRE: AtomicBool = AtomicBool::new(true);
        if SHOULD_FIRE.swap(false, Ordering::Relaxed) {
            $expression;
        }
    }};
}

/// Call [`trace!`](crate::tracing::trace) once per call site.
///
/// Useful for logging within systems which are called every frame.
#[macro_export]
macro_rules! trace_once {
    ($($arg:tt)+) => ({
        $crate::once!($crate::tracing::trace!($($arg)+))
    });
}

/// Call [`debug!`](crate::tracing::debug) once per call site.
///
/// Useful for logging within systems which are called every frame.
#[macro_export]
macro_rules! debug_once {
    ($($arg:tt)+) => ({
        $crate::once!($crate::tracing::debug!($($arg)+))
    });
}

/// Call [`info!`](crate::tracing::info) once per call site.
///
/// Useful for logging within systems which are called every frame.
#[macro_export]
macro_rules! info_once {
    ($($arg:tt)+) => ({
        $crate::once!($crate::tracing::info!($($arg)+))
    });
}

/// Call [`warn!`](crate::tracing::warn) once per call site.
///
/// Useful for logging within systems which are called every frame.
#[macro_export]
macro_rules! warn_once {
    ($($arg:tt)+) => ({
        $crate::once!($crate::tracing::warn!($($arg)+))
    });
}

/// Call [`error!`](crate::tracing::error) once per call site.
///
/// Useful for logging within systems which are called every frame.
#[macro_export]
macro_rules! error_once {
    ($($arg:tt)+) => ({
        $crate::once!($crate::tracing::error!($($arg)+))
    });
}

Alternatives

We can keep these within bevy if there's no interest in having these features.