Macro span

macro_rules! span {
    ($args:expr, $block:expr) => { ... };
    ($args:expr, $a:expr, $block:expr) => { ... };
    ($args:expr, $a:expr, $b:expr, $block:expr) => { ... };
    ($args:expr, $a:expr, $b:expr, $c:expr, $block:expr) => { ... };
    ($args:expr, $a:expr, $b:expr, $c:expr, $d:expr, $block:expr) => { ... };
    ($args:expr, $a:expr, $b:expr, $c:expr, $d:expr, $e:expr, $block:expr) => { ... };
    ($args:expr, $a:expr, $b:expr, $c:expr, $d:expr, $e:expr, $f:expr, $block:expr) => { ... };
    ($args:expr, $a:expr, $b:expr, $c:expr, $d:expr, $e:expr, $f:expr, $g:expr, $block:expr) => { ... };
}

Create a tracing span.

Spans are pairs of trace! logs. Every span wraps a block, and logs a message at the start, of the block and a message after the block has finished. This works in asynchronous contexts too.

Each span takes a name, a block, and optionally a list of key-value pairs in between those. Once structured logging becomes part of log (currently feature gated as kv_unstable), we’ll move to support arbitrary key-value pairs.

Because of the way this macro is constructed, we currently support up to 9 key-value pairs. Which makes a total of 12 arguments.

Examples

use async_log::span;
use log::info;

span!("main", {
    let x = "foo";
    info!("this {}", x);

    span!("inner, x={}", x, {
        info!("we must go deeper {}", x);
    });
})