Macro debug_span 

macro_rules! debug_span {
    (target: $target:expr, parent: $parent:expr, $name:expr, $($field:tt)*) => { ... };
    (target: $target:expr, parent: $parent:expr, $name:expr) => { ... };
    (parent: $parent:expr, $name:expr, $($field:tt)*) => { ... };
    (parent: $parent:expr, $name:expr) => { ... };
    (target: $target:expr, $name:expr, $($field:tt)*) => { ... };
    (target: $target:expr, $name:expr) => { ... };
    ($name:expr, $($field:tt)*) => { ... };
    ($name:expr) => { ... };
}

Constructs a span at the debug level.