Macro span

macro_rules! span {
    ($lvl:expr, target: $target:expr, parent: $parent:expr, $name:expr, $($($k:ident).+ $( = $val:expr )* ),*,) => { ... };
    ($lvl:expr, target: $target:expr, parent: $parent:expr, $name:expr, $($($k:ident).+ $( = $val:expr )* ),*) => { ... };
    ($lvl:expr, target: $target:expr, $name:expr,$($($k:ident).+ $( = $val:expr )* ),*) => { ... };
    ($lvl:expr, target: $target:expr, parent: $parent:expr, $name:expr) => { ... };
    ($lvl:expr, parent: $parent:expr, $name:expr, $($($k:ident).+ $( = $val:expr)*),*,) => { ... };
    ($lvl:expr, parent: $parent:expr, $name:expr, $($($k:ident).+ $( = $val:expr)*),*) => { ... };
    ($lvl:expr, parent: $parent:expr, $name:expr) => { ... };
    ($lvl:expr, target: $target:expr, $name:expr, $($($k:ident).+ $( = $val:expr )* ),*,
    ) => { ... };
    ($lvl:expr, target: $target:expr, $name:expr, $($($k:ident).+ $( = $val:expr )* ),*) => { ... };
    ($lvl:expr, target: $target:expr, $name:expr) => { ... };
    ($lvl:expr, $name:expr, $($($k:ident).+ $( = $val:expr)*),*,) => { ... };
    ($lvl:expr, $name:expr, $($($k:ident).+ $( = $val:expr)*),*) => { ... };
    ($lvl:expr, $name:expr) => { ... };
}

Constructs a new span.

Examples

Creating a new span with no fields:

let span = span!(Level::TRACE, "my span");
span.enter(|| {
    // do work inside the span...
});

Creating a span with fields:

span!(Level::TRACE, "my span", foo = 2, bar = "a string").enter(|| {
    // do work inside the span...
});

Note that a trailing comma on the final field is valid:

span!(
    Level::TRACE,
    "my span",
    foo = 2,
    bar = "a string",
);

Creating a span with custom target and log level:

span!(
    Level::TRACE,
    target: "app_span",
    "my span",
    foo = 3,
    bar = "another string"
);

Field values may be recorded after the span is created:

let my_span = span!(Level::TRACE, "my span", foo = 2, bar);
my_span.record("bar", &7);

Note that a span may have up to 32 fields. The following will not compile:

span!(
    Level::TRACE,
    "too many fields!",
    a = 1, b = 2, c = 3, d = 4, e = 5, f = 6, g = 7, h = 8, i = 9,
    j = 10, k = 11, l = 12, m = 13, n = 14, o = 15, p = 16, q = 17,
    r = 18, s = 19, t = 20, u = 21, v = 22, w = 23, x = 24, y = 25,
    z = 26, aa = 27, bb = 28, cc = 29, dd = 30, ee = 31, ff = 32, gg = 33
);