Macro format_in 

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

Formats its arguments into an arena, returning &mut str.

The macro form of Arena::alloc_fmt, reading like format! but with no intermediate global allocation:

use mago_allocator::prelude::*;

let arena = LocalArena::new();
let label = format_in!(arena, "{}::{}", "App", "VERSION");
assert_eq!(label, "App::VERSION");