Macro vec_in 

macro_rules! vec_in {
    ($arena:expr $(,)?) => { ... };
    ($arena:expr; $element:expr; $count:expr) => { ... };
    ($arena:expr; $($element:expr),* $(,)?) => { ... };
}

Builds an arena-allocated Vec, analogous to std::vec!.

The first argument is the arena (or any &allocator); the elements follow a ;:

use mago_allocator::prelude::*;

let arena = LocalArena::new();
let evens = vec_in![&arena; 0, 2, 4];
assert_eq!(evens.as_slice(), &[0, 2, 4]);