Macro vec_in 

macro_rules! vec_in {
    (in $alloc:expr $(,)?) => { ... };
    (in $alloc:expr; $elem:expr; $n:expr) => { ... };
    (in $alloc:expr; $($x:expr),+ $(,)?) => { ... };
}

A bumpalo::vec!-style constructor for the arena Vec, generic over the allocator backend.

• vec_in![in alloc] -> empty
• vec_in![in alloc; elem; n] -> n clones of elem
• vec_in![in alloc; a, b, c] -> the listed elements, in order