Macro vstack 

macro_rules! vstack {
    (spacing: $spacing:expr; $($child:expr),* $(,)?) => { ... };
    ($($child:expr),* $(,)?) => { ... };
}

Build a VStack from a list of children.

VStack(spacing: 12) { … } is written vstack![spacing: 12.0; …]: the configuration first, then a semicolon, then the children.