Macro set 

macro_rules! set {
    { } => { ... };
    { $($e:expr),* $(,)? } => { ... };
    { let $s:expr; $($e:expr),* $(,)? } => { ... };
}

new a HashSet<V>

set![a, b]

equivalent to

{
    let mut s = HashSet::new();
    s.insert(a);
    s.insert(b);
    s
}

---

set like

set![let BTreeSet::new(); 1, 2]

equivalent to

{
    let mut s = BTreeSet::new();
    s.insert(a);
    s.insert(b);
    s
}