Macro poly 

macro_rules! poly {
    ({ $($traits:tt)+ }, $v:ident, $alloc:ident) => { ... };
    ($trait:path, $v:ident, $alloc:ident) => { ... };
    ({ $($traits:tt)+ }, $poly:ident) => { ... };
    ($trait:path, $poly:ident) => { ... };
    ([$($x:expr),* $(,)?], $alloc:ident) => { ... };
}