Macro newtype 

macro_rules! newtype {
    ($( #[$attrs:meta] )* $name:ident $( : $default:ty )?) => { ... };
}

A macro for easy creation of newtypes.

Note

Newtypes created by this macro are sealed, meaning they do not implement AsMut, AsRef, Deref, or DerefMut.

Doing so would invalidate any use of a newtype.