Macro link 

macro_rules! link {
    ($library:literal $abi:literal $($link_name:literal)? fn $name:ident($($params:tt)*) $(-> $ret:ty)?) => { ... };
}

Defines an external function to import.

Expands to an extern block declaring the function as well as a pub type alias matching the function’s signature. The type alias has the same name as the function, lives in the type namespace, and is useful for storing or passing around the function pointer (for example, after resolving the symbol at runtime via GetProcAddress).