containerof::containerof_intrusive!

macro_rules! containerof_intrusive {
    ($nt:ident = $container:ty : $field:ident :: $fieldtype:ty) => (
        containerof_intrusive!(_decl $nt);
        containerof_intrusive!(_impl $nt = $container : $field :: $fieldtype);
        );
    (pub $nt:ident = $container:ty : $field:ident :: $fieldtype:ty) => (
        containerof_intrusive!(_decl pub $nt);
        containerof_intrusive!(_impl $nt = $container : $field :: $fieldtype);
        );
    // below are implementation details. you should not invoke these
    // macro variants directly.
    (_decl $nt:ident) => (
        struct $nt($crate::IntrusiveAlias);
        );
    (_decl pub $nt:ident) => (
        pub struct $nt($crate::IntrusiveAlias);
        );
    (_impl $nt:ident = $container:ty : $field:ident :: $fieldtype:ty) => (
        impl $crate::IntrusiveBase for $nt {
            type Container = $container;
            type Field = $fieldtype;
            #[inline]
            fn offset() -> usize {
                containerof_field_offset!($container : $field)
            }
            #[inline]
            unsafe fn new(ia: $crate::IntrusiveAlias) -> $nt {
                $nt(ia)
            }
            #[inline]
            unsafe fn as_alias<'a>(&'a self) -> &'a IntrusiveAlias {
                ::std::mem::transmute(self as *const _)
            }
        }
        );
}

Define a type representing the translation between an intrusive field and its containing structure.