Macro slotmap_dec_ref

macro_rules! slotmap_dec_ref {
    { $slotmap:expr, $ref_:expr, $refcount:expr } => { ... };
}

Decrement a refcount and maybe remove a corresponding slotmap entry

fn slotmap_dec_ref!<K, V>(
   slotmap: &mut SlotMap<K, V>,
   ref_: Ref<K>,
   refcount: &mut Count<K>, // (typically) borrows from slotmap
)