/// Inserts a default-constructed value in the entry if it is vacant and returns a mutable
/// reference to it. Otherwise a mutable reference to an already existent value is returned.
///
/// Computes in **O(1)** time (amortized average).
Inserts a default-constructed value in the entry if it is vacant and returns a mutable
reference to it. Otherwise a mutable reference to an already existent value is returned.

Computes in **O(1)** time (amortized average).
Inserts a default - constructed value in the entry if it is vacant and returns a mutable reference to it . Otherwise a mutable reference to an already existent value is returned . Computes in ** O ( 1 ) ** time ( amortized average ) .
pub fn or_default(self) -> &'a mut V
    where
        V: Default,
    {
        match self {
            Entry::Occupied(entry) => entry.into_mut(),
            Entry::Vacant(entry) => entry.insert(V::default()),
        }
    }
pub fn or_default ( self ) -> & ' a mut V where V : Default , { match self { Entry :: Occupied ( entry ) => entry . into_mut ( ) , Entry :: Vacant ( entry ) => entry . insert ( V :: default ( ) ) , } }
Inserts a default-constructed value in the entry if it is vacant and returns a mutable
reference to it.
Inserts a default - constructed value in the entry if it is vacant and returns a mutable reference to it .
self