pub struct VacantEntry<'a> { /* fields omitted */ }A vacant Entry. It is part of the Entry enum.
 
                    
                Gets a reference to the key that would be used when inserting a value
through the VacantEntry.
use serde_json::map::Entry;
let mut map = serde_json::Map::new();
match map.entry("serde") {
    Entry::Vacant(vacant) => {
        assert_eq!(vacant.key(), &"serde");
    }
    Entry::Occupied(_) => unimplemented!(),
}
Sets the value of the entry with the VacantEntry's key, and returns a
mutable reference to it.
use serde_json::map::Entry;
let mut map = serde_json::Map::new();
match map.entry("serde") {
    Entry::Vacant(vacant) => {
        vacant.insert(json!("hoho"));
    }
    Entry::Occupied(_) => unimplemented!(),
}