.or_insert_default() for collections' Entry

[lukaslueg]

Option (since 1.0) and Result(since 1.16) have a handy .unwrap_or_default() for every type that is Default. The collection types have .or_insert() and .or_insert_with() but no .or_insert_default(). This would be nice to have and would free people from writing things like .or_insert_with(HashSet::new) multiple times.

Something along the line of

use std::collections::hash_map::{HashMap, Entry};

struct DefaultedEntry<'a, K:'a, V:'a> (Entry<'a, K, V>);

impl<'a, K:'a, V:'a + Default> DefaultedEntry<'a, K, V> {
    fn or_insert_default(self) -> &'a mut V {
        match self.0 {
            Entry::Occupied(entry) => entry.into_mut(),
            Entry::Vacant(entry) => entry.insert(Default::default()),
        }
    }
}

fn main() {
    let foos = [("foo", "bar"), ("foobar", "ping"), ("foo", "barfoo")];

    let mut foomap: HashMap<_, Vec<_>> = HashMap::new();
    for &(ref foo, ref bar) in &foos {
        DefaultedEntry(foomap.entry(foo)).or_insert_default().push(bar);
    }

    println!("{:?}", foomap);
}

[kennytm]

Duplicate of #44324, and already implemented in #44344! 😃 (The method name is or_default.)

[lukaslueg]

👍