How do I create a HashMap literal?

Question

How I can create a HashMap literal in Rust? In Python I can do it so:

hashmap = {
   'element0': {
       'name': 'My New Element',
       'childs': {
           'child0': {
               'name': 'Child For Element 0',
               'childs': {
                   ...
               }
           }
       }
   },
   ...
}

And in Go like this:

type Node struct {
    name string
    childs map[string]Node
}

hashmap := map[string]Node {
    "element0": Node{
        "My New Element",
        map[string]Node {
            'child0': Node{
                "Child For Element 0",
                map[string]Node {}
            }
        }
    }
}

Answer 1

There isn't a map literal syntax in Rust. I don't know the exact reason, but I expect that the fact that there are multiple data structures that act maplike (such as both BTreeMap and HashMap) would make it hard to pick one.

However, you can create a macro to do the job for you, as demonstrated in Why does this rust HashMap macro no longer work?. Here is that macro simplified a bit and with enough structure to make it runnable in the playground:

macro_rules! map(
    { $($key:expr => $value:expr),+ } => {
        {
            let mut m = ::std::collections::HashMap::new();
            $(
                m.insert($key, $value);
            )+
            m
        }
     };
);

fn main() {
    let names = map!{ 1 => "one", 2 => "two" };
    println!("{} -> {:?}", 1, names.get(&1));
    println!("{} -> {:?}", 10, names.get(&10));
}

This has no additional allocation and is maximally efficient.

---

In a nightly version of Rust, you can avoid both unneeded allocation and the need for a macro:

#![feature(array_value_iter)]

use std::array::IntoIter;
use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
use std::iter::FromIterator;

fn main() {
    let s = Vec::from_iter(IntoIter::new([1, 2, 3]));
    println!("{:?}", s);

    let s = BTreeSet::from_iter(IntoIter::new([1, 2, 3]));
    println!("{:?}", s);

    let s = HashSet::<_>::from_iter(IntoIter::new([1, 2, 3]));
    println!("{:?}", s);

    let s = BTreeMap::from_iter(IntoIter::new([(1, 2), (3, 4)]));
    println!("{:?}", s);

    let s = HashMap::<_, _>::from_iter(IntoIter::new([(1, 2), (3, 4)]));
    println!("{:?}", s);
}

This logic can then be wrapped back into a macro as well:

#![feature(array_value_iter)]

use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};

macro_rules! collection {
    // map-like
    ($($k:expr => $v:expr),* $(,)?) => {
        std::iter::Iterator::collect(std::array::IntoIter::new([$(($k, $v),)*]))
    };
    // set-like
    ($($v:expr),* $(,)?) => {
        std::iter::Iterator::collect(std::array::IntoIter::new([$($v,)*]))
    };
}

fn main() {
    let s: Vec<_> = collection![1, 2, 3];
    println!("{:?}", s);

    let s: BTreeSet<_> = collection! { 1, 2, 3 };
    println!("{:?}", s);

    let s: HashSet<_> = collection! { 1, 2, 3 };
    println!("{:?}", s);

    let s: BTreeMap<_, _> = collection! { 1 => 2, 3 => 4 };
    println!("{:?}", s);

    let s: HashMap<_, _> = collection! { 1 => 2, 3 => 4 };
    println!("{:?}", s);
}

See also:

• Add hashmap, hashset, treemap, and treeset macros