Code shared from the Rust Playground

playground.rs

#![feature(unboxed_closures)]
#![feature(fn_traits)]
use std::collections::BTreeMap;

struct IncIdGen(i32);

impl FnOnce<()> for &mut IncIdGen {
    type Output = i32;
    extern "rust-call" fn call_once(self, _args: ()) -> Self::Output {
        self.0 += 1;
        self.0
    }
}

fn main() {
    let mut id_gen = IncIdGen(0);
    let mut map = BTreeMap::new();
    map.insert("c".into(), 3i32);
    for id in "abcdefghijklmnopqrs".chars() {
        let id = format!("{}", id);
        map.entry(id).or_insert_with(&mut id_gen);
    }
    dbg!(map);
}