How to get the number of keys in a HashMap after inserting or updating a value?

Question

I want to insert or update a value in the map, and then get the number of keys.

 use std::collections::HashMap;

fn main() {
    let mut map = HashMap::new();
    let count = map.entry("Tom").or_insert(0);
    *count += 1;

    let size = map.keys().len();
    println!("{} men found", size);
}

The compiler error:

error[E0502]: cannot borrow `map` as immutable because it is also borrowed as mutable
  --> src/main.rs:8:16
   |
5  |     let count = map.entry("Tom").or_insert(0);
   |                 --- mutable borrow occurs here
...
8  |     let size = map.keys().len();
   |                ^^^ immutable borrow occurs here
9  |     println!("{} men found", size);
10 | }
   | - mutable borrow ends here

Is there any way to work around this? Is the way I wrote it wrong?

Answer 1

Choose one of:

1. Use Rust 2018 or another version of Rust with non-lexical lifetimes:

   use std::collections::HashMap;
   
   fn main() {
       let mut map = HashMap::new();
       let count = map.entry("Tom").or_insert(0);
       *count += 1;
   
       let size = map.keys().len();
       println!("{} men found", size);
   }
   

2. Don't create a temporary value:

   *map.entry("Tom").or_insert(0) += 1;
   

3. Add a block to constrain the borrow:

   {
       let count = map.entry("Tom").or_insert(0);
       *count += 1;
   }