How to add optional comma support in Rust?

Question

I started learning macros in Rust. I decided to practice a little and write a simple macro for hashmap. If call the macro this way, then everything works

let hashmap = hashmap!{ 1 => String::from("1"), 2 => String::from("2") };

But I wanted to add the ability to support an optional comma at the end.

For example, so that I can call a macro like this

let hashmap = hashmap!{ 1 => String::from("1"), };

My macro looks like this

macro_rules! hashmap {
   ($($key:expr => $val:expr),*) => {
       {
           let mut map = ::std::collections::HashMap::new();
           $(
               map.insert($key, $val);
           )*
           map
       }
   }
}

I was trying to add something like ,? or this (,)?, but it doesn't work and outputs an error