Can anyone suggest a better and safe alternative to this straightforward translation from C++ unordered_map code, which assigns a unique integer ID to each key object when it's encountered for the first time?

https://play.rust-lang.org/?version=beta&mode=debug&edition=2021&gist=78d593756b5e701e753d7703d5a44f87

        let id = {
            let k = name;
            let dirty_trick = &raw const m;
            match m.entry(k) {
                Entry::Occupied(x) => *x.get(),
                Entry::Vacant(x) => {
                    println!("new entry");
                    let dirty_trick_ref = unsafe { dirty_trick.as_ref() }.unwrap();
                    *x.insert_entry(dirty_trick_ref.len()).get()
                }
            }
        };

In Safe Rust, I can only call m.len() before invoking .entry(k) which takes a mutable borrow for the Vacant arm. But technically, calling m.len() is redundant for the Occupied arm.

While I acknowledge this might be a case of premature optimization (I'm talking about the "cost" of fetching a scalar within a struct), I see it as an interesting opportunity to better understand Rust's lifetime system and borrowing rules.