Inserting references to variables into HashSet then using variables afterwards

Question

I am trying insert pairs of a variable into a HashSet, after checking if the pair is present already. I then need to do downstream work with the pair, within the same context. Here is a playground reproducing my problem:

https://play.rust-lang.org/?gist=5eecfa9c1e54b468d79955aab916dd84&version=stable&mode=debug&edition=2015

use std::collections::HashSet;

fn main() {
    let mut h = HashSet::new();
    let a = 1;
    let b = 2;
    if h.contains(&(&a, &b)) {
        println!("fail");
    }
    h.insert(&(&a, &b));
}

error[E0597]: borrowed value does not live long enough
  --> src/main.rs:10:15
   |
10 |     h.insert(&(&a, &b));
   |               ^^^^^^^^ - temporary value dropped here while still borrowed
   |               |
   |               temporary value does not live long enough
11 | }
   | - temporary value needs to live until here
   |
   = note: consider using a `let` binding to increase its lifetime

error[E0597]: `a` does not live long enough
  --> src/main.rs:10:17
   |
10 |     h.insert(&(&a, &b));
   |                 ^ borrowed value does not live long enough
11 | }
   | - `a` dropped here while still borrowed
   |
   = note: values in a scope are dropped in the opposite order they are created

error[E0597]: `b` does not live long enough
  --> src/main.rs:10:21
   |
10 |     h.insert(&(&a, &b));
   |                     ^ borrowed value does not live long enough
11 | }
   | - `b` dropped here while still borrowed
   |
   = note: values in a scope are dropped in the opposite order they are created

How can I check if a and b are in the set, then insert them afterwards if they are not, and then do other things with them? How are they being borrowed if I am loading in a reference to them within the same scope?

Answer 1

Leaving "why"s aside for a moment, the example as presented has three issues:

1. The tuple &(...) ceases to exist once the function call ends. So inserting a reference to something that will not exist once HashSet::insert returns is an error, and one that the compiler thankfully catches.

2. You declared the HashSet before the variables you want to reference, which the compiler sees as an error (likely due to the fact that, at some point in the HashSet's scope, the variables a and b do not exist). This is something I've been burnt by on numerous occasions and is a limitation you should keep in mind until the compiler improves.

3. As pointed out in the comments above, HashSets take T when inserting and &T when accessing. Your code uses T for both.

The following code fixes these issues:

use std::collections::HashSet;

fn main() {
    let a = 1;
    let b = 2;
    let mut h = HashSet::new();
    if h.contains(&(&a, &b)) {
        println!("fail");
    }
    h.insert((&a, &b));
}

playground

Compare the lines that declare the HashSet and insert the tuple.