Optimize HashSet contains+insert usage

[nyurik]

What it does

Optimize hashset (and possibly similar HashMap?) usage when the user first uses contains() to check the non-existence of a value, followed by insertion.

Advantage

• Faster code
• Shorter code

Drawbacks

No response

Example

use std::collections::HashSet;

fn main() {
    let mut vals = HashSet::new();
    insert_if(&mut vals, 10);
}

fn insert_if(set: &mut HashSet<i32>, value: i32) {
    if !set.contains(&value) {
        set.insert(value);
        println!("inserted {value:?}");
    }
}

Could be written as:

fn insert_if(set: &mut HashSet<i32>, value: i32) {
    if set.insert(value) {
        println!("inserted {value:?}");
    }
}

[lochetti]

Interesting! I will try to do it. I might need some help, though :)
If you know some other lint that has similar logic (looking if two expressions happen one after another and then linting) let me know.

@rustbot claim

[Centri3]

I believe unnecessary_unwrap, @lochetti ^^

[Jarcho]

map_entry does this for HashMap/BTreeMap.