Lifetime trouble when slice filtering - rust

Question

I want to optionally filter a slice without changing the type. The following approach fails:
use std::collections::HashSet;
fn main() {
let include: Option<HashSet<i8>> = Some(HashSet::new());
let y: &[i8] = &[1, 2, 3];
let z: &[i8] = match include {
Some(set) => {
let filtered: Vec<i8> = y.iter().filter(|&i| set.contains(i)).map(|&i| i).collect();
filtered.as_slice()
},
None => y
};
println!("{:?}", z);
}
with error
error: `filtered` does not live long enough
--> <anon>:10:9
|
9 | filtered.as_slice()
| -------- borrow occurs here
10 | },
| ^ `filtered` dropped here while still borrowed
...
14 | }
| - borrowed value needs to live until here
-- playground
I understand that filtered doesn't live long enough because the slice is simply a reference into it. I have tried to copy or clone or otherwise make a static version of the resulting filtered.as_slice(), but nothing compiles.
Declaring filtered outside of the definition of z won't work, because it requires set to do the filtering.
I expect this dance of converting to an iterator, then to a vector, and then to a slice is probably not idiomatic. So I'm interested to know of a better approach.

Answer 1

Declaring filtered outside of the definition of z won't work, because it requires set to do the filtering.
I don't understand why not. In the statement filtered.as_slice(), you borrow from filtered, so you must make sure filtered lives longer than the variable you assign it to (z):
use std::collections::HashSet;
fn main() {
let mut h = HashSet::new();
h.insert(2);
let exclude: Option<HashSet<i8>> = Some(h);
let y: &[i8] = &[1, 2, 3];
let filtered: Vec<i8>;
let z: &[i8] = match exclude {
Some(set) => {
filtered = y.iter().filter(|&i| !set.contains(i)).map(|&i| i).collect();
filtered.as_slice()
},
None => y
};
println!("{:?}", z); // outputs [1, 3]
}
Note I inverted the logic in filter.