Build HashSet from a vector in Rust

The following should work nicely; it fulfills your requirements:

use std::collections::HashSet;
use std::iter::FromIterator;

fn vec_to_set(vec: Vec<u8>) -> HashSet<u8> {
    HashSet::from_iter(vec)
}

from_iter() works on types implementing IntoIterator, so a Vec argument is sufficient.

Additional remarks:

• you don't need to explicitly return function results; you only need to omit the semi-colon in the last expression in its body

• I'm not sure which version of Rust you are using, but on current stable (1.12) to_iter() doesn't exist

Moving data ownership

let vec: Vec<usize> = vec![1, 2, 3, 4];
let hash_set: HashSet<usize> = vec.into_iter().collect();

Cloning data

let vec: Vec<usize> = vec![1, 2, 3, 4];
let hash_set: HashSet<usize> = vec.iter().cloned().collect();

Because the operation does not need to consume the vector¹, I think it should not consume it. That only leads to extra copying somewhere else in the program:

use std::collections::HashSet;
use std::iter::FromIterator;

fn hashset(data: &[u8]) -> HashSet<u8> {
    HashSet::from_iter(data.iter().cloned())
}

Call it like hashset(&v) where v is a Vec<u8> or other thing that coerces to a slice.

There are of course more ways to write this, to be generic and all that, but this answer sticks to just introducing the thing I wanted to focus on.

¹This is based on that the element type u8 is Copy, i.e. it does not have ownership semantics.