How to write a Rust function that takes an iterator?

I'd like to write a function that accepts an iterator and returns the results of some operations on it. Specifically, I'm trying to iterate over the values of a HashMap:

use std::collections::HashMap;

fn find_min(vals: Iterator) -> Option {
    vals.min()
}

fn main() {
    let mut map = HashMap::new();
    map.insert("zero", 0u32);
    map.insert("one", 1u32);
    println!("Min value {:?}", find_min(map.values()));
}

But alas:

error: the min method cannot be invoked on a trait object
--> src/main.rs:4:10
|
4 | vals.min()
| ^^^

error[E0277]: the trait bound std::iter::Iterator + 'static: std::marker::Sized is not satisfied
--> src/main.rs:3:17
|
3 | fn find_min(vals: Iterator) -> Option {
| ^^^^ std::iter::Iterator + 'static does not have a constant size known at compile-time
|
= help: the trait std::marker::Sized is not implemented for std::iter::Iterator + 'static
= note: all local variables must have a statically known size

error[E0308]: mismatched types
--> src/main.rs:11:41
|
11 | println!("Min value {:?}", find_min(map.values()));
| ^^^^^^^^^^^^ expected trait std::iter::Iterator, found struct std::collections::hash_map::Values
|
= note: expected type std::iter::Iterator + 'static
found type std::collections::hash_map::Values


I get the same error if I try to pass by reference; if I use a Box, I get lifetime errors.

You want to use generics here:

fn find_min(vals: I) -> Option
where
    I: Iterator,
{
    vals.min()
}

Traits can be used in two ways: as bounds on type parameters and as trait objects. The book The Rust Programming Language has a chapter on traits and a chapter on trait objects that explain these two use cases.

Additionally, you often want to take something that implements IntoIterator as this can make the code calling your function nicer:

fn find_min(vals: I) -> Option
where
    I: IntoIterator,
{
    vals.into_iter().min()
}

Stack Overflow Q#34969902, score: 119