Hi Rustaceans,
I'm just started learning Rust so probably do not know right keywords to search my question, so I'm sorry if this question already was answered.
As my first excercise I decided to implement program, that prints table of function values for given arguments. And I came up 2 problems for me:
 I would like to work with arbitrary integers;
 I would like to have identity function.
To solve this problem I declared this struct:
struct MyFun<'a> {
name: &'a str,
body: fn(&BigInt) > ???,
}
And I have found 4 ways to declare return type of body
function:

BigInt
 this way I like the most, but I do not like that I should clone argument in identity function:
fn n(x: &BigInt) > BigInt {
return x.clone();
}

Cow<BigInt>
 I think it's the most efficient working way, but looks cumbersome for me 
&BigInt
 I can not find way to implement other functions in this way:
fn nn(x: &BigInt) > &BigInt {
let res = x * x;
return &res; // error: `res` does not live long enough
}

Box<BigInt>
 Will solve lifetime problem in previous way, but again I need to clone argument in identity function and it looks cumbersome
So what is idiomatic way in Rust to implement those two functions?
Thanks.
Full code:
extern crate num;
use num::FromPrimitive;
use num::bigint::*;
use std::borrow::Cow;
struct MyFun<'a> {
name: &'a str,
body: fn(&BigInt) > Cow<BigInt>,
}
fn n(x: &BigInt) > Cow<BigInt> {
return Cow::Borrowed(x);
}
fn nn(x: &BigInt) > Cow<BigInt> {
let res = x * x;
return Cow::Owned(res);
}
fn main() {
let functions = [
MyFun {
name: "n",
body: n,
},
MyFun {
name: "nn",
body: nn,
}];
let second: BigInt = FromPrimitive::from_u64(1).unwrap();
let minute: BigInt = &second * &(FromPrimitive::from_u64(60).unwrap());
let hour = &minute * &(FromPrimitive::from_u64(60).unwrap());
let day = &hour * &(FromPrimitive::from_u64(24).unwrap());
let month = &day * &(FromPrimitive::from_u64(30).unwrap());
let year = &month * &(FromPrimitive::from_u64(12).unwrap());
let century = &year * &(FromPrimitive::from_u64(100).unwrap());
let arguments = [second, minute, hour, day, month, year, century];
for f in functions.iter() {
print!("{}: ", f.name);
for a in arguments.iter() {
print!("{} ", (f.body)(a));
}
println!("");
}
}