rust - How do I require a generic type implement an operation like Add, Sub, Mul, or Div in a generic function?

Question

I'm trying to implement a generic function in Rust where the only requirement for the argument is that the multiplication operation should be defined. I'm trying to implement a generic "power", but will go with a simpler cube function to illustrate the problem:

use std::ops::Mul;

fn cube<T: Mul>(x: T) -> T {
    x * x * x
}

fn main() {
    println!("5^3 = {}", cube(5));
}

When compiling I get this error:

error[E0369]: binary operation `*` cannot be applied to type `<T as std::ops::Mul>::Output`
 --> src/main.rs:4:5
  |
4 |     x * x * x
  |     ^^^^^^^^^
  |
  = note: an implementation of `std::ops::Mul` might be missing for `<T as std::ops::Mul>::Output`

What does this mean? Did I choose the wrong trait? How can I resolve this?

Answer 1

Let's break down your example a bit:

fn cube<T: Mul>(x: T) -> T {
    let a = x * x;
    let b = a * x;
    b
}

What are the types of a and b? In this case, the type of a is <T as std::ops::Mul>::Output — sound familiar from the error message? Then, we are trying to multiply that type by x again, but there's no guarantee that Output is able to be multiplied by anything!

Let's do the simplest thing and say that T * T needs to result in a T:

fn cube<T: Mul<Output = T>>(x: T) -> T {
    x * x * x
}

Unfortunately, this gives two similar errors:

error[E0382]: use of moved value: `x`
 --> src/lib.rs:6:9
  |
6 |     x * x * x
  |     -   ^ value used here after move
  |     |
  |     value moved here
  |
  = note: move occurs because `x` has type `T`, which does not implement the `Copy` trait

Which is because the Mul trait takes arguments by value, so we add the Copy so we can duplicate the values.

I also switched to the where clause as I like it better and it is unwieldy to have that much inline:

fn cube<T>(x: T) -> T
where
    T: Mul<Output = T> + Copy
{
    x * x * x
}

See also:

• How do I implement the Add trait for a reference to a struct?
• How to write a trait bound for adding two references of a generic type?