How do I require a generic type implement an opera

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
}

Answer 2

The bound T: Mul does not imply that the result of the binary operator is also of type T. The result type is an associated type of this trait: Output.

The other issue is that at some point in time the operator traits switched from pass-by-referece to pass-by-value. In generic code this can be a bit of a pain in the butt (for now at least) because these operators consume their operands unless you also require the types to be Copy.

Just for completeness (in case you don't like to require Copy), let me add some information about a possible alternative direction.

For the sake of generic code, authors of "numeric types" are encouraged to provide additional non-consuming implementations of these operator traits so that you don't need Copy or Clone. For example, the standard library already provides the following implementations:

 f64 implements Mul< f64>
 f64 implements Mul<&f64>
&f64 implements Mul< f64>
&f64 implements Mul<&f64>

with each implementation having f64 as Output type. But making use of these traits directly is not pretty:

fn cube<T>(x: &T) -> T
    where for<'a> T: Mul<&'a T, Output = T>,
          for<'a,'b> &'a T: Mul<&'b T, Output = T>
{
    x * x * x
}

Eventually, we might get some (slightly) higher level traits, which would reduce the noise. For example: T: Mul2 could imply T: Mul<T> + Mul<&T> and &T: Mul<T> + Mul<&T>. But at the time of writing this, the Rust compiler does not seem able to handle this. At least I could not successfully compile the following code:

use std::ops::Mul;

pub trait Mul2 where
    Self: Mul<Self, Output=Self>,
    Self: for<'a> Mul<&'a Self, Output=Self>,
    for<'a> &'a Self: Mul<Self, Output=Self>,
    for<'a,'b> &'a Self: Mul<&'b Self, Output=Self> {}

impl<T> Mul2 for T where
    T: Mul<T, Output=T>,
    T: for<'a> Mul<&'a T, Output=T>,
    for<'a> &'a T: Mul<T, Output=T>,
    for<'a,'b> &'a T: Mul<&'b T, Output=T> {}

fn cube<T: Mul2>(x: &T) -> T {
    x * x * x
}

fn main() {
    let c = cube(&2.3);
    println!("Hello, world! {}", c)
}

I think it's safe to say that things will improve in this area. For now, the ability to generically implement numeric algorithms in Rust is not as good as I would like it to be.