Rust trait and its default implementation - traits

Question

Wrote a trait that checks a given number(u16 or u32) is even or not.
v1.0
trait EvenOdd {
fn is_even(&self) -> bool;
}
impl EvenOdd for u16 {
fn is_even(&self) -> bool {
self % 2 == 0
}
}
impl EvenOdd for u32 {
fn is_even(&self) -> bool {
self % 2 == 0
}
}
fn main() {
let x: u16 = 11;
let y: u32 = 44;
println!("x = {}, y = {}", x.is_even(), y.is_even());
}
This runs fine. But since is_even is repeated for u16 and u32, moved it into the trait as a default method.
v2.0
trait EvenOdd {
fn is_even(&self) -> bool {
self % 2 == 0
}
}
impl EvenOdd for u16 {
}
impl EvenOdd for u32 {
}
fn main() {
let x: u16 = 11;
let y: u32 = 44;
println!("x = {}, y = {}", x.is_even(), y.is_even());
}
This produces compiler error:
error[E0369]: binary operation `%` cannot be applied to type `&Self`
--> trait_arithmetic_v2.rs:3:9
|
3 | self % 2 == 0
| ^^^^^^^^
|
= note: an implementation of `std::ops::Rem` might be missing for `&Self`
Attempting to constrain &self results in compiler error:
fn is_even<T: std::ops::Rem> (&self: T) -> bool
self % 2 == 0
}
error: expected one of `)` or `,`, found `:`
--> trait_arithmetic_v2.rs:2:44
|
2 | fn is_even<T: std::ops::Rem> (&self: T) -> bool {
| ^ expected one of `)` or `,` here
The only way I could apply the constraint is by changing the is_even api.
v3.0
use std::ops::Rem;
trait EvenOdd {
fn is_even<T: Rem<Output = T> + PartialEq + From<u8>> (&self, other: T) -> bool {
other % 2.into() == 0.into()
}
}
And use it like x.is_even(x) which is un-natural. How to fix my v2.0? Thanks.

Answer 1

In v2.0 the compiler don't know the type that you are trying to apply the arithmetic operation and trait EvenOdd: Rem doesn't work because Rem takes Self.
This code is somewhat limited but works for your example and it is functional because there is a default implementation of impl From<u16> for u32.
use std::ops::Rem;
trait EvenOdd {
fn is_even(&self) -> bool;
}
impl<T> EvenOdd for T
where
T: Copy + From<u16> + PartialEq + Rem<Output=T>
{
fn is_even(&self) -> bool {
*self % From::from(2u16) == From::from(0u16)
}
}
fn main() {
let x: u16 = 11;
let y: u32 = 44;
println!("x = {}, y = {}", x.is_even(), y.is_even());
}