Compare `Foo<i32>` and `Foo<u32>` - compare

Question

Need realize the possibility of comparison Foo<i32> and Foo<u32>.
struct Foo<T> {
id: usize,
data: T
}
impl<T> Foo<T> {
fn new_i32(i: i32) -> Foo<i32> {
Foo {
id: 0,
data: i
}
}
fn new_u32(u: u32) -> Foo<u32> {
Foo {
id: 1,
data: u
}
}
}
The problem is that trait PartialEq can only compare the same types.
impl<T> PartialEq for Foo<T> {
fn eq(&self, other: &Self) -> bool {
self.id == other.id
}
}
And this dont work:
let a = Foo::new_u32(123);
let b = Foo::new_i32(123);
println!("{}", a == b);
Full code in play.rust-lang.org

Answer 1

The problem is that trait PartialEq can only compare the same types.
That's not true. It just defaults the Rhs to the same type as Self:
pub trait PartialEq<Rhs = Self> where Rhs: ?Sized {
...
}
Here's how you would implement PartialEq for Foo<L> and Foo<R> for any L and R:
impl<L, R> PartialEq<Foo<R>> for Foo<L> {
fn eq(&self, other: &Foo<R>) -> bool {
self.id == other.id
}
}
Note that I also had to change the impl block to fix some type inference errors. Final code:
struct Foo<T> {
id: usize,
data: T,
}
impl Foo<i32> {
fn new_i32(i: i32) -> Foo<i32> {
Foo { id: 0, data: i }
}
}
impl Foo<u32> {
fn new_u32(u: u32) -> Foo<u32> {
Foo { id: 1, data: u }
}
}
impl<L, R> PartialEq<Foo<R>> for Foo<L> {
fn eq(&self, other: &Foo<R>) -> bool {
self.id == other.id
}
}
fn main() {
let a = Foo::new_u32(123);
let b = Foo::new_i32(123);
println!("{}", a == b);
}
Output:
false
https://play.rust-lang.org/?gist=51166880a9ab0c49d7650588c4ed8290&version=stable&backtrace=0