When should I not implement a trait for references to implementors of that trait?

Question

If I have a trait, and a function that accepts a generic type constrained to that type, everything works fine. If I try to pass in a reference to that type, I get a compilation error.

trait Trait {
    fn hello(&self) -> u32;
}

struct Struct(u32);

impl Trait for Struct {
    fn hello(&self) -> u32 {
        self.0
    }
}

fn runner<T: Trait>(t: T) {
    println!("{}", t.hello())
}

fn main() {
    let s = Struct(42);

    // Works
    runner(s);

    // Doesn't work
    runner(&s);
}

error[E0277]: the trait bound `&Struct: Trait` is not satisfied
  --> src/main.rs:24:5
   |
24 |     runner(&s);
   |     ^^^^^^ the trait `Trait` is not implemented for `&Struct`
   |
   = help: the following implementations were found:
             <Struct as Trait>
note: required by `runner`
  --> src/main.rs:13:1
   |
13 | fn runner<T: Trait>(t: T) {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^

I can fix the issue by implementing the trait for any reference to a type that implements the trait:

impl<'a, T> Trait for &'a T
where
    T: Trait,
{
    fn hello(&self) -> u32 {
        (*self).hello()
    }
}

The piece of information that I'm missing is when shouldn't I implement this? Asked another way, why doesn't the compiler automatically implement this for me? Since it currently doesn't, I assume there must be cases where having this implementation would be disadvantageous.

swizard swizard · Accepted Answer · 2015-03-02T00:06:49

when shouldn't I implement this? Asked another way, why doesn't the compiler automatically implement this for me? Since it currently doesn't, I assume there must be cases where having this implementation would be disadvantageous.

As an example, the Default trait immediately came to mind.

pub trait Default {
    fn default() -> Self;
}

I could implement it for T, but there is no way to automatically implement it for &T.

When should I not implement a trait for references to implementors of that trait?

2 Answers