Call non-object-safe method from object-safe one

help
1

The following code:

trait Trait {
    fn foo()
    where
        Self: Sized,
    {
    }

    fn bar(&self) {
        Self::foo();
    }
}

Fails with:

error[E0277]: the size for values of type `Self` cannot be known at compilation time
 --> src/lib.rs:9:9
  |
9 |         Self::foo();
  |         ^^^^^^^^^ doesn't have a size known at compile-time
  |
note: required by a bound in `Trait::foo`
 --> src/lib.rs:4:15
  |
2 |     fn foo()
  |        --- required by a bound in this
3 |     where
4 |         Self: Sized,
  |               ^^^^^ required by this bound in `Trait::foo`
help: consider further restricting `Self`
  |
8 |     fn bar(&self) where Self: Sized {
  |                   +++++++++++++++++

Is there a specific reason we don't allow calling a non-object-safe method from within an object-safe one within the trait? I think it should be possible; we will always know the concrete type within the implementation of the trait. Do I miss something?

2

If you remove the following the snippet compiles.

3

(a) And makes Trait non object safe. (b) This was not the question.

4

This doesn't seem related to object safety to me. foo has a trait bound that bar doesn't, the same error will occur no matter what the trait is.

trait Trait {
    fn foo()
    where
        Self: Clone,
    {
    }

    fn bar(&self) {
        Self::foo();
    }
}

error[E0277]: the trait bound `Self: Clone` is not satisfied
 --> src/lib.rs:9:9
  |
9 |         Self::foo();
  |         ^^^^^^^^^ the trait `Clone` is not implemented for `Self`
  |
note: required by a bound in `Trait::foo`
 --> src/lib.rs:4:15
  |
2 |     fn foo()
  |        --- required by a bound in this
3 |     where
4 |         Self: Clone,
  |               ^^^^^ required by this bound in `Trait::foo`
help: consider further restricting `Self`
  |
8 |     fn bar(&self) where Self: Clone {
  |                   +++++++++++++++++
5

Sure, but there there is a reason. where Self: Sized is already special, so if this could work why not?

6

It can't work because you can write

impl Trait for str {}

in which case, what's the default implementation of <str as Trait>::bar going to call? <str as Trait>::foo doesn't exist.

I think you want something like trait NotDyn with Sized: NotDyn, but sadly, for now we have the hack of relying on Sized to mean NotDyn.

You can work around parts of it in unsatisfying ways.

4 Likes
7

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