Generic over dyn upcasting?

language design
1

dyn Supertrait and dyn Subtrait are distinct types, the second is not a subtype of the first. This is necessary because they have different vtable layouts. In addition, dyn Trait is not a supertype of dyn Trait + AutoTrait. These do have the same memory layout, and therefore the second could theoretically be a subtype of the first. However, backward compatibility prevents us from changing this, and the current design does have advantages (you can't specialize a trait impl for a subtype). However, this lack of subtyping also causes problems, in that trait implementations for trait objects don't apply to closely related objects even when it would make sense.

A simplified example from a rust-lang/rust GitHub issue:

use core::fmt;

pub trait ConnectionDriver {
    // ...
}

impl fmt::Debug for dyn ConnectionDriver {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "A conection driver")
    }
}

#[derive(Debug)] // Error, doesn't work!
// `dyn ConnectionDriver + Send` is not `Debug`
// even though `dyn ConnectionDriver` is
pub struct SendConnection(Box<dyn ConnectionDriver + Send>);

Ideally, the Debug impl could be generic over any auto-traits the dyn ConnectionDriver might have, or it could even apply to dyn SubtraitOfConnectionDriver.

2

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