Strategies for dyn trait objects with optional behavior?

Дата публикации: 06-10-2026 03:51:10

I have a trait with optional behavior, and I'm trying to figure out how to use it behind a trait object.
trait Trait {
fn required_behavior(&self);
fn optional_behavior(&self)
where
Self: Optional;
}
trait Optional: Trait {}
Unfortunately, this trait is not dyn compatible. If I change the bound to Self: Optional + Sized then it is dyn compatible, but the optional_behavior method is understandably non-dispatchable.
If I move the optional method to the subtrait:
trait Trait {
fn required_behavior(&self);
}
trait Optional: Trait {
fn optional_behavior(&self);
}
then the trait is dyn compatible, but I need some way to determine whether the Trait is also Optional, and if so downcast a &dyn Trait to a &dyn Optional. OO languages provide a way to downcast, but AFAIK Rust does not.
I could provide a way to access the downcasted subtrait object from the supertrait:
trait Trait {
fn required_behavior(&self);
fn as_optional(&self) -> Option<&dyn Optional> {
None
}
}
trait Optional: Trait {
fn optional_behavior(&self);
}
However, this is a bit awkward and feels non-idiomatic. (Iterator doesn't do this for DoubleEndedIterator, so rev() is unusable behind dyn Iterator AFAIK.)
Is there a better way?
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