Tracking Issue for downcast_trait

[ivarflakstad]

Feature gate: #![feature(downcast_trait)]

This is a tracking issue for the downcast_trait and downcast_trait_mut functions.

This feature will let us try to turn any value into a dyn MyTrait. If the value does not implement MyTrait then we simply return None so that the user can have some default functionality happen instead.

We expect to be able to remove many cases of specialization from the compiler and instead perform this kind of downcasting in the bodies of functions that were relying on specialization before.

A simple example could be

// Look ma, no `T: Debug`
fn downcast_debug_format<T: 'static>(t: &T) -> String {
    match std::any::downcast_trait::<_, dyn Debug>(t) {
        Some(d) => format!("{d:?}"),
        None => "default".to_string()
    }
}

Public API

pub const fn downcast_trait<
    T: Any + 'static,
    U: ptr::Pointee<Metadata = ptr::DynMetadata<U>> + ?Sized + 'static,
>(
    t: &T,
) -> Option<&U>

pub const fn downcast_trait_mut<
    T: Any + 'static,
    U: ptr::Pointee<Metadata = ptr::DynMetadata<U>> + ?Sized + 'static,
>(
    t: &mut T,
) -> Option<&mut U>

Steps / History

(Remember to update the S-tracking-* label when checking boxes.)

• Implementation: [WIP] Add downcast_trait and downcast_trait_mut #144363
• Final comment period (FCP)¹
• Stabilization PR

Unresolved Questions

• Where should these functions live?
  Currently the implementations are in the any module, which can be very confusing as these downcast concepts are for statically known types, while the downcast concepts in Any proper are for dynamic runtime types.

Footnotes

1. https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html ↩

[Jules-Bertholet]

Going from a concrete type to a trait object is an upcast, not a downcast. I would suggest the names try_upcast/try_upcast_mut.

[ivarflakstad]

Going from a concrete type to a trait object is an upcast, not a downcast.

Oh yeah I think you're right hehe

Both upcast and downcast make more sense in OOP imo, where you typically have real inheritance like Cat: Animal and move up and down in levels of abstraction.
Since rust is more oriented toward capturing behaviour with traits (not up or down but more like sideways) maybe it should simply be trait_cast or something like that?
We have already started using downcast in any for runtime casts though, so perhaps best to just stick to the established terminology and use upcast.

[oli-obk]

Other APIs suggest this would be more a try_as_trait or sth like that. No mentioning of down or up ast, just naming it for its intended use case