A trait (& utilities) for downcasting trait objects back to their original types.

Add to your Cargo.toml:

[dependencies]
downcast = "0.11"

Add to your crate root:

#[macro_use]
extern crate downcast;

• simple showcases the most simple usage of this library.
• with_params showcases how to deal with traits who have type parameters.
• sync_service showcases how to downcast Arc-pointers.

• std (default) enables all functionality requiring the standard library (Downcast::downcast()).
• nightly enables all functionality requiring rust nightly (Any::type_name()).

Q: I'm getting the size for values of type XXX cannot be known at compile time errors, what am i doing wrong?

A: Make sure you use the corresponding Any bound along with the Downcast traits. So, Any for Downcast and AnySync for DowncastSync.

Q: Can i cast trait objects to trait objects?

A: No, that is currently no possible in safe rust - and unsafe solutions are very tricky, as well. If you found a solution, feel free to share it!

Q: What is the difference between this and the downcast-rs crate on crates.io?

A: At the moment, there isn't one, really. There was an unfortunate naming clash. You may consider using the other crate, as it is more actively maintained. This one is considered feature-complete and frozen in functionality. Hopefully, one day, the Rust language will make downcasting easier and we will need neither of these crates anymore!