hot elm
I am trying to create a function that has a closure that has a fixed argument, but can return any error type.
Problem is Box<dyn Error> doesn't implement Error and instead delegates to dyn Error which is ?Sized.
See playground:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=e0420d394f32875319a2762ad393e376
Are there any solutions to this?
How to create function with closure argument that returns generic Error?
lilac moon
That's not exactly what the problem is, but it's basically accurate
The actual issue is that the impl is impl<E: Error> Error for Box<E>
Only boxes of sized errors are errors
As for the solution: do you need pass_closure to have an Error bound?
hot elm
Well minimally I could work with Display orDebug but it feels kind of ugly to make it a public facing API. I might also need Error::source to provide extra debug information as the actual use case goes pretty deep (moving the closure as Lua closures and then called from C whenever the binary needs to call it, ofcourse with a bunch of wrappers in between for ABI compatibility)
Though does it make sense to accept a Display instead of Error in a public API?
Assuming I need the Error bound what would my options be?
noble arrow
you can write a type that contains Box<dyn Error> and implements Error and forwards
it's only Box<dyn Error> exactly that is stuck with this doesn't-implement-Error thing
struct MyOwnDynError(Box<dyn Error>) can do whatever you want
hot elm
That would work but it means a user would have to know and use this niche type.
Since its a public API I want to keep it as simple as possible.
hot elm
Actually the more I think about it introducing a struct is making more sense. Still open to other solutions though.
hidden ore
hyper and friends work around this with the bound E: Into<Box<dyn Error + Send + Sync + 'static>> instead of E: Error + 'static, and the blanket impl<T> Into<T> for T allows using Box<dyn Error + Send + Sync + 'static> exactly
see the bounds on S::Error and Bd::Error in hyper::server::conn::http2::Builder::serve_connection
the tradeoff is that you cannot use E as a impl Error, the only thing you can do with it is turn it into a box