axum-option

Better handling of optional extractors in axum.

This crate exposes a trait, and an extractor, that allows you to extract a potentially-missing value in your handlers.

Crates may implement FromRequestPartsOptional for their extractors to signify that the extractor can have a missing value, and then the ValidOption extractor can be used to extract either a valid T, or a missing T, but reject an invalid T.

How does this differ from using Option<T> directly?

Unfortunately, axum's default 'Option' handler means either 'T passed validation' or 'T failed validation', essentially meaning the same as Result<T, T::Rejection>.ok(), which is typically not what you want to do. It means that supporting handlers that, for example, need either a valid cookie, or no cookie at all, requires per-crate newtype wrapper boilerplate and knowledge over the internal workings of the crate itself. ValidOption solves this M + N problem.

Now you can do ValidOption<Session> and get either a valid session, or no session at all, but reject an invalid session.