OptionalPath extractor

[jplatte]

• I have looked for existing issues (including closed) about this

Feature Request

Motivation

We've been getting many people asking about an easy way of extracting a path segment value only if a placeholder exists at the route the handler was called from.

Proposal

Add an extractor struct OptionalPath<T>(Option<T>); that succeeds with None if there are no placeholders for the given route, and if there are placeholders, attempts to deserialize the arguments to T and return Some(T) on success, rejecting the request on failure.

Alternatives

🤷🏼

[simbleau]

            .route("/", get(handler)
            .route("/:path", get(handler))

pub(crate) async fn handler(
    path: Option<Path<String>>,
) -> impl IntoResponse
{
..
}

[jplatte]

Yes, that works most of the time, but it doesn't reject the request for /:path requests with invalid UTF-8. Or maybe it does for the wrong reason. Anyways if using u32 instead of String in your example, a non-integer value for :path will still call the handler.

@davidpdrsn wdyt about this idea?

[davidpdrsn]

I'm wondering if we can change PathRejection so you can tell if a param was missing, without needing a new extractor. An OptionalPath might still be worth it but in that case we could put it in axum-extra as a simple wrapper around Path + PathRejection::MissingParam (or smth)

[jplatte]

Yeah, the idea was to put it in axum-extra, and implement it in terms of Path. I didn't realize that isn't yet possible 😄

[davidpdrsn]

Then I think it's a good idea!

I don't specifically remember if its possible today but looking at PathRejection seems like we're missing a variant for it:

pub enum PathRejection {
    FailedToDeserializePathParams(FailedToDeserializePathParams),
    MissingPathParams(MissingPathParams),
}

MissingPathParams is for the internal extension missing and responds with Internal Server Error so that's a different thing. Though the name is a bit confusing in this context.

[davidpdrsn]

Looking at PathRejection a bit more closely it seems we might be able to use WrongNumberOfParameters from the axum::extract::path::ErrorKind that you get from FailedToDeserializePathParams::into_kind.