Nested fallback routing

[kylewlacy]

I noticed that, at least after #408, Axum doesn't allow having a .nest router that has a fallback (well as of 0.3.4 it's possible to add, but it never gets routed to-- because of #488 / #529 this looks like it will become a panic soon).

We have a use case where having a nested fallback is useful, specifically we have a structure like this:

• /v1
  • /swagger
    • /schema.json → Return fixed response
    • Other requests → Route to StaticDir
  • (... other routes ...)

So, if .nest() could take a Router with a .fallback(), then we could easily model the above. Without it, it would start to get messy. For now, we came up with the following simple workaround: using service_method_routing::any to wrap the Router. Here's what it looks like in code:

let swagger = axum::Router::new()
    .route("/schema.json", routing::get(|| async { "schema.json" }))
    .fallback(
        tower_http::services::ServeDir::new(path)
            .handle_error(|_| (StatusCode::INTERNAL_SERVER_ERROR, "Internal server error")),
    );

let router = axum::Router::new()
    .nest("/v1/swagger", routing::service_method_routing::any(swagger)) // <---- Wrapping the router
    .fallback(fallback_handler.into_service());

(Here's a full working example: https://github.com/kylewlacy/test-axum-nested-routing)

---

So there's the context basically. For purposes of discussion, here are 2 questions to start:

1. Is the above use of service_method_routing::any to wrap a router intended? That is, can we rely on the above behavior indefinitely?
2. Would it be possible to get Axum to do the more intuitive thing here, and use a nested router's fallback? As a new user of Axum, it was pretty surprising to me that this didn't work out of the box (but I also understand it's likely a tradeoff that was made while balancing competing priorities).

[jplatte]

I think the main / only reason nested fallbacks aren't supported is that matchit doesn't support having both a /foo/bar and a /foo/*rest route. If this could be supported there, it should be super easy to translate nested fallbacks to it.

[davidpdrsn]

Yes I believe thats correct.