leptos 0.6.2 on Rust Cargo

This is release for our new server functions rewrite and Axum 0.7 support.
This should be a relatively feature-rich release, with limited breaking changes.

Migration

Actix

You can remove any explicit .handle_server_fns() call in your main.rs, as server functions are now handled in .leptos_routes()
The current extract function has been removed, and replaced with a new extract that has the same API as the current extractor. I think this API is strictly better, but please share feedback if you disagree.

Axum

This release supports Axum 0.7, so you'll need to migrate from Axum 0.6. The easiest way to do this is probably to consult the diff on one of the examples. (Note that along with Axum 0.7, you'll need to update to http 1.0 and tower_http 0.5.)
You can remove any explicit .handle_server_fns() call in your main.rs, as server functions are now handled in .leptos_routes()
The current extract function has been removed, and replaced with a new extract that has the same API as the current extractor. I think this API is strictly better, but please share feedback if you disagree.
RequestParts has been removed, as http::request::Parts now implements Clone: any use_context::<RequestParts>() should be updated to use Parts directly instead.

`ServerFnError::new()`

The addition of custom error types means that constructing ServerFnError inside server functions can cause type inference errors:

let oops = Err(ServerFnError::ServerError("No server state".to_string()));
return oops;  // this is fine
oops? // this is not: cannot infer type of the type parameter `E` declared on the enum `ServerFnError`

As a result, we've added a helper ServerFnError::new which simply constructs a ServerFnError::<NoCustomError>::ServerError:

let oops = Err(ServerFnError::new("No server state"));
return oops;  // this is fine
oops? // this is also fine now

This has the benefit of being more concise than the earlier pattern in any case.

Features

A rewritten server function system that is backwards-compatible, but reduces binary size and increases flexibility, specifically by allowing

automatic setup of server fn handlers with .leptos_routes() from the integrations
a variety of additional built-in encodings (rkyv, multipart forms/file uploads) in addition to the current set (GET URL, POST URL, CBOR, Rkyv) (#1868, #1989)
support for streaming responses from server functions (#1284)
ability to create custom user encodings for input and output by deriving IntoReq, FromReq, IntoRes, and/or FromRes traits
ability to mix and match encodings easily: This server function should be a JSON POST request and a ByteStream response, this one should be GET URL request and an rkyv response, etc.; any combination of the encodings above is supported
custom error types (#1657)
a #[middleware] macro to add per-server-function middleware from the Tower or Actix ecosystems (#1461)

Note: The additional included encodings (serde_lite, rkyv, multipart form data) are all enabled by additive features on the server_fn crate. If you want to use them you can just add that crate as a dependency and enable the required features.

Example: You can find a comprehensive example of these new features in the new server_fns_axum example.

Full Changelog: v0.5.7...0.6.2