replace #[async_trait] with native async trait for trait Resolver

[AbnerZheng]

Refer to: #6657

The native async trait in Rust has a limitation where it requires explicit declaration if it implements the Send or non-Send version of a trait.

For instance, consider the following trait:

trait HttpService {
    async fn fetch(&self, url: Url) -> HtmlBody;
}

This desugars to:

trait HttpService {
    fn fetch(&self, url: Url) -> impl Future<Output = HtmlBody>;
}

By default, the returned Future does not have the Send bound, which is necessary for a multithreaded runtime. Without using trait_variant to create a multithreaded version of the trait, we encounter errors as below indicating that the future cannot be sent between threads safely.

error: future cannot be sent between threads safely
   --> crates/net/dns/src/query.rs:62:51
    |
62  |         self.queued_queries.push_back(Query::Root(Box::pin(resolve_root(resolver, link, timeout))))
    |                                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ future returned by `resolve_root` is not `Send`
    |
    = help: within `impl Future<Output = Result<(TreeRootEntry, LinkEntry<K>), (LookupError, LinkEntry<K>)>>`, the trait `Send` is not implemented for `impl Future<Output = std::option::Option<std::string::String>>`
note: future is not `Send` as it awaits another future which is not `Send`

For more details on this issue, refer to the following resources:
https://blog.rust-lang.org/inside-rust/2023/05/03/stabilizing-async-fn-in-trait.html#mvp-part-2-send-bounds-and-associated-return-types
https://blog.rust-lang.org/2023/12/21/async-fn-rpit-in-traits.html#async-fn-in-public-traits

On the other hand, by utilizing the #[async_trait] attribute, async functions are transformed into methods that return Pin<Box<dyn Future + Send + 'async_trait>>. This results in a Future that is thread-safe by default.

[AbnerZheng]

So it is beneficial to introduce #[trait_variant] to replace #[aync_trait]?

[AbnerZheng]

By using -> impl trait, we can avoid using #[trait_variant]. For example we can rewrite the Resolver as below:

pub trait Resolver: Send + Sync + Unpin + 'static {
    /// Performs a textual lookup and returns the first text
    fn lookup_txt(&self, query: &str) -> impl Future<Output = Option<String>> + Send;
}

By adding bound Send manually, we can avoid the error mentioned above. And when implementing the trait Resolver, we can either use async fn or ->impl trait, both are ok.

impl<P: ConnectionProvider> Resolver for AsyncResolver<P> {
    async fn lookup_txt(&self, query: &str) -> Option<String> {
      ...
    }
}

impl<P: ConnectionProvider> Resolver for AsyncResolver<P> {
    fn lookup_txt(&self, query: &str) -> impl Future<Output=Option<String>> + Send {
        todo!()
    }
}

[AbnerZheng]

WDYT? @mattsse