docs(examples): Update web_api example to use async await

[messense]

Ref #1849

[messense]

This does not compile yet, error:

error[E0495]: cannot infer an appropriate lifetime for borrow expression due to conflicting requirements
   --> examples/web_api.rs:118:40
    |
118 |                 response_examples(req, &client)
    |                                        ^^^^^^^
    |
note: first, the lifetime cannot outlive the lifetime '_ as defined on the body at 117:46...
   --> examples/web_api.rs:117:46
    |
117 |             Ok::<_, GenericError>(service_fn(move |req| {
    |                                              ^^^^^^^^^^
note: ...so that closure can access `client`
   --> examples/web_api.rs:118:40
    |
118 |                 response_examples(req, &client)
    |                                        ^^^^^^^
note: but, the lifetime must be valid for the expression at 117:35...
   --> examples/web_api.rs:117:35
    |
117 |             Ok::<_, GenericError>(service_fn(move |req| {
    |                                   ^^^^^^^^^^
note: ...so type `fn([closure@examples/web_api.rs:117:46: 119:14 client:hyper::Client<hyper::client::HttpConnector>]) -> hyper::service::service::ServiceFn<[closure@examples/web_api.rs:117:46: 119:14 client:hyper::Client<hyper::client::HttpConnector>], hyper::Body> {hyper::service::service_fn::<[closure@examples/web_api.rs:117:46: 119:14 client:hyper::Client<hyper::client::HttpConnector>], hyper::Body, impl std::future::Future>}` of expression is valid during the expression
   --> examples/web_api.rs:117:35
    |
117 |             Ok::<_, GenericError>(service_fn(move |req| {
    |                                   ^^^^^^^^^^

error: aborting due to previous error

error: Could not compile `hyper`.

Not sure how to fix it.

[seanmonstar]

I think it's complaining that the future returned in service_fn has a borrow on &client, but the service_fn closure owns client. So, if the service_fn were dropped after return a future but before that future started, the borrow would be invalid.

It's holding a borrow because any references passed to an async fn are held for the duration of the returned future.

You could either pass in clones of the client, or change the async fns to be normal functions, and use an internal async block. Like:

let fut = client.request(req);
async move {
    let res = fut.await?;
}

[weihanglo]

@messense
The lifetime issue was fixed on #1872. Just clone the client again as @seanmonstar said.

[messense]

Closing in favor of #1872