Async version of LazyLock

[tisonkun]

I remember I don't provide LazyLock before because Deref's method is not async.

But perhaps I can try to figure out an async LazyLock with a named async getter but not implement Deref.

Originally posted by @tisonkun in apache/opendal#8108 (comment)

[mhambre]

So there's an existing library that has this core idea implemented but it's older so it may either be complete or dead:

Here's an example:

use async_once_cell::Lazy;
use std::future::Future;
use std::pin::Pin;

type BoxFut<T> = Pin<Box<dyn Future<Output = T> + Send>>;

struct Service {
    client: Lazy<DbClient, BoxFut<DbClient>>,
}

impl Service {
    fn new(url: String, creds: Credentials) -> Self {
        Self {
            client: Lazy::new(Box::pin(async move {
                // `url` and `creds` are moved into the future
                DbClient::connect(&url, creds).await.expect("connect failed")
            })),
        }
    }

    async fn client(&self) -> &DbClient {
        self.client.get_unpin().await
    }
}

We might be able to implement this similarly. In reference to usage in OpenDAL the only problem would be that for the issues this came about because of this we'd need to be able to construct an owned instance of the OperatorContext (e.g. for the HTTP transporter).

[mhambre]

Thinking about it a bit more, we should definitely make sure to include try semantics for Error types since the major use case for this would be IO-based futures susceptible to blips that should be retriable at the user's discretion.

I can try taking a swing at it.

[tisonkun]

@mhambre Brilliant! Just go ahead :D