Support Future Transformation for FluentBuilders

[jinyoung-cs]

Describe the feature

aws-sdk-rust's codegen produces FluentBuilders which provide future-like builders which must be send() off in order to be awaited. Arguably the send itself is boilerplate since the builder can be treated like a future. I am proposing to implement IntoFuture for all builders.

Use Case

Remove boiler plate.

Proposed Solution

According to rust docs: https://doc.rust-lang.org/std/future/trait.IntoFuture.html

The .await keyword desugars into a call to IntoFuture::into_future first before polling the future to completion. IntoFuture is implemented for all T: Future which means the into_future method will be available on all futures.

Therefore this is the proposed solution

impl IntoFuture for FooFluentBuilder {
    type Output = Result<FooOutput, SdkError<FooError, HttpResponse>>;
    type IntoFuture = Pin<Box<dyn Future<Output = Self::Output>>>;

    fn into_future(self) -> Self::IntoFuture {
        Box::pin(self.send())
    }
}

Key notes: This has somewhat a trivial performance consequence as this is making a heap alloc for the future along side a dynamic network call. If such a penalty is not acceptable, then send may still be used as an alternative. If the sdk team would not like to introduce a disparity between the two calls then RFC 2515, "Permit impl Trait in type aliases" is required before implementation.

Proposed implementation would be the implementation linked directly above, and reimplementing with RFC 2515 when stabilized. Since the current implementation returns an opaque future, introduction of RFC 2515 shouldn't be a breaking change.

Other Information

This maintains backwards compatibility and allows eventual deprecation of send if such a breaking change is desired, and also allows new usage of immediately awaiting the builder.

Acknowledgements

• I may be able to implement this feature request
• This feature might incur a breaking change

A note for the community

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