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Rust Error E0727: Yield Keyword in Async Context

Understanding Rust Error E0727

Rust Error E0727 occurs when a yield clause is used within an asynchronous context. This is currently not supported within generators, as async and the yield keyword have different behaviors and use cases.

Error E0727 Code Example

The erroneous code below demonstrates using the yield keyword inside an async block:

#![feature(generators)]

fn main() {
    let generator = || {
        async {
            yield;
        }
    };
}

To fix Rust Error E0727, you must move the yield statement out of the async block. In the corrected example below, the yield keyword is moved outside of the async block:

#![feature(generators)]

fn main() {
    let generator = || {
        yield;
    };
}

Reasoning and Constraints

Async functions in Rust return a Future, which represents a value that may not yet be computed. On the other hand, Generators with yield produce an Iterator or Stream to generate new values on-the-fly. Mixing these two constructs is tricky and generally not allowed due to potential inconsistencies and behavior alignment.

When to Use Async or Yield

Select async functions when you're dealing with asynchronous and potentially concurrent computations, like waiting for data from a network request. Meanwhile, use generators along with the yield keyword when you need to lazily generate new values on-the-fly as they are requested by the consumer.

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