# async/await (Rust)

> async functions, Future trait, tokio runtime, async blocks, .await syntax, Streams, Pin<T>, cancellation, executor patterns

- 22 interview questions
- Mid-Level
- [Interview Questions: Rust](https://sharpskill.dev/en/technologies/rust/interview-questions.md)

## 1. What does a function declared with the async keyword return in Rust?

**Answer**

An async function automatically returns a type that implements the Future trait. The compiler transforms the function body into a state machine that implements Future. The return value is not computed immediately: it will only be calculated when the Future is awaited with .await or executed by a runtime.

## 2. What is the main role of the Future trait in Rust?

**Answer**

The Future trait represents an asynchronous computation that may not be finished yet. Its poll method allows the executor to check if the Future is ready (Poll::Ready) or needs to wait (Poll::Pending). This is the fundamental mechanism of asynchrony in Rust, enabling non-blocking and efficient execution.

## 3. Why is a runtime like tokio necessary to execute async code in Rust?

**Answer**

Rust does not include an async runtime in its standard library to keep the language minimal and allow choosing the runtime based on needs. A runtime like tokio provides the executor that polls Futures, the scheduler to manage tasks, and async I/O. Without a runtime, Futures would never be executed.

## 19 more questions available

- What is the difference between an async block and an async function in Rust?
- What happens when using .await on a Future in Rust?

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## Other Rust interview topics

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