Скачать или смотреть Understanding the Differences Between Closure Returns and Local Closures in Rust

Explore the differences between closure returns and locally generated closures in Rust, along with how these affect multi-threading capabilities and type-safety.
---
This video is based on the question https://stackoverflow.com/q/71158797/ asked by the user 'progquester' ( https://stackoverflow.com/u/7556091/ ) and on the answer https://stackoverflow.com/a/71158942/ provided by the user 'Masklinn' ( https://stackoverflow.com/u/8182118/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: What is the diffrence between closure return by function and generated locally in rust?

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding the Differences Between Closure Returns and Local Closures in Rust

Rust, a systems programming language known for its focus on safety and speed, has unique features like closures that can sometimes lead to confusions, especially when dealing with threading. This guide aims to clarify the difference between closures returned by a function and locally generated closures in Rust, focusing on why one can be sent to a thread while the other cannot.

The Problem: Differentiating Between Two Types of Closures

In Rust, closures are anonymous functions that capture their environment. However, the way these closures are created can influence their traits, particularly their ability to be transferred between threads (Send). The following scenario encapsulates our question:

Closure c1 is created and returned by a function.

Closure c2 is defined locally within the main thread.

Now, when attempting to spawn a new thread that uses c1, we encounter a compilation error, while c2 works perfectly fine.

Code Example

[[See Video to Reveal this Text or Code Snippet]]

Analyzing the Solution: Understanding Traits

To unpack why c1 cannot be sent to a thread while c2 can, we need to delve into the underlying mechanics of Rust's type system and its handling of traits.

Closure Types

Local Closure (c2):

The local closure defined in the main function has a specific type that the compiler recognizes.

Because c2 doesn’t capture any variables or context (it closes over nothing), the Rust compiler can infer that it implements the Send trait. Thus, it can be safely transferred between threads.

Closure Returned by a Function (c1):

When c1 is returned as Box<dyn Fn(&str)>, it introduces the concept of type erasure. In this case, the exact type of c1 is unknown at compile time.

The Rust compiler can only recognize that it has the Fn trait but has no assurance that c1 also implements the Send trait, leading to the compilation error when attempting to spawn a thread.

Making c1 Sendable

To extend the functionality of c1 so it can be sent to threads, we need to guarantee that it is indeed Send. This can be achieved by specifying it explicitly in the function signature:

[[See Video to Reveal this Text or Code Snippet]]

This guarantees that c1 can be sent safely to another thread as long as it adheres to the contract established: it has to implement the Send trait.

Error Example

If we try to create c1 with a closure that captures a non-Send variable (like an Rc<T>), Rust will return an error:

[[See Video to Reveal this Text or Code Snippet]]

This results in an error similar to:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

Understanding the difference between closures returned by functions and those created locally is essential when working with Rust, especially in multi-threaded contexts. By being aware of traits and ownership, developers can write safer, more efficient Rust code. Always remember, ensuring your closures are Send can prevent threading errors and unlock the full power of Rust's concurrency model.

With this knowledge, you can confidently work with closures in Rust and leverage their power in multi-threaded applications. Happy coding!