Скачать или смотреть Understanding When to Make a Closure mut in Rust

Discover why you need to declare closures as `mut` in Rust when modifying captured variables and learn how to resolve errors related to mutable borrows.
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Understanding When to Make a Closure mut in Rust

Rust is known for its strict rules regarding memory safety and ownership, which often leads to errors that can be puzzling for developers. One such scenario arises when dealing with closures and mutable variables. You might encounter a compile-time error that makes you question why a closure must be declared as mutable even if you do not intend to change its definition. In this guide, we will unpack the key concepts behind this issue and provide a detailed explanation of how closures work with mutable references.

The Problem: Mutable Borrowing Error

Consider the following Rust code:

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When you compile this code, you are likely to face an error such as:

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Why Does This Error Occur?

The message prompts you to consider changing the definition of c to let mut c. But why is this necessary? After all, you're not modifying the closure itself, just executing it. To answer this, we need to delve deeper into how closures operate in Rust.

The Explanation: Closures as Structures

In Rust, a closure can be seen as a struct that captures its environment. When a closure captures a variable, it holds a reference to it, and each captured variable becomes a field in the closure's struct.

Understanding the Structural Equivalent

The code snippet above can be roughly translated into a struct that represents the closure:

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This code defines a struct S that takes a mutable reference to a string. The closure c is essentially an instance of this struct. If you attempt to call c.call(), the compiler will throw the same mutable borrowing error unless you declare c as mutable:

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

The Root of the Issue: Ownership and References

You might wonder why it's necessary to define methods that interact with mutable references using &mut. If you don't, like in this line:

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You will run into another similar error:

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This reinforces the key rule in Rust's semantics:

Rust's Ownership Rules

At any given time, you can have either one mutable reference or any number of immutable references. Thus, without the mut declaration, borrowing a mutable reference through an immutable one is not allowed, which prevents potential data races and ensures memory safety.

Conclusion: The Importance of mut in Closures

When you define a closure that captures a mutable variable, you must also declare the closure itself as mut. This is because the closure, when called, attempts to borrow its captured variables mutably. By following this rule, you can ensure that your Rust code compiles successfully and operates safely.

Understanding the patterns of ownership, borrowing, and mutability in Rust can be challenging, but with practice and careful attention to these principles, you will master the art of writing robust and efficient Rust programs.