Borrowed Data Escapes Outside of Closure Reference in Rust

Understanding the Error

The error "Borrowed data escapes outside of closure reference escapes the closure body here" occurs when a value borrowed immutably (shared) within a closure escapes the closure's lifetime. This means that the closure attempts to reference the borrowed data after its lifetime has ended. In the code snippet mentioned, the issue might arise from a mutable borrow of the `a` and `b` variables within the closure. Additionally, the closure re-borrows the mutable borrow immutably for the `cal` function, potentially causing the borrowed data to escape the closure's lifetime.

Example

Consider the following code: ```rust fn main() { let a = 10; let b = 20; let closure = || { println!("a: {}, b: {}", a, b); }; // This re-borrows a and b immutably for the cal function let cal = || { closure(); }; cal(); // Borrowed data escapes outside of closure reference } ``` In this example, the `closure` borrows `a` and `b` immutably. However, the `cal` function re-borrows `a` and `b` immutably for its own execution. This means that the closure's reference to `a` and `b` escapes the closure's lifetime, leading to the error.

Solution

To resolve the error, ensure that the borrowed data does not escape the closure's lifetime. This can be achieved by either: * Borrowing the data mutably within the closure and ensuring that it is returned before the closure's lifetime ends. * Moving the borrowed data into the closure, ensuring that it remains within its lifetime. For instance, in the example above, the error can be fixed by moving the `a` and `b` variables into the closure: ```rust fn main() { let a = 10; let b = 20; let closure = move || { println!("a: {}, b: {}", a, b); }; closure(); } ``` By moving `a` and `b` into the closure, they become part of the closure's environment and their lifetime is extended to match the closure's lifetime.