Cannot Borrow As Mutable

Rust Mutable Borrow Error

Understanding the Error Message

The Rust error message "Cannot borrow `row` as mutable as it is behind a `&` borrow" indicates an attempt to mutate data that is currently borrowed as immutable. In Rust, data can be borrowed as either mutable or immutable, and attempting to borrow data as mutable while it is already borrowed as immutable results in this error.

Mutable Borrowing in Loops

The error often occurs when attempting to mutate data within a loop where the data is borrowed as immutable. For example, consider the following code: ```rust for row in &data { row.value = 10; } ``` In this code, we are attempting to mutate the `value` field of each row in the `data` slice. However, the slice is borrowed as immutable, so we are not allowed to mutate its contents. To fix this error, we can borrow the slice as mutable instead: ```rust for row in &mut data { row.value = 10; } ``` This will allow us to mutate the data within the loop.

Mutable References

Mutable data can also be borrowed using mutable references. A mutable reference is written as `&mut T`, where `T` is the type of the data being borrowed. Mutable references give the borrower read-write access to the data, but they must be used with caution as they can lead to data races if not handled carefully.