How to borrow a field while mutating another field

You cannot borrow one field immutably while mutating another field in the same struct because Rust requires a mutable borrow of the entire struct to modify any part of it. Use RefCell<T> to enable interior mutability, allowing you to borrow fields independently at runtime.

use std::cell::RefCell;

struct Data {
    a: RefCell<i32>,
    b: RefCell<String>,
}

fn main() {
    let data = Data {
        a: RefCell::new(5),
        b: RefCell::new(String::from("hello")),
    };

    let _borrow_a = data.a.borrow();
    data.b.borrow_mut().push_str(" world");
}

Rust prevents you from reading one part of a data structure while changing another to avoid crashes from conflicting access. Think of it like a library book: you can't check out a chapter to read while someone else is writing in the same book. RefCell acts like a librarian who checks these rules at runtime instead of when you write the code, allowing you to read and write different parts safely.