Rust error[E0499]: 'cannot borrow `x` as mutable more than once at a time' (and the related E0502 'cannot borrow as immutable because it is also borrowed as mutable'). How do I restructure the code?

Cause: Rust's borrowing rules allow either one mutable reference or any number of shared references to a value at a time. E0499 means two mutable borrows of the same value are alive at once; E0502 means a shared and a mutable borrow overlap. Common shapes: holding a &mut from one method call while calling another &mut method on the same struct, pushing to a Vec while holding a reference to one of its elements, mutating a collection while iterating over it, or two closures that both capture the same variable mutably. Fix (pick the one that matches): 1. Shorten the first borrow: finish using the first reference before taking the second. Since non-lexical lifetimes, a borrow ends at its last use, so often just reordering statements or moving a use earlier fixes it. 2. Copy or clone the small piece you need instead of keeping a reference: let len = v.len(); or let key = item.id.clone(); then mutate. 3. Borrow different fields separately: the compiler lets you borrow self.a and self.b mutably at the same time when you access the fields directly, but not through two &mut self method calls. Split the method or pass the fields as arguments. 4. For two elements of one slice, use split_at_mut or get_many_mut-style helpers instead of indexing twice. 5. For mutating while iterating, collect the changes first (e.g. indices or new items into a Vec) and apply them after the loop, or use retain, iter_mut, or the Entry API for maps. 6. If shared mutable state is truly needed, use interior mutability deliberately (Cell/RefCell in single-threaded code, Mutex/RwLock across threads); RefCell moves the check to run time and panics on violations, so keep borrows short there too. Verify: cargo check passes, and cargo clippy shows no new warnings about needless clones. Sources: https://doc.rust-lang.org/error_codes/E0499.html https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html