RefCell & Cell

Cell<T> for Copy values without borrow tracking. RefCell<T> runtime-enforces & / &mut rules for non-Copy data.

Recipe

use std::cell::Cell;
 
fn main() {
    let c = Cell::new(0);
    c.set(c.get() + 1);
    println!("{}", c.get());
}

When to reach for this: Mutate through shared handle in single-threaded code when borrow checker blocks.

Working Example

use std::cell::RefCell;
 
struct Cache {
    hits: RefCell<u32>,
}
 
impl Cache {
    fn hit(&self) {
        *self.hits.borrow_mut() += 1;
    }
    fn hits(&self) -> u32 {
        *self.hits.borrow()
    }
}
 
fn main() {
    let c = Cache { hits: RefCell::new(0) };
    c.hit();
    println!("{}", c.hits());
}

What this demonstrates:

• &self methods mutating interior via RefCell
• borrow / borrow_mut runtime checks
• Panic on double borrow_mut

Deep Dive

try_borrow for non-panicking. Ref/RefMut guards deref to inner. Not Sync - single thread.

Gotchas

• Panic on reentrant borrow - Runtime borrow conflict. Fix: Shorter borrows, restructure.
• RefCell across threads - Not Sync. Fix: Mutex.
• Cell for non-Copy - Use RefCell or UnsafeCell (unsafe).
• Holding RefMut too long - Blocks other borrows. Fix: Scope limits.
• RefCell as first choice - Try compile-time borrows first.

Alternatives

FAQs

Related

• Rc<RefCell<T>> & Arc<Mutex<T>>
• Fighting the Borrow Checker
• Rc & Arc
• Mutex & RwLock

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.