Rust's Stateful Closures: Passing and Mutating Across Multiple Calls

To pass a closure to a Rust function that needs to call it multiple times while maintaining state between calls, the closure must implement the FnMut trait to allow mutation of its captured environment. I’ll explain how to design this, using Rust’s ownership, traits, and lifetimes, and highlight when to use simple closures versus structured approaches.

Solution: Use FnMut and Mutable Closure

A closure that mutates state must implement FnMut, which allows multiple calls with mutable access to captured variables. The function receiving the closure takes it as &mut impl FnMut to retain ownership while enabling mutation.

Example:

fn call_repeatedly<F: FnMut() -> i32>(f: &mut F) {
    println!("First call: {}", f());  // 1
    println!("Second call: {}", f()); // 2
}

fn main() {
    let mut counter = 0; // State stored outside the closure
    let mut closure = || {
        counter += 1; // Mutates captured state → `FnMut`
        counter
    };
    
    // Pass as `&mut closure` to retain ownership
    call_repeatedly(&mut closure);
    // closure can still be used here
    println!("After: {}", closure()); // 3
}

Key Mechanics

• Mutable State: The closure captures counter via a mutable borrow (&mut i32). The closure itself is declared mut to allow mutation.
• Function Signature: fn call_repeatedly<F: FnMut() -> i32>(f: &mut F) ensures the closure can be called multiple times with mutable access.
• Lifetime Safety: The closure borrows counter, so it cannot outlive counter, enforced by Rust’s borrow checker.

Alternative: Encapsulate State in a Struct

For complex state, encapsulate it in a struct with an explicit FnMut implementation:

struct Counter {
    count: i32,
}

impl Counter {
    fn new() -> Self {
        Counter { count: 0 }
    }
    
    fn call(&mut self) -> i32 {
        self.count += 1;
        self.count
    }
}

fn main() {
    let mut counter = Counter::new();
    call_repeatedly(|| counter.call()); // Closure captures `counter`
    println!("After: {}", counter.call()); // Continues state
}

Why Not FnOnce or Fn?

• FnOnce: Can only be called once, consuming the closure. Unsuitable for multiple calls.
• Fn: Uses immutable borrows, preventing state mutation, so it can’t modify captured variables.

Pitfalls

• Forgetting mut:

  let closure = || { /* ... */ }; // Not `mut` → compile error
  call_repeatedly(&mut closure);
  

  The closure and parameter must be mut to implement FnMut.
• Dangling References: Ensure captured variables live as long as the closure. For example:

  fn bad() -> impl FnMut() -> i32 {
      let counter = 0;
      || { counter += 1; counter } // ERROR: `counter` doesn’t live long enough
  }
  

Key Takeaways

✅ Use FnMut for closures that mutate state across multiple calls.
✅ Mark closures and parameters as mut to enable mutation.
✅ Prefer simple closures for basic state; use structs for complex state management.

Real-World Example: Stateful closures are common in event loops or async tasks (e.g., tokio) where a closure maintains counters or buffers across iterations.

Experiment: Try passing a non-mut closure to call_repeatedly.
Answer: Compile error! The closure must be mutable to implement FnMut.