Using closures versus regular functions ?

Performance Overhead

Closures in Rust have zero runtime overhead in most cases due to static dispatch and compiler optimizations. However, specific scenarios can introduce costs:

When Closures May Be Less Efficient

Heap-Allocated Trait Objects (Box)

Using dynamic dispatch (e.g., Box) adds overhead:

• Vtable Lookups: Indirect calls via function pointers.
• Cache Misses: Fat pointers (data + vtable) reduce locality.

let closures: Vec<Box<dyn Fn(i32) -> i32>> = vec![
    Box::new(|x| x + 1),
    Box::new(|x| x * 2),
]; // Heap-allocated, slower to call

Large Captured Environments

Closures storing large structs (e.g., 1KB buffer) increase memory usage and may inhibit inlining:

let data = [0u8; 1024]; // 1KB array
let closure = move || data.len(); // Closure size = 1KB + overhead

A closure is just an anonymous struct holding its captures, so "how expensive is it" is really a question about that struct's layout:

Excessive Monomorphization

Generic closures with many instantiations (e.g., in a hot loop) can bloat binary size:

(0..1_000).for_each(|i| { /* Unique closure per iteration */ });

Zero-Cost Abstractions in Practice

Static Dispatch (impl Fn)

Closures are as fast as regular functions when:

• Captured data is small (e.g., primitives).
• Monomorphization doesn't cause code bloat.

let add = |x, y| x + y; // Same ASM as `fn add(x: i32, y: i32) -> i32`

Example: Inlining

fn main() {
    let x = 5;
    let closure = || x * 2; // Inlined → no function call
    println!("{}", closure()); // ASM: `mov eax, 10`
}

What the overhead actually is

✅ Use impl Fn for zero-cost static dispatch. 🚫 Avoid Box in performance-critical code. ⚠️ Optimize large captures: Prefer borrowing or minimizing captured data.

Real-World Impact

• rayon uses closures with static dispatch for parallel iterators (no overhead).
• GUI frameworks like iced leverage closures for event handlers efficiently.

If you want to see the zero-cost claim rather than take my word for it: cargo rustc -- --emit asm and diff the closure against the equivalent function.