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:

Aspect	Closures	Regular Functions
Dispatch	Static (via monomorphization)	Always static (direct call)
Memory	May store captured data (size varies)	No captured data (fixed size)
Heap Allocation	Only if boxed (Box)	Never
Optimization	Inlined aggressively	Inlined aggressively

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

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`
}

Key Takeaways

✅ 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.

Try This: Compare the assembly output of a closure and a function with cargo rustc -- --emit asm!

#static-dispatch

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