Understanding the distinction between functions and closures is fundamental to mastering Rust's ownership system and performance characteristics.

Key Differences

Functions	Closures
Defined at compile time with `fn`	Anonymous, created at runtime
Static dispatch (no runtime overhead)	May involve dynamic dispatch (trait objects)
Cannot capture environment variables	Can capture variables from enclosing scope
Always have a known type	Type is unique and inferred (each closure has its own type)

Underlying Mechanics

Closures Are Structs + Traits

Rust models closures as structs that:

Store captured variables (as fields)
Implement one of the closure traits (Fn, FnMut, or FnOnce)

For example, this closure:

let x = 42;
let closure = |y| x + y;

Is desugared to something like:

struct AnonymousClosure {
    x: i32,  // Captured variable
}

impl FnOnce<(i32,)> for AnonymousClosure {
    type Output = i32;
    fn call_once(self, y: i32) -> i32 {
        self.x + y
    }
}

Dynamic Dispatch (Vtables)

When closures are trait objects (e.g., Box<dyn Fn(i32) -> i32>), Rust uses vtables for dynamic dispatch:

Vtable: A lookup table storing function pointers, enabling runtime polymorphism
Overhead: Indirect function calls (~2–3x slower than static dispatch)

When to Use Each

Use Functions when:

You need zero-cost abstractions (e.g., mathematical operations)
No environment capture is required

fn add(a: i32, b: i32) -> i32 { a + b }

Use Closures when:

You need to capture state from the environment
Writing short, ad-hoc logic (e.g., callbacks, iterators)

let threshold = 10;
let filter = |x: i32| x > threshold;  // Captures `threshold`

Performance Considerations

Scenario	Static Dispatch (Closures)	Dynamic Dispatch (dyn Fn)
Speed	Fast (inlined)	Slower (vtable lookup)
Memory	No overhead	Vtable + fat pointer
Use Case	Hot loops, embedded	Heterogeneous callbacks

Example: Static vs. Dynamic Dispatch

// Static dispatch (compile-time)
fn static_call<F: Fn(i32) -> i32>(f: F, x: i32) -> i32 {
    f(x)  // Inlined
}

// Dynamic dispatch (runtime)
fn dynamic_call(f: &dyn Fn(i32) -> i32, x: i32) -> i32 {
    f(x)  // Vtable lookup
}

Key Takeaways

✅ Functions: Predictable performance, no captures
✅ Closures: Flexible, capture environment, but may involve vtables
🚀 Prefer static dispatch (impl Fn) unless you need trait objects

Try This: What happens if a closure captures a mutable reference and is called twice?
Answer: The borrow checker ensures exclusive access—it won't compile unless the first call completes!

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Functions or Closures in Rust? Know the Difference!