What is the difference between fn and Fn in Rust?

Question

Accepted Answer

**fn** — function pointer. **Fn** — closure trait.

**fn:**
- Function pointer type.
- Named function.
- No environment capture.

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

let f: fn(i32, i32) -> i32 = add;
```

**Fn (closure traits):
- Closure traits.
- Can capture environment.
- Fn, FnMut, FnOnce.

```rust
let x = 5;

let closure = |y| x + y;  // Fn
println!("{}", closure(10));  // 15
```

**Key differences:**
- **fn** — function pointer.
- **Fn** — closure trait.
- **fn** — no capture.
- **Fn** — can capture.

Rust: What is the difference between fn and Fn in Rust?