if

https://doc.rust-lang.org/book/if.html

カッコが無くても問題ない。

fn main() {
    let x = 5;

    if x == 5 {
        println!("x is five!");
    }
    if (x == 5) {
        println!("x is five!");
    }
}

逆にカッコをつけると警告が・・・

$ cargo run
warning: unnecessary parentheses around `if` condition, #[warn(unused_parens)] on by default
 --> src/main.rs:7:8
  |
7 |     if (x == 5) {
  |        ^^^^^^^^

x is five!
x is five!