Grouped expressions

^Syntax
GroupedExpression :
( Expression )

A parenthesized expression wraps a single expression, evaluating to that expression. The syntax for a parenthesized expression is a (, then an expression, called the enclosed operand, and then a ).

Parenthesized expressions evaluate to the value of the enclosed operand. Unlike other expressions, parenthesized expressions are both place expressions and value expressions. When the enclosed operand is a place expression, it is a place expression and when the enclosed operand is a value expression, it is a value expression.

Parentheses can be used to explicitly modify the precedence order of subexpressions within an expression.

An example of a parenthesized expression:

#![allow(unused)]
fn main() {
let x: i32 = 2 + 3 * 4;
let y: i32 = (2 + 3) * 4;
assert_eq!(x, 14);
assert_eq!(y, 20);
}

An example of a necessary use of parentheses is when calling a function pointer that is a member of a struct:

#![allow(unused)]
fn main() {
struct A {
   f: fn() -> &'static str
}
impl A {
   fn f(&self) -> &'static str {
       "The method f"
   }
}
let a = A{f: || "The field f"};

assert_eq!( a.f (), "The method f");
assert_eq!((a.f)(), "The field f");
}