Grouped expressions - The Rust Reference

# Grouped expressions

> Syntax
> GroupedExpression :
> `(` InnerAttribute* Expression `)`

An expression enclosed in parentheses evaluates to the result of the enclosed expression. Parentheses can be used to explicitly specify evaluation order within an expression.

An example of a parenthesized expression:

    #![allow(unused_variables)]
    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_variables)]
    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");
    }

## Group expression attributes

Inner attributes are allowed directly after the opening parenthesis of a group expression in the same expression contexts as attributes on block expressions.