^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");
#}

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