r[expr.paren]

Grouped expressions

r[expr.paren.syntax]

GroupedExpression -> `(` Expression `)`

r[expr.paren.intro] 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 ).

r[expr.paren.evaluation] Parenthesized expressions evaluate to the value of the enclosed operand.

r[expr.paren.place-or-value] 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.

r[expr.paren.override-precedence] Parentheses can be used to explicitly modify the precedence order of subexpressions within an expression.

An example of a parenthesized expression:

let x: i32 = 2 + 3 * 4; // not parenthesized
let y: i32 = (2 + 3) * 4; // parenthesized
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:

# 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");