For now, this reference is a best-effort document. We strive for validity and completeness, but are not yet there. In the future, the docs and lang teams will work together to figure out how best to do this. Until then, this is a best-effort attempt. If you find something wrong or missing, file an issue or send in a pull request.

Grouped expressions

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.