Statements

Terminated by ;

Statements are terminated by semicolons ; and they are mandatory, except for the last statement in a block (enclosed by { .. } pairs) where it can be omitted.

Semicolons can also be omitted for statement types that always end in a block – for example the if, while, for and loop statements.


#![allow(unused)]
fn main() {
let a = 42;             // normal assignment statement
let a = foo(42);        // normal function call statement
foo < 42;               // normal expression as statement

let a = { 40 + 2 };     // 'a' is set to the value of the statement block, which is the value of the last statement
//              ^ the last statement does not require a terminating semicolon (but also works with it)
//                ^ semicolon required here to terminate the 'let' statement
//                  it is a syntax error without it, even though it ends with '}'
//                  that is because the 'let' statement doesn't end in a block

if foo { a = 42 }
//               ^ no need to terminate an if-statement with a semicolon
//                 that is because the 'if' statement ends in a block

4 * 10 + 2              // a statement which is just one expression - no ending semicolon is OK
                        // because it is the last statement of the whole block
}

Statement Expression

A statement can be used anywhere where an expression is expected. These are called, for lack of a more creative name, “statement expressions.”

The last statement of a statement block is always the block’s return value when used as a statement, regardless of whether it is terminated by a semicolon or not. This is different from Rust where, if the last statement is terminated by a semicolon, the block’s return value is taken to be ().

If the last statement has no return value (e.g. variable definitions, assignments) then it is assumed to be ().