rhai/doc/src/language/statements.md

Statements

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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 if the statement contains a block itself (e.g. the if, while, for and loop statements).

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 (although it also works with it)
//                ^ semicolon required here to terminate the assignment statement; it is a syntax error without it

if foo { a = 42 }
//               ^ there is no need to terminate an if-statement with a semicolon

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.

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