NoReturn

Declares that a function never returns to its caller — it ends the program, loops forever, or transfers control elsewhere. The compiler uses this to check control flow: code after a call to a #NoReturn function is known to be unreachable, and a branch that ends in one satisfies a function that must produce a value.

Syntax

#NoReturn()
func Exit(code: int32);

The attribute takes no arguments — the parentheses are always empty.

Effect on Analysis

Because the compiler knows control never comes back, a #NoReturn call can stand in for a missing return:

#NoReturn()
func Panic(message: Slice<char8>);

func MustParse(text: Slice<char8>) -> int32 {
    if let value = TryParse(text) {
        return value;
    }
    Panic("invalid number");   // no return needed — `Panic` never comes back
}

It is the right annotation for process-exit routines, panic/abort handlers, and infinite event loops. The standard library's Panic is declared this way.

See Also

• Panic — the standard-library function that uses this attribute
• Functions — declarations and return types
• Attributes — the full attribute set