The require function

This function creates a custom constraint in any place of your code. Using require() you can check whether some condition is true and make the application exit with an error if otherwise.

The full function description is here.

Example

const BAD_VALUE: u8 = 42;

fn wrong(a: u8, b: u8) -> u8 {
    let c = a + b - BAD_VALUE;
    require(a + b == c, "always fails");
    c
}