requires

States a precondition, checked when the function is entered: requires cond, "message".

A failed requires panics and blames the caller (the message names the call site). Clauses come after the signature and before the body. Contextual: only a keyword in that position.

fn isqrt(int n) -> int
    requires n >= 0, "negative input"
{
    let r = 0;
    while (r + 1) * (r + 1) <= n {
        r = r + 1;
    }
    return r;
}

See also#

• fn: Declares a function, a method, or (written in an expression) an anonymous function.
• let: Declares a mutable local variable.
• const: Declares a binding that cannot be reassigned: a local, a module constant, or a class field that is set only by new.
• static: Declares a module-level or class-level slot shared by the whole program, or a method without self.
• readonly: A read-only view of a value: nothing reachable through it can be changed.
• if: Runs a block when a bool condition is true.
• else: Introduces the branch of an if that runs when its condition is false (or its pattern does not match).
• while: Repeats a block while a bool condition is true.