old

The value an expression had when the function was entered, inside an ensures clause: old(e).

Each old(e) is evaluated once, right after the requires checks. Contextual: only special inside ensures.

class Account {
    pub balance: int,
}

impl Account {
    pub fn deposit(int amount)
        requires amount > 0
        ensures self.balance == old(self.balance) + amount
    {
        self.balance = self.balance + amount;
    }
}

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.