defer

Schedules a block to run when the enclosing function exits.

Deferred blocks run on every return, on falling off the end, and while a panic or task cancellation unwinds, in reverse order of the defer statements that were reached. The block sees outer locals only when they are listed in defer use (…).

fn work(Vec<string> log) -> int {
    log.push("start");
    defer use (log) {
        log.push("cleanup");
    }
    return log.len(); // 1: the defer runs after the value is computed
}

fn main() {
    let log: Vec<string> = Vec::new();
    let n = work(log); // log is now ["start", "cleanup"]
}

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.