RAII & Drop Guards

Resource Acquisition Is Initialization: acquire resources in constructors, release in Drop when scope ends. Scope guards run cleanup even on panic (with unwinding).

Recipe

struct TempDirGuard(tempfile::TempDir);
impl Drop for TempDirGuard {
    fn drop(&mut self) { /* tempdir deletes on drop */ }
}

When to reach for this: Locks, files, transactions, metric timers, restoring previous state.

Working Example

struct ResetFlag<'a> { flag: &'a AtomicBool, prev: bool }
 
impl<'a> ResetFlag<'a> {
    fn guard(flag: &'a AtomicBool, value: bool) -> Self {
        let prev = flag.swap(value, Ordering::SeqCst);
        ResetFlag { flag, prev }
    }
}
 
impl Drop for ResetFlag<'_> {
    fn drop(&mut self) {
        self.flag.store(self.prev, Ordering::SeqCst);
    }
}
 
fn work() {
    let _g = ResetFlag::guard(&IN_PROGRESS, true);
    // flag restored when _g drops
}

What this demonstrates:

• _g binding keeps guard alive until scope end
• Drop runs on normal return and panic (unwinding)
• Encapsulate restore logic once
• scopeguard crate for closure-based guards

Deep Dive

defer pattern with scopeguard::guard((), |_| cleanup()). Mutex guards (MutexGuard) are RAII locks. tokio::spawn tasks should use RAII for in-flight counters.

Gotchas

• Drop order - fields dropped in declaration reverse order. Fix: structure fields if order matters.
• panic=abort - no unwinding drop in some builds. Fix: critical cleanup may need finally style explicit paths too for abort builds.
• async Drop - not automatic. Fix: explicit shutdown future or Drop only for sync resources.
• Forgotten _ bind - guard dropped immediately. Fix: let _guard = ... for full scope.
• Deadlock on drop - lock order in nested guards. Fix: consistent lock ordering.

Alternatives

FAQs

Related

• Smart Pointers Basics - Drop trait
• Mutex and RwLock - lock guards
• Common Anti-Patterns - leak patterns
• 30 Unsafe Rules

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.