RAII & Drop Guards

Q: forget guard?

mem::forget leaks resource intentionally; document why.

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

Alternative	Use When	Don't Use When
`try/finally` manual	abort builds critical path	Normal Rust RAII
`scopeguard` crate	Quick closure guard	Custom type clearer
Explicit `close()`	Async shutdown	Sync file handles

FAQs

Drop on panic?

Yes with unwinding; use catch_unwind at boundary if needed.

Manually drop early?

drop(guard) before scope end.

Drop and Copy?

Copy types can implement Drop with care; rare pattern.

union Drop order fields?

Use nested guards or single struct owning multiple resources.

Avoid logging in Drop that may panic; keep Drop infallible.

transaction rollback?

DB transaction guard rolls back unless committed.

metrics timer guard?

Record duration in Drop of timer struct.

forget guard?

mem::forget leaks resource intentionally; document why.

Send/Sync guard?

Guard fields determine auto traits; watch RefCell in guard.

test Drop?

Use flag atomics or counters asserted after scope.

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

Standard Library (std) Docs - Complete API reference for the standard library.
The Rustonomicon - Deep dive into unsafe Rust.
The Rust Programming Language (The Book) - Canonical beginner-to-intermediate guide.
Programming Rust: Fast, Safe Systems Development (Amazon) - Blandy, Orendorff, Tindall.
Rust for Rustaceans (Amazon) - Jon Gjengset.

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+.

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

Alternative	Use When	Don't Use When
`try/finally` manual	abort builds critical path	Normal Rust RAII
`scopeguard` crate	Quick closure guard	Custom type clearer
Explicit `close()`	Async shutdown	Sync file handles

FAQs

Drop on panic?

Yes with unwinding; use catch_unwind at boundary if needed.

Manually drop early?

drop(guard) before scope end.

Drop and Copy?

Copy types can implement Drop with care; rare pattern.

union Drop order fields?

Use nested guards or single struct owning multiple resources.

Avoid logging in Drop that may panic; keep Drop infallible.

transaction rollback?

DB transaction guard rolls back unless committed.

metrics timer guard?

Record duration in Drop of timer struct.

forget guard?

mem::forget leaks resource intentionally; document why.

Send/Sync guard?

Guard fields determine auto traits; watch RefCell in guard.

test Drop?

Use flag atomics or counters asserted after scope.

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

Standard Library (std) Docs - Complete API reference for the standard library.
The Rustonomicon - Deep dive into unsafe Rust.
The Rust Programming Language (The Book) - Canonical beginner-to-intermediate guide.
Programming Rust: Fast, Safe Systems Development (Amazon) - Blandy, Orendorff, Tindall.
Rust for Rustaceans (Amazon) - Jon Gjengset.

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+.

RAII & Drop Guards

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RAII & Drop Guards