Pattern	When	Implementation
RAII	Auto cleanup	`Drop` trait
Lazy init	Deferred creation	`OnceLock`, `LazyLock`
Pool	Reuse expensive resources	`r2d2`, `deadpool`
Guard	Scoped access	`MutexGuard` pattern
Scope	Transaction boundary	Custom struct + Drop

Thinking Prompt

Before designing lifecycle:

What's the resource cost?
- Cheap → create per use
- Expensive → pool or cache
- Global → lazy singleton
What's the scope?
- Function-local → stack allocation
- Request-scoped → passed or extracted
- Application-wide → static or Arc
What about errors?
- Cleanup must happen → Drop
- Cleanup is optional → explicit close
- Cleanup can fail → Result from close

Trace Up ↑

To domain constraints (Layer 3):

"How should I manage database connections?"
    ↑ Ask: What's the connection cost?
    ↑ Check: domain-* (latency requirements)
    ↑ Check: Infrastructure (connection limits)

Question	Trace To	Ask
Connection pooling	domain-*	What's acceptable latency?
Resource limits	domain-*	What are infra constraints?
Transaction scope	domain-*	What must be atomic?

Trace Down ↓

To implementation (Layer 1):

"Need automatic cleanup"
    ↓ m02-resource: Implement Drop
    ↓ m01-ownership: Clear owner for cleanup

"Need lazy initialization"
    ↓ m03-mutability: OnceLock for thread-safe
    ↓ m07-concurrency: LazyLock for sync

"Need connection pool"
    ↓ m07-concurrency: Thread-safe pool
    ↓ m02-resource: Arc for sharing

Quick Reference

Pattern	Type	Use Case
RAII	`Drop` trait	Auto cleanup on scope exit
Lazy Init	`OnceLock`, `LazyLock`	Deferred initialization
Pool	`r2d2`, `deadpool`	Connection reuse
Guard	`MutexGuard`	Scoped lock release
Scope	Custom struct	Transaction boundaries

Lifecycle Events

Event	Rust Mechanism
Creation	`new()`, `Default`
Lazy Init	`OnceLock::get_or_init`
Usage	`&self`, `&mut self`
Cleanup	`Drop::drop()`

Pattern Templates

RAII Guard

struct FileGuard {
    path: PathBuf,
    _handle: File,
}

impl Drop for FileGuard {
    fn drop(&mut self) {
        // Cleanup: remove temp file
        let _ = std::fs::remove_file(&self.path);
    }
}

Lazy Singleton

use std::sync::OnceLock;

static CONFIG: OnceLock<Config> = OnceLock::new();

fn get_config() -> &'static Config {
    CONFIG.get_or_init(|| {
        Config::load().expect("config required")
    })
}

Common Errors

Error	Cause	Fix
Resource leak	Forgot Drop	Implement Drop or RAII wrapper
Double free	Manual memory	Let Rust handle
Use after drop	Dangling reference	Check lifetimes
E0509 move out of Drop	Moving owned field	`Option::take()`
Pool exhaustion	Not returned	Ensure Drop returns

Anti-Patterns

Anti-Pattern	Why Bad	Better
Manual cleanup	Easy to forget	RAII/Drop
`lazy_static!`	External dep	`std::sync::OnceLock`
Global mutable state	Thread unsafety	`OnceLock` or proper sync
Forget to close	Resource leak	Drop impl

Related Skills

When	See
Smart pointers	m02-resource
Thread-safe init	m07-concurrency
Domain scopes	m09-domain
Error in cleanup	m06-error-handling

m12-lifecycle

Resource Lifecycle

Core Question

Lifecycle Pattern → Implementation