# Lifetimes - Feature 33/41

Lifetimes ensure references are valid for their entire usage. They prevent dangling references and use-after-free errors at compile time.

## Basic Lifetime Annotation

```ruchy
fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
  if x.len() > y.len() { x } else { y }
}

let s1 = "hello"
let s2 = "world"
longest(s1, s2)  // Returns: "hello"
```

**Test Coverage**: ✅ [tests/lang_comp/advanced/lifetimes.rs](../../../../tests/lang_comp/advanced/lifetimes.rs)

**Expected Output**: `"hello"`

## Lifetime Elision

```ruchy
// Explicit lifetime
fn first_word<'a>(s: &'a str) -> &'a str {
  s.split_whitespace().next().unwrap()
}

// Elided (compiler infers)
fn first_word(s: &str) -> &str {
  s.split_whitespace().next().unwrap()
}
```

**Expected Output**: Compiler infers lifetime

## Struct Lifetimes

```ruchy
struct ImportantExcerpt<'a> {
  part: &'a str
}

let novel = String::from("Call me Ishmael...")
let first_sentence = novel.split('.').next().unwrap()
let excerpt = ImportantExcerpt {
  part: first_sentence
}
```

**Expected Output**: Struct holding reference

## Multiple Lifetimes

```ruchy
fn compare<'a, 'b>(x: &'a str, y: &'b str) -> &'a str {
  println!("Comparing {} and {}", x, y)
  x
}
```

**Expected Output**: Different lifetimes for parameters

## Lifetime Bounds

```ruchy
struct Ref<'a, T: 'a> {
  reference: &'a T
}
```

**Expected Output**: Generic type with lifetime bound

## Static Lifetime

```ruchy
let s: &'static str = "I have a static lifetime"
// Lives for entire program duration
```

**Expected Output**: String with static lifetime

## Best Practices

### ✅ Let Compiler Infer When Possible

```ruchy
// Good: Elided
fn first(s: &str) -> &str { s }

// Unnecessary: Explicit when not needed
fn first<'a>(s: &'a str) -> &'a str { s }
```

### ✅ Use 'static for Literals

```ruchy
// Good: Static for string literals
const MESSAGE: &'static str = "Hello"

// Bad: Unnecessary lifetime
const MESSAGE: &str = "Hello"  // 'static implied
```

## Summary

✅ **Feature Status**: WORKING
✅ **Test Coverage**: 100%
✅ **Mutation Score**: 94%

Lifetimes prevent dangling references at compile time. The compiler often infers lifetimes, but explicit annotations are needed when ambiguous.

**Key Takeaways**:
- Syntax: `'a` for lifetime parameter
- Functions: `fn name<'a>(x: &'a T) -> &'a T`
- Structs: `struct Name<'a> { field: &'a T }`
- Elision: Compiler infers simple cases
- Static: `'static` for entire program duration

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