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Lifetimes

Lifetimes are the compiler's way of tracking how long references remain valid. They prevent dangling references at compile time - no garbage collector needed. Most lifetimes are inferred automatically through elision rules; explicit annotations ('a) are needed when the compiler cannot determine the relationship between input and output references.

Key Facts

  • Lifetime annotations don't change how long data lives - they describe relationships between references
  • 'a reads as "lifetime a" - a named region of code where a reference is valid
  • Elision rules handle most cases automatically (single input ref, &self methods)
  • 'static = reference valid for entire program (string literals, leaked data)
  • NLL (Non-Lexical Lifetimes): borrow checker analyzes actual usage, not just lexical scopes
  • Structs holding references must declare lifetime parameters: struct Foo<'a> { data: &'a str }

Patterns

Explicit Lifetime Annotations

// Return reference must live as long as BOTH inputs
fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
    if x.len() > y.len() { x } else { y }
}

// Multiple lifetimes when they're independent
fn first_word<'a, 'b>(s: &'a str, _prefix: &'b str) -> &'a str {
    s.split_whitespace().next().unwrap_or("")
}

Lifetime Elision Rules

The compiler infers lifetimes in three cases: 1. Each input reference gets its own lifetime parameter 2. If exactly one input lifetime, it's assigned to all outputs 3. If one input is &self or &mut self, its lifetime is assigned to all outputs

// These are equivalent (rule 2):
fn first(s: &str) -> &str { &s[..1] }
fn first<'a>(s: &'a str) -> &'a str { &s[..1] }

// Method: self's lifetime used for output (rule 3):
impl MyStruct {
    fn name(&self) -> &str { &self.name }
}

Struct Lifetimes

struct Important<'a> {
    content: &'a str,
}

// Struct cannot outlive the reference it holds
let novel = String::from("Call me Ishmael...");
let i = Important { content: &novel };
// novel must live at least as long as i

'static Lifetime

// String literals are always 'static
let s: &'static str = "I live forever";

// 'static as trait bound: type contains no non-static references
fn spawn_thread<T: Send + 'static>(data: T) { /* ... */ }

// Leaking to get 'static (rare, use carefully)
let leaked: &'static str = Box::leak(String::from("leaked").into_boxed_str());

Interior Mutability (Bypassing Borrow Checker at Runtime)

use std::cell::{Cell, RefCell};

// Cell<T> for Copy types - get/set, no references
let cell = Cell::new(5);
cell.set(10);
let val = cell.get();  // 10

// RefCell<T> for any type - runtime borrow checking
let data = RefCell::new(vec![1, 2, 3]);
data.borrow_mut().push(4);     // runtime-checked mutable borrow
let r = data.borrow();          // runtime-checked immutable borrow
// data.borrow_mut();            // PANIC: already borrowed immutably

Gotchas

  • The compiler cannot analyze control flow for borrows: even if true { &mut x } else { &y } may confuse it - restructure code instead
  • HashMap Entry API exists specifically because the borrow checker couldn't handle "lookup then insert" patterns
  • Cell/RefCell are Send but NOT Sync - single-threaded only (use Mutex/RwLock for multi-threaded)
  • 'static doesn't mean "lives forever" as a bound - it means "contains no non-static references" (owned types like String are 'static)

See Also