# Lifetimes

> Annotations like 'a describe how long references stay valid and how outputs relate to inputs; they never extend anything's life.

Concept · Rust · https://levelcode.org/concepts/rust-lifetimes

A **lifetime** is the span of code during which a reference is valid. The compiler infers them inside function bodies. Across function boundaries it only reads signatures, so when a function returns a reference, the signature has to say which input it borrows from:

```rust
fn longest<'a>(a: &'a str, b: &'a str) -> &'a str {
    if a.len() >= b.len() { a } else { b }
}
```

Read it as: the result is valid as long as *both* inputs are. Callers can't use the result after either argument is dropped.

## Elision: when you don't write them

Three rules fill in most lifetimes for you:

1. Each reference parameter gets its own lifetime.
2. If there's exactly one input lifetime, outputs get it.
3. If there's `&self` or `&mut self`, outputs get its lifetime.

So `fn trim(s: &str) -> &str` and `fn name(&self) -> &str` need no annotations. You write them when there are several reference inputs, or when a method's result borrows from something other than `self` — the classic case is a parser struct whose tokens should borrow from the source text, not from the parser.

## 'static

`&'static T` is a reference valid for the whole program (string literals are `&'static str`). The bound `T: 'static`, which `thread::spawn` and `tokio::spawn` require, means something different: the type holds no borrowed data, so owned values like `String` satisfy it. Fix "borrowed value does not live long enough" in spawned work by moving owned data in or sharing it with `Arc`, not by annotating references `'static`.

> [!IMPORTANT]
> Annotations describe relationships the compiler then checks. They can't make a value live longer.

See the [lifetimes lesson](https://levelcode.org/roadmaps/rust/lifetimes) for structs that borrow and when to prefer owned data, and [the borrow checker](https://levelcode.org/concepts/borrow-checker) for the rule lifetimes serve.