Rust Lifetimes & Borrowing

Comprehensive guide to Rust's ownership system, borrowing rules, and lifetime annotations based on The Rust Programming Language, The Rust Reference, and Common Rust Lifetime Misconceptions.

When to Use This Skill

• Borrow checker errors you don't understand
• Deciding between owned types and references
• Annotating lifetime parameters on structs/functions
• Understanding 'static, 'a, and elision rules
• Working with self-referential structures
• Debugging "does not live long enough" or "borrowed value dropped" errors

Ownership Rules

1. Each value has exactly one owner.
2. When the owner goes out of scope, the value is dropped.
3. Ownership can be transferred (moved) but not duplicated (unless Copy).

let s1 = String::from("hello");
let s2 = s1;          // s1 is MOVED into s2; s1 is invalid
// println!("{s1}");   // compile error: use of moved value

let x: i32 = 5;
let y = x;            // i32 is Copy; both x and y are valid

Borrowing Rules

1. You can have either one &mut T or any number of &T — never both simultaneously.
2. References must always be valid (no dangling).

fn calculate(data: &[u32]) -> u32 { data.iter().sum() }  // immutable borrow
fn push_item(v: &mut Vec<u32>, x: u32) { v.push(x); }   // mutable borrow

Lifetime Annotations

Lifetimes describe the scope during which a reference is valid. They don't change how long data lives — they help the compiler verify correctness.

// Explicit: both inputs and output share lifetime 'a
fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
    if x.len() > y.len() { x } else { y }
}

// Struct holding a reference must declare the lifetime
struct Excerpt<'a> {
    part: &'a str,
}

Lifetime Elision Rules

The compiler infers lifetimes when unambiguous:

1. Each reference parameter gets its own lifetime.
2. If there is exactly one input lifetime, it is assigned to all output lifetimes.
3. If one of the parameters is &self or &mut self, its lifetime is assigned to all output lifetimes.

// Elided (compiler infers)
fn first_word(s: &str) -> &str { ... }
// Desugared
fn first_word<'a>(s: &'a str) -> &'a str { ... }

Common Misconceptions

1. T only contains owned types — WRONG

T is a superset of &T and &mut T. A generic T can be instantiated with reference types.

fn print_it<T: Display>(val: T) { println!("{val}"); }
print_it(&42);  // T = &i32 — perfectly valid

2. T: 'static means "lives forever" — WRONG

T: 'static means T can live for the entire program if needed — it contains no short-lived borrows. Owned types like String, Vec<u8> all satisfy 'static.

fn spawn_thread<T: Send + 'static>(val: T) { ... }
spawn_thread(String::from("hi"));  // String: 'static ✓

3. &'a T and T: 'a are the same — WRONG

• T: 'a — T outlives 'a (T's internal references live at least as long as 'a)
• &'a T — a reference to T that is valid for 'a

4. Lifetimes grow/shrink at runtime — WRONG

Lifetimes are compile-time constructs. They describe static regions of code, not runtime durations.

5. Downgrading &mut to & is always safe — WRONG

Reborrowing as &*x is fine, but holding both the shared reborrow and the original &mut simultaneously is UB-adjacent (compiler rejects it).

6. Closures follow function elision rules — WRONG

Closures do NOT get the same elision rules as fn items. You often need explicit annotations or for<'a> bounds.

Variance

Invariance means the lifetime/type cannot be shortened or lengthened — matters for mutable references and interior mutability.

Practical Patterns

Splitting borrows

let mut v = vec![1, 2, 3];
let (left, right) = v.split_at_mut(1);
left[0] = 10;
right[0] = 20;  // both halves borrowed mutably — safe because disjoint

Returning references from methods

impl MyStruct {
    fn name(&self) -> &str { &self.name }  // ties output lifetime to &self
}

NLL (Non-Lexical Lifetimes)

Borrows end at the point of last use, not at the end of the lexical scope:

let mut v = vec![1, 2, 3];
let first = &v[0];
println!("{first}");  // last use of `first`
v.push(4);            // OK — `first` borrow already ended (NLL)

Reference Map

• references/ownership-moves.md — move semantics, Copy, Clone, drop order
• references/borrowing-rules.md — borrow checker mechanics, reborrowing, two-phase borrows
• references/lifetime-annotations.md — elision, explicit annotations, HRTB
• references/common-misconceptions.md — the 10 misconceptions with solutions

Key References

• The Rust Programming Language, Ch. 4 & 10
• The Rust Reference: Lifetime Elision
• Common Rust Lifetime Misconceptions