Lifetimes & Variance Lab

Visualize lifetime relationships ('a), the 3 lifetime elision rules, and subtyping variance rules (Covariance vs Invariance).

Lifetime Lifespan & Variance Timeline

Analyze lifespan overlap constraints and verify when a reference outlives its source data.

Source CodeValid Lifetimes

1fn main() {
2    let outer_string = String::from("Safe & Sound"); // Lifetime 'a
3    let r: &str;
4    {
5        r = &outer_string; // Reference lives in 'b, data lives in 'a
6        println!("Inner read: {}", r);
7    }
8    println!("Outer read: {}", outer_string);
9}

Lifespan Timeline (Lines 1 to 9)Rule: Data life ≥ Loan life

'a (outer_string)Lines 2 → 8

7 lines

Stack life of outer_string

'b (r reference)Lines 5 → 6

2 lines

Active loan of r

Lifetime Subtyping & Variance Rules

&'a T (Covariant)

Can substitute longer lifetime for shorter

&'a mut T (Invariant)

Exact lifetime match required (no substitution)

The 3 Lifetime Elision Rules Desugared

Rule 1: Input Parameters

Each elided lifetime in function arguments is assigned its own distinct lifetime parameter.

fn f(x: &i32, y: &i32) → fn f<'a, 'b>

Rule 2: Single Input

If there is exactly one input lifetime, that lifetime is assigned to all output references.

fn f(x: &str) → &str → fn f<'a>('a) → 'a

Rule 3: &self / &mut self

If there are multiple input parameters and one is &self, the lifetime of self is assigned to all outputs.

fn get(&self, x: &str) → &str → output borrows self

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