Lifetime Variance and Covariance in References

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Lifetime Variance and Covariance in References

Variance determines how subtyping between lifetimes affects the compatibility of complex types. Immutable references are covariant over their lifetime, meaning you can pass a longer-lived reference to a function expecting a shorter one. However, mutable references are invariant to prevent memory unsafety where a short-lived reference could be written into a long-lived location.

snippet.rs
rust
fn subtyping_demo<'a, 'b: 'a>(short: &'a str, long: &'b str) {
    // &'b str is a subtype of &'a str because 'b outlives 'a
    let x: &'a str = long; // Covariance: we can use a longer lifetime where a shorter is expected
    
    // This would fail if we used mutable references:
    // let mut m_long: &'b str = "...";
    // let r: &mut &'a str = &mut m_long; // Error: &mut T is invariant over T
}

Breakdown

'b: 'a

A lifetime constraint indicating that 'b must last at least as long as 'a.

let x: &'a str = long;

Demonstrates covariance: a subtype (&'b) is assigned to a supertype (&'a).

&mut T is invariant

Ensures that the exact type T is maintained, preventing invalid lifetime assignments.

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Lifetime Variance and Covariance in References

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