Covariant and Contravariant Lifetimes

Variance describes how subtyping relationships change when types are nested. Covariant types preserve ordering (&'long T is subtype of &'short T), contravariant reverses it, invariant requires exact matches. Affects how lifetimes work with references, boxes, and function parameters.

Visit the following resources to learn more:

• @official@Subtyping and Variance
• @article@Demystifying Covariant and Contravariant Lifetimes in Rust