1.12.1. Lifetime Variance
Variance is a concept in Rust’s type system. It describes how generic parameters — especially lifetime parameters — relate to one another in the type hierarchy.
We can think of it simply as variance describes which types are “subtypes” of other types, where “subtype” is somewhat similar to the concept used in Java and C#.
In addition, variance also cares about when a “subtype” can replace a “supertype” and vice versa.
In general, if A is a subtype of B, then A is at least as useful as B. Here is a Rust example: if a function takes &'a str, then &'static str can be passed in. Because 'static is a subtype of 'a, 'static lives at least as long as any 'a (and 'static can remain valid for the entire program).
