Reference lifetimes

Here's something you'll utilize in Rust all the time without thinking about it:

• A &'long T coerces to a &'short T
• A &'long mut T coerces to a &'short mut T

The technical term is "covariant (in the lifetime)" but a practical mental model is "the (outer) lifetime of references can shrink".

The property holds for values whose type is a reference, but it doesn't always hold for other types. For example, we'll soon see that this property doesn't always hold for the lifetime of a reference nested within another reference. When the property doesn't hold, it's usually due to invariance.

Even if you never really think about covariance, you'll grow an intuition for it -- so much for so that you'll eventually be surprised when you encounter invariance and the property doesn't hold. We'll look at some cases soon.