Subtyping and Variance

As a warning, this section will be complex and type-theory heavy, but the gist for this scope is that there are three types of lifetime relationships:

• Covariant: ‘a can be used where a ‘b is expected if ‘a is as long or longer than ‘b. Shared references are covariant because a longer-living reference than required can always be given. Tree structures where you can only delete leaves kind of act like this (so a RefCell<&T> chain of references follows this).

• Contravariant: ‘a can be used where a ‘b is expected if ‘a lives as long or shorter than ‘b. This only applies to arguments inside of functions or closures, so those should be banned from use to avoid any headaches. Closures aren’t application binary interface safe so they are already banned, and functions as arguments can be replaced with Java upcalls where less care is needed.

• Invariant: ‘a cannot be used, the thing you pass in must live exactly as long as ‘b. This applies to exclusive references, because Rust allows you to modify data behind an exclusive reference and potentially change its lifetime, and the caller would have no idea its lifetime got changed, so that would fail once the caller tries to use it within its old lifetime, but outside its new lifetime. If an exclusive reference is checked for validity first before every time it is used, this can work (it’s effectively RefCell<&mut T>), but that then still bans every function that touches an exclusive reference directly. Honestly, this may not be truly solvable, it might just have to be invasive to the programmer.

To learn more about subtyping and variance, it is recommended to read the official Rust resource The Rustonomicon chapter 3.8.