[Manishearth]

Rust follows multiple-readers or single-writer even in single-threaded situations. You can hold multiple immutable references to an object, or a single mutable one.

Initially this gets annoying, but as time passes one realizes how awesome this is. It prevents iterator invalidation (and vector-move-invalidation), for one (it's also necessary to get memory safety for our ADT enums since you can otherwise change the variant and invalidate a reference to its contents). In general you realize that in a large codebase, it's almost as bad as a multithreaded situation -- you don't know what objects are being modified by the methods being called and the code is too large to easily figure it out.