[wasted_intel]

For vanilla references (&), Rust will prevent you from having multiple mutable references. Beyond that, there are several types in the stdlib[0] that progressively increase flexibility, but always safely.

I think it's bad advice because you need to know what is potentially dangerous. That alienates newcomers to systems programming (which is why one of Rust's goals is to be able to "hack without fear"). There are also some designs that I'd never try to build without safety checks, because they're notoriously hard to get right. Even knowing the fundamentals of memory safety, without a way of enforcing lifetimes and mutability, writing zero-allocation implementations that share immutably can be daunting.

[0] Cell, RefCell, Mutex, Rc, Arc