The seed of a mental model

Some find it helpful to think of shared (&T) and exclusive (&mut T) references like so:

• &T is a compiler-checked RwLockReadGuard
  • You can have as many of these at one time as you want
• &mut T is a compiler-checked RwLockWriteGuard
  • You can only have one of these at one time, and when you do, you can have no RwLockReadGuard

The exclusivity is key.

&mut T are often called “mutable references” for obvious reasons. And following from that, &T are often called “immutable references”. However, I find it more accurate and consistent to call &mut T an exclusive reference and to call &T a shared reference.

This guide doesn’t currently go into great depth on the topic of shared mutability (more commonly called interior mutability), but you’ll run into the concept sometime in your Rust journey. One thing I will highlight here is that it enables mutation behind a &T, and thus “immutable reference” is a misleading name.

There are also situations where the important quality of a &mut T is the exclusivity that it guarantees, and not the ability to mutate through it. If you find yourself annoyed at getting borrow errors about &mut T when you performed no actual mutation, it may help to instead consider &mut T as a directive to the compiler to ensure exclusive access instead of mutable access.