The big clue that'll jump out to any rust dev is the type signature:

pub const fn toggle(&mut self)

The "&mut" says that toggle takes a mutable reference to the instance it is called off of. This is also called a mutable borrow. You might think that would get buried in the program because you don't always check the type signature of every method you call, but there are a few reasons that muts really jump out in rust. The languages immutable by default nature is one and the other is that several IDE have features to specifically draw attention to mutable instances/references. It's a fairly common pattern in rust to have helper methods on a type that take a mutable reference or ownership of self.

TL;DR: Rust's ownership model is very unique. Once it clicks its the cat's pajamas, but experience in other languages don't map well to this part of the language.