What is the usage of the asterisk symbol in Rust?

If *y is a reference

*y is not a reference. y is a reference; *y dereferences y, allowing you access to the referred-to value.

what is the difference [between += and println!]

println! is a macro that automatically references the arguments given to it. In addition, the Display trait (used via {} in the format string) is implemented for all references to types that themselves implement Display (impl<'a, T> Display for &'a T where T: Display + ?Sized).

Thus, println!("{}", y); is actually printing out a reference to a reference to a value. Those intermediate references are automatically dereferenced due to the implementation of Display.

+=, on the other hand, is implemented via the AddAssign trait. The standard library only implements adding an integer type to itself (impl AddAssign<i32> for i32). That means that you have to add an appropriate level of dereferencing in order to get both sides to an integer.