Understanding struct-field mutation

Question

From the Rust book about how to mutate struct fields:

let mut point = Point { x: 0, y: 0 };
point.x = 5;

and later:

Mutability is a property of the binding, not of the structure itself.

This seems counter-intuitive to me because point.x = 5 doesn't look like I'm rebinding the variable point. Is there a way to explain this so it's more intuitive?

The only way I can wrap my head around this is to "imagine" that I'm rebinding point to a copy of the original Point with a different x value (not even sure that's accurate).

Answer 1

This seems counter-intuitive to me because point.x = 5 doesn't look like I'm rebinding the variable point. Is there a way to explain this so it's more intuitive?

All this is saying is that whether or not something is mutable is determined by the let- statement (the binding) of the variable, as opposed to being a property of the type or any specific field.

In the example, point and its fields are mutable because point is introduced in a let mut statement (as opposed to a simple let statement) and not because of some property of the Point type in general.

As a contrast, to show why this is interesting: in other languages, like OCaml, you can mark certain fields mutable in the definition of the type:

type point =
   { x: int;
     mutable y: int;
   };

This means that you can mutate the y field of every point value, but you can never mutate x.