How can I force a struct's field to always be immutable in Rust? - immutability

Question

In Rust, you don't specify mutability inside a struct, but it is inherited from the variable binding. That's great, but is it possible to force a field to be always immutable, even when the root is mutable?
Something like this hypothetical syntax:
struct A {
immut s: Shape, // immutable by design
bla: Bla, // this field inheriting (im)mutability
}
let mut a = make_a();
a.s = x/*...*/; // illegal
This would help to maintain nice semantic restrictions in a program, just like Java's final does (in a very limited way).
Also, we could imagine this kind of struct having some non-owning references to internal immutable data, taking advantage of this immutability...

Answer 1

It's impossible to have immutability of a single field. That was an option in an ancient version of Rust (think before 0.8), but it was dropped because the rules confused a LOT of people. How was it confusing, you might ask? Think about it like this: if a field is declared mutable and struct is declared mutable and the reference used was an immutable reference (&) then the field is _______.
The best, as Lily Ballard noted, is that you can declare your Shape field as private and make a getter method using impl A {...}.
mod inner {
pub struct A {
s: i32, // can't be seen outside of module
pub bla: i32,
}
impl A {
pub fn new() -> Self {
Self { s: 0, bla: 42 }
}
pub fn get_s(&self) -> i32 {
self.s
}
}
}
let mut a = inner::A::new();
a.s = 42; // illegal
println!("{}", a.s); // also illegal
println!("{}", a.get_s()); // could be made to serve as a read-only method
error[E0616]: field `s` of struct `main::inner::A` is private
--> src/main.rs:20:5
|
20 | a.s = 42; // illegal
| ^^^
error[E0616]: field `s` of struct `main::inner::A` is private
--> src/main.rs:21:20
|
21 | println!("{}", a.s); // also illegal
| ^^^
There is proposition that might drop notions of mutability and immutability completely (you can't say a struct never changes). See Niko's explanation for that change.

Answer 2

You can create a struct and only implement the Deref trait for it. Without the DerefMut trait it won't be possible for contained values to be mutated.
https://doc.rust-lang.org/std/ops/trait.Deref.html
This way the compiler will make the member usable as if it's not wrapped in another struct, no need for any written getter method call.
use std::ops::Deref;
/// A container for values that can only be deref'd immutably.
struct Immutable<T> {
value: T,
}
impl<T> Immutable<T> {
pub fn new(value: T) -> Self {
Immutable { value }
}
}
impl<T> Deref for Immutable<T> {
type Target = T;
fn deref(&self) -> &Self::Target {
&self.value
}
}
struct Foo {
bar: Immutable<Vec<u8>>,
baz: usize,
}
impl Foo {
pub fn new(vec: Vec<u8>) -> Self {
Foo {
bar: Immutable::new(vec),
baz: 1337,
}
}
pub fn mutate(&mut self) {
self.bar.push(0); // This will cause a compiler error
}
}
|
| self.bar.push(0);
| ^^^^^^^^ cannot borrow as mutable
|
= help: trait `DerefMut` is required to modify through a dereference, but it is not implemented for `runnable::immutable::Immutable<std::vec::Vec<u8>>`

Answer 3

You can't force immutability on a field. How would the struct mutate its own value when necessary?
What you can do is make the field private and expose a getter method to return a reference to it (or to copy/clone the value).

Answer 4

A Solution could be to have a more general approach:
pub struct Immutable<T> {
value : T ,
}
impl<T> Immutable<T> {
pub fn new(value : T) -> Immutable<T> {
Immutable { value : value }
}
pub fn get( &self) -> &T { &self.value }
}
Now it is possible to use the Immutable struct in every case for every other type.
Given this into a module avoids changing the content of the Immutable object. It is still possible to change the variable which holds the Immutable object itself by overwriting it by a new Object but you should notice it by the Immutable::new statement and so you can avoid using it.

Answer 5

In many cases an associated constant achieves the desired behaviour:
struct Foo { blah: Blah }
impl Foo {
const S: Shape = Shape { x: 1, y: 1 };
}
Of course, constants cannot be set on creation, they are set at compile-time. Making the field private as explained in other answers will work if dynamicism is required.

Answer 6

Probably the easiest way to achieve this today is by using the readonly crate from prolific developer David Tolnay (author of serde and many others). From the github documentation:
This crate provides an attribute macro to expose struct fields that are readable and writable from within the same module but readable only outside the module.
#[readonly::make]
pub struct S {
// This field can be read (but not written) by super.
#[readonly]
pub(super) readable: i32,
// This field can be neither read nor written by other modules.
private: i32,
}