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

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.

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).

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>>`