Separating mutability within a struct

[PFaas]

Has anyone ever had any success splitting the mutability to only one field of a struct like this? Playground

[Hyeonu]

Yes, this is the hard problem. Common pattern is to split the struct into two, the shared half and the mutable half each stored in their own variable.

[Yandros]

In these situations it is useful to be strict about the terminology and the semantics: you want to have a struct for which you want to restrict exclusive access (&mut) to some of its fields. This is just another form of API limitation, and is thus generally achieved through privacy and modules:

pub use privacy_boundary::Struct;
mod privacy_boundary {
    pub
    struct Struct {
        pub
        mutable_field: A,

        immutable_field: B,
    }

    impl Struct {
        pub
        fn new (…) -> Self
        { … }

        pub
        fn immutable_field (self: &'_ Struct) -> &'_ B
        {
            &self.immutable_field
        }
    }
}

so that

let mut s = Struct::new(…);
s.mutable_field = …; // OK
s.immutable_field = …; // Error, private field
dbg!(&s.immutable_field()); // OK

The main drawback here are ergonomics, but for that you can go and use handy proc-macros, such as:

readonly

---

Finally, going back to the start of my post, there could be a different problem when talking about "separating mutability within a struct": instead of talking about publicly forwarding (or not) the exclusive access (&mut), we can talk about a struct we know is gonna be shared (& access only), and at that point will only be mutable the fields that feature Shared Mutability / Interior Mutability, such as:

use ::core::cell::Cell as Mut; // Single-threaded mutation

struct Shared {
    mutable_field: Mut<i32>,
    immutable_when_shared: i32,
}

so that, if you have, say, an s: Rc<Shared> or a s: &Shared, you can do:

s.mutable_field.set(0);