readonly 0.1.6

Struct fields that are made read-only accessible to other modules


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 = "0.1"


Place #[readonly::make] on a braced struct or tuple struct. This will make all fields of the struct publicly readable according to their individual visibility specifiers, but not writable from other modules.

mod m {
    pub struct S {
        pub n: i32,

    impl S {
        pub fn demo(&mut self) {
            // Can read and write from inside the same module.
            println!("{}", self.n);
            self.n += 1;

fn demo(s: &mut m::S) {
    // From outside the module, can only read.
    println!("{}", s.n);

    // Does not compile:
    //s.n += 1;

The error appears as follows.

error[E0594]: cannot assign to data in a dereference of `m::S`
21 |     s.n += 1;
   |     ^^^^^^^^ cannot assign

Optionally, place #[readonly] on individual struct fields to make just those fields publicly readable, without affecting other fields of the struct.

pub struct S {
    // This field can be read (but not written) by super.
    pub(super) readable: i32,

    // This field can be neither read nor written by other modules.
    private: i32,