[Pre-RFC] read-only visibility

[RustyYato]

As I said in the linked thread, I am extremely against using get/set terminology because it is actively misleading.

Also note, there is no way to specify read-only in Rust due to the existence of UnsafeCell and all things built on top of it (modulo the compiler internal Freeze auto trait). I don't think that read-only fields is the correct way to sell this feature. It is better to lean into the notion that &T is a shared reference to T, so these fields would be shareable fields.

@Aloso

Make nonsensical visibility a hard error , e.g. pub(crate, &crate) or pub(&self) . I think supporting this syntax might be desirable in edge cases (e.g. macros), so a warning should be enough.

I think a deny by default warning is good for this.

@Aloso

• Also have a notion of write-only . This wouldn't be very useful: It wouldn't allow you to borrow it, and there aren't many use cases anyway

Similarly to how Rust doesn't have a notion of read only, it doesn't have a notion of write only, so it would be impossible to enforce this other than just allowing assignment. Not allowing references to these fields is not significantly better than just having a setter for these fields.

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link to previous thread where I spoke about this,

Idea: Properties

But this is wrong. You can mutate through a shated reference, for example, atomics. That us the entire point of staying away from the mutability view. Using get/set terminology is actively misleading because it gives the impression that you can't mutate fields that you only have get access to. I think that this will be a source of bugs that will only be explainable by using the exclusivity view, so we should just stay aligned to that from the start. This is why I used the ref keyword in my sy…