Definitive guide on `pub` keyword visibility?

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#1

Hi -- I've been trying to understand the visibility of structs in rust and what the "pub" keyword does. By experimenting, I have reversed-engineered some rules but I couldn't verify them. Is there a pointer to a definitive guide so I can check if my findings are correct?

My current version of the rules:

  • A non-public struct is visible to the module where it's defined and all its descendants
  • A public struct is only visible to the immediate parent of a module, but not higher.

Is this correct?

Say we have module tree structure being
Crate -> (Mod1, Mod2), and Mod1 -> (SubMod1, SubMod2).

  • If we have a struct Foo in Mod1, then it is visible from SubMod1, but not visible in any other places.

  • If we have a pub struct Bar in the lowest submodule SubMod1, then it is visible from Mod1 (its immediate parent), but not visible anywhere else.

Please advise. Thanks!

#2

The definitive reference is here: https://doc.rust-lang.org/reference/visibility-and-privacy.html

With the notion of an item being either public or private, Rust allows item accesses in two cases:

  1. If an item is public, then it can be accessed externally from some module m if you can access all the item's ancestor modules from m . You can also potentially be able to name the item through re-exports. See below.
  2. If an item is private, it may be accessed by the current module and its descendants.

In the second case, it mentions that a private item "can be accessed" by the current module and its descendants, but the exact meaning of accessing an item depends on what the item is. Accessing a module, for example, would mean looking inside of it (to import more items). On the other hand, accessing a function would mean that it is invoked. Additionally, path expressions and import statements are considered to access an item in the sense that the import/expression is only valid if the destination is in the current visibility scope.

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#3

A pub item inside module x is visible in any place where module x is visible. This always includes x’s parent module (including its descendants), but if x is also pub, it can include other modules too.

#4

Thanks! I've a question on the point2. Say I have a nested module structure A->B->C->D, and I have a private struct defined in B. Then what's the syntax to access it in D? I know structs in C can be accessed via super keyword, but what about the module B which is two level up?

#5

You can keep adding supers to the path: super::super::SomeBStruct. If you're familar with unix file paths you can think of it like one, where super is ../, crate is / and self is ./

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#6

You can also use the absolute path crate::A::B::SomeBStruct which is the same no matter where you use it.

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#7

Note also that re-exports are important here. If you have a public type in a private module, the parent module can still re-export it and make it available in other places.

#8

Somewhat related, but, if you enable -W unreachable-pub warning, the semantics of pub becomes clearer -- pub are the things visible outside of the crate, they are exported public API.

#9

Yes -- but isn't that only okay if I'm calling it in a descendant (e.g. D) but not okay if I call it from a sibling module (say A'), due to mod A is private?

#10

Interesting! could you give an example of re-export?

#11

Cool! Where should I put the -W unreachable-pub?

#12

A private item is visible everywhere within its parent module. This means that a private module is visible to its siblings.

Public/private only controls whether an item is "exported" beyond its parent module. (At the top level of a crate, it only controls whether the item is visible to external crates.)

#13

Yeah sorry I didn't express myself clearly: I meant say we have crate->A->B->C->D. and B has a private struct Foo. Say we also have crate->A'. Then we can use Foo in D by use crate::A::B::Foo, but the same command will fail in A', is that right?

#14

A reexport is written like this:

// mod1.rs

pub use self::submod1::Bar;

Now you can access Bar as if it was defined in Mod1.

#15

If you really choose to use it (which imo makes sense for larger projects, but is not worth it for the smaller ones), I'd go with setting RUSTFLAGS="-W unreachable-pub" environment variable on CI.

You can also use this attribute in the crate root(lib.rs/main.rs): #![warn(unreachable_pub)].

#16

Right.

#17

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