`unreachable_pub` false negative when type is referenced in a public field, even if it is not reachable

[djmitche]

Code

#![deny(unreachable_pub)]
mod config {
    pub struct AccessMode;
}

pub enum StorageConfig {
    OnDisk(config::AccessMode),
}

Current output

Compiling playground v0.0.1 (/playground)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.50s

Desired output

error: unreachable `pub` item
 --> src/lib.rs:4:5
  |
4 |     pub struct AccessMode;
  |     ---^^^^^^^^^^^^^^^^^^
  |     |
  |     help: consider restricting its visibility: `pub(crate)`
  |
  = help: or consider exporting it for use by other crates

Rationale and extra context

Changing StorageConfig to

pub struct StorageConfig(config::AccessMode);

produces the desired output, so this is a difference between a reference to a type from an enum variant and a reference to a type from a struct.

Other cases

The same occurs if the enum variant and struct are defined with a named field (`access_mode: config::AccessMode`)

Rust Version

rustc 1.83.0 (90b35a623 2024-11-26)
binary: rustc
commit-hash: 90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf
commit-date: 2024-11-26
host: x86_64-unknown-linux-gnu
release: 1.83.0
LLVM version: 19.1.1

Anything else?

No response

[djmitche]

In fact,

pub struct StorageConfig(pub config::AccessMode);

shows the incorrect output. Enum variants have an implicit pub, so the issue is not the enum variant.

The issue is that a type that is referenced publicly but not reachable publicly is not detected. So, this may be a duplicate of #116604.

[cyrgani]

Why do you think this is a false positive? In your first example, AccessMode types can be extracted from the StorageConfig:

fn f(config: StorageConfig) {
    match config {
        StorageConfig::OnDisk(mode) => todo!(),
    }
}

In fact, removing the pub on AccessMode or reducing it to pub(crate) would be incorrect and cause compiler errors with my example usage:

warning: type `AccessMode` is more private than the item `StorageConfig::OnDisk::0`
 --> src/lib.rs:7:12
  |
7 |     OnDisk(config::AccessMode),
  |            ^^^^^^^^^^^^^^^^^^ field `StorageConfig::OnDisk::0` is reachable at visibility `pub`
  |
note: but type `AccessMode` is only usable at visibility `pub(crate)`
 --> src/lib.rs:3:5
  |
3 |     pub(crate) struct AccessMode;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = note: `#[warn(private_interfaces)]` on by default

error: type `abcd::config::AccessMode` is private
 --> src/main.rs:9:31
  |
9 |         StorageConfig::OnDisk(mode) => todo!(),
  |                               ^^^^ private type

[Urgau]

What you want is done by the unnameable_types lint which:

detects types for which you can get objects of that type, but cannot name the type itself.

which is the case here as one can get AccessMode from the enum but cannot name it.

error: struct `AccessMode` is reachable but cannot be named
 --> src/lib.rs:3:5
  |
3 |     pub struct AccessMode;
  |     ^^^^^^^^^^^^^^^^^^^^^ reachable at visibility `pub`, but can only be named at visibility `pub(crate)`
  |
note: the lint level is defined here
 --> src/lib.rs:1:9
  |
1 | #![deny(unnameable_types)]
  |         ^^^^^^^^^^^^^^^^

While the unreachable_pub description state that it does not lint on "leaked types":

The unreachable_pub lint triggers for pub items not reachable from other crates - that means neither directly accessible, nor reexported, nor leaked through things like return types.

@rustbot label +C-discussion

[djmitche]

Ah, thank you! That's an important misunderstanding of the term "reachable" on my part.

[Urgau]

@djmitche We added a reference to the unnameable_types lint in the unreachable_pub lint documentation in #135145. Do you think that addresses your issue here?

[djmitche]

Thanks -- that's great! ❤