`unreachable_pub` false negative when an unreachable type is mentioned in a type alias

[kpreid]

I tried this code:

#![deny(unreachable_pub)]

mod foo {
    pub enum FooError {}
    pub type FooResult<T> = Result<T, FooError>;
}

pub use foo::FooResult;

I expected an unreachable_pub lint because FooError is unreachable. Instead, there is no diagnostic output.

The FooError type is in principle nameable via defining a suitable trait and associated type, but this is probably not the intent of the unreachable_pub lint; the author probably wants to be told to reexport FooError.

Meta

rustc --version --verbose:

rustc 1.73.0 (cc66ad468 2023-10-03)
binary: rustc
commit-hash: cc66ad468955717ab92600c770da8c1601a4ff33
commit-date: 2023-10-03
host: x86_64-apple-darwin
release: 1.73.0
LLVM version: 17.0.2

rustc 1.75.0-nightly (2bbb61989 2023-10-04)
binary: rustc
commit-hash: 2bbb6198933abf193c0ef1d18d9d70f99eed59ba
commit-date: 2023-10-04
host: x86_64-apple-darwin
release: 1.75.0-nightly
LLVM version: 17.0.2

@rustbot label +A-lint +A-visibility

[Urgau]

I don't think this is a false positive, as your case is detected by the unnameable_types lint which:

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

error: enum `FooError` is reachable but cannot be named
 --> src/lib.rs:4:5
  |
4 |     pub enum FooError {}
  |     ^^^^^^^^^^^^^^^^^ 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 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.

[kpreid]

That makes sense. Given this, I think it would be good if unreachable_pub’s documentation mentioned the unnameable_types lint. I will start a PR for that (#135145).