unreachable_pub lint thinks a pub fn in a pub(crate) struct is public

[mpalmer]

I tried this code:

// src/foo.rs
pub(crate) struct Foo();

impl Foo {
    pub fn foo() {
        println!("foo!");
    }
}

// src/lib.rs
#[warn(unreachable_pub)]

mod foo;

use foo::Foo;

pub fn bar() {
    Foo::foo();
}

I expected that this code would compile without warnings, because the Foo struct isn't visible outside the crate, and thus its associated functions won't be visible either (I'm making a bit of an assumption on this point; maybe you can access a private struct's functions outside the crate, and I just don't know enough Rust to know how?)

Instead, the compiler output this warning:

   Compiling pub_crate v0.1.0 ([...]/pub_crate)
warning: unreachable `pub` item
 --> src/foo.rs:4:5
  |
4 |     pub fn foo() {
  |     ---^^^^^^^^^
  |     |
  |     help: consider restricting its visibility: `pub(crate)`
  |
note: the lint level is defined here
 --> src/lib.rs:1:8
  |
1 | #[warn(unreachable_pub)]
  |        ^^^^^^^^^^^^^^^

warning: `pub_crate` (lib) generated 1 warning (run `cargo fix --lib -p pub_crate` to apply 1 suggestion)
    Finished dev [unoptimized + debuginfo] target(s) in 0.11s

Meta

rustc --version --verbose:

rustc 1.69.0 (84c898d65 2023-04-16)
binary: rustc
commit-hash: 84c898d65adf2f39a5a98507f1fe0ce10a2b8dbc
commit-date: 2023-04-16
host: x86_64-unknown-linux-gnu
release: 1.69.0
LLVM version: 15.0.7

The output is the same if I run with rustup run nightly cargo build, where rustup run nightly rustc --version --verbose says:

rustc 1.71.0-nightly (1a6ae3d69 2023-04-27)
binary: rustc
commit-hash: 1a6ae3d692cfb52b21d0f45ba50b659486e53d6c
commit-date: 2023-04-27
host: x86_64-unknown-linux-gnu
release: 1.71.0-nightly
LLVM version: 16.0.2

[Noratrieb]

The lint is correct, albeit a little confusingly. pub means "publically accessible outside the crate", which your function is indeed not. You need to change it to pub(crate).

[mpalmer]

Ooooooooooh, I understand the lint's purpose better now. The pub keyword is both an action ("make this available outside this namespace") and also a declaration of intent ("I want this to be available outside this crate"). Since pub has this dual meaning, but in order to make it actually accessible outside the crate you need a "chain" of pubs from the the identifier to the crate root, the lint is identifying situations where the actions don't match the intention. Since the function is in a non-pub struct, it lacks the "chain", and so fwackoom.

I can't think of a pithy way to summarise all of the above such that it is suitable for inclusion in the compiler's error message itself, but would a PR to modify the lint's documentation to add something like the above be welcomed?