False negatives for missing_docs_in_private_items since 1.79.

Summary

Apparently there are circumstances where missing_docs_in_private_items fails to report errors; they seem to have strange, non-local behavior.

Lint Name

missing_docs_in_private_items

Reproducer

I ran cargo clippy --all-features with Rust 1.85 using this code as my lib.rs.

//! Documented.

#![deny(clippy::missing_docs_in_private_items)]

/// Documented.
#[derive(Debug)]
#[allow(dead_code)]
struct Mine {
    //// Note use of four /s.  No warning here.
    undocumented_1: String,
}

#[derive(serde::Deserialize)]
pub(crate) struct Undocumented2 {}

Here's the Cargo.toml I used:

[package]
name = "clippy-repro"
version = "0.1.0"
edition = "2021"

[features]
f = []

[dependencies]
serde = { version = "1.0.218", features = ["derive"] }

I expected to see warnings from undocumented_1 and Undocumented2.

Instead, no warnings were produced.

I tried removing #[derive(Debug)] from Mine: It caused a warning to start appearing on Undocumented2!

I tried with older versions of Clippy: Rust version 1.78 and earlier correctly give warnings; Rust 1.79 is the first version that fails to warn at all.

Version

rustc 1.85.0 (4d91de4e4 2025-02-17)
binary: rustc
commit-hash: 4d91de4e48198da2e33413efdcd9cd2cc0c46688
commit-date: 2025-02-17
host: x86_64-unknown-linux-gnu
release: 1.85.0
LLVM version: 19.1.7

Start with the supplied lib.rs and Cargo.toml reproducer, running Clippy with Rust 1.78 and 1.79 or later to isolate the regression. Investigate the missing_docs_in_private_items lint behavior around the derive attributes; done means warnings are reported for both undocumented_1 and Undocumented2 without depending on unrelated derives.