add new lint, pub_underscore_fields

- add a new late pass lint, with config options
- add ui tests for both variations of config option
- update CHANGELOG.md

github feedback

bump version to 1.77 and run cargo collect-metadata

Change `,` to `;` in `conf.rs`

clippy_lints/src/declared_lints.rs

@@ -576,6 +576,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::ptr::MUT_FROM_REF_INFO,
     crate::ptr::PTR_ARG_INFO,
     crate::ptr_offset_with_cast::PTR_OFFSET_WITH_CAST_INFO,
+    crate::pub_underscore_fields::PUB_UNDERSCORE_FIELDS_INFO,
     crate::pub_use::PUB_USE_INFO,
     crate::question_mark::QUESTION_MARK_INFO,
     crate::question_mark_used::QUESTION_MARK_USED_INFO,

clippy_lints/src/lib.rs

@@ -272,6 +272,7 @@ mod permissions_set_readonly_false;
 mod precedence;
 mod ptr;
 mod ptr_offset_with_cast;
+mod pub_underscore_fields;
 mod pub_use;
 mod question_mark;
 mod question_mark_used;
@@ -571,6 +572,7 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
         verbose_bit_mask_threshold,
         warn_on_all_wildcard_imports,
         check_private_items,
+        pub_underscore_fields_behavior,
 
         blacklisted_names: _,
         cyclomatic_complexity_threshold: _,
@@ -1081,6 +1083,11 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
     store.register_late_pass(|_| Box::new(uninhabited_references::UninhabitedReferences));
     store.register_late_pass(|_| Box::new(ineffective_open_options::IneffectiveOpenOptions));
     store.register_late_pass(|_| Box::new(unconditional_recursion::UnconditionalRecursion));
+    store.register_late_pass(move |_| {
+        Box::new(pub_underscore_fields::PubUnderscoreFields {
+            behavior: pub_underscore_fields_behavior,
+        })
+    });
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

clippy_lints/src/pub_underscore_fields.rs

@@ -0,0 +1,83 @@
+use clippy_config::types::PubUnderscoreFieldsBehaviour;
+use clippy_utils::attrs::is_doc_hidden;
+use clippy_utils::diagnostics::span_lint_and_help;
+use clippy_utils::is_path_lang_item;
+use rustc_hir::{FieldDef, Item, ItemKind, LangItem};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::impl_lint_pass;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks whether any field of the struct is prefixed with an `_` (underscore) and also marked
+    /// `pub` (public)
+    ///
+    /// ### Why is this bad?
+    /// Fields prefixed with an `_` are inferred as unused, which suggests it should not be marked
+    /// as `pub`, because marking it as `pub` infers it will be used.
+    ///
+    /// ### Example
+    /// ```rust
+    /// struct FileHandle {
+    ///     pub _descriptor: usize,
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// struct FileHandle {
+    ///     _descriptor: usize,
+    /// }
+    /// ```
+    ///
+    /// OR
+    ///
+    /// ```rust
+    /// struct FileHandle {
+    ///     pub descriptor: usize,
+    /// }
+    /// ```
+    #[clippy::version = "1.77.0"]
+    pub PUB_UNDERSCORE_FIELDS,
+    pedantic,
+    "struct field prefixed with underscore and marked public"
+}
+
+pub struct PubUnderscoreFields {
+    pub behavior: PubUnderscoreFieldsBehaviour,
+}
+impl_lint_pass!(PubUnderscoreFields => [PUB_UNDERSCORE_FIELDS]);
+
+impl<'tcx> LateLintPass<'tcx> for PubUnderscoreFields {
+    fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'_>) {
+        // This lint only pertains to structs.
+        let ItemKind::Struct(variant_data, _) = &item.kind else {
+            return;
+        };
+
+        let is_visible = |field: &FieldDef<'_>| match self.behavior {
+            PubUnderscoreFieldsBehaviour::PublicallyExported => cx.effective_visibilities.is_reachable(field.def_id),
+            PubUnderscoreFieldsBehaviour::AllPubFields => {
+                // If there is a visibility span then the field is marked pub in some way.
+                !field.vis_span.is_empty()
+            },
+        };
+
+        for field in variant_data.fields() {
+            // Only pertains to fields that start with an underscore, and are public.
+            if field.ident.as_str().starts_with('_') && is_visible(field)
+                // We ignore fields that have `#[doc(hidden)]`.
+                && !is_doc_hidden(cx.tcx.hir().attrs(field.hir_id))
+                // We ignore fields that are `PhantomData`.
+                && !is_path_lang_item(cx, field.ty, LangItem::PhantomData)
+            {
+                span_lint_and_help(
+                    cx,
+                    PUB_UNDERSCORE_FIELDS,
+                    field.vis_span.to(field.ident.span),
+                    "field marked as public but also inferred as unused because it's prefixed with `_`",
+                    None,
+                    "consider removing the underscore, or making the field private",
+                );
+            }
+        }
+    }
+}