branch start, add new lint pub_underscore_fields, add ui tests

CHANGELOG.md

@@ -4673,6 +4673,7 @@ Released 2018-09-13
 [`ptr_eq`]: https://rust-lang.github.io/rust-clippy/master/index.html#ptr_eq
 [`ptr_offset_with_cast`]: https://rust-lang.github.io/rust-clippy/master/index.html#ptr_offset_with_cast
 [`pub_enum_variant_names`]: https://rust-lang.github.io/rust-clippy/master/index.html#pub_enum_variant_names
+[`pub_underscore_fields`]: https://rust-lang.github.io/rust-clippy/master/index.html#pub_underscore_fields
 [`pub_use`]: https://rust-lang.github.io/rust-clippy/master/index.html#pub_use
 [`question_mark`]: https://rust-lang.github.io/rust-clippy/master/index.html#question_mark
 [`range_minus_one`]: https://rust-lang.github.io/rust-clippy/master/index.html#range_minus_one

clippy_lints/src/declared_lints.rs

@@ -504,6 +504,7 @@ 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::ranges::MANUAL_RANGE_CONTAINS_INFO,

clippy_lints/src/lib.rs

@@ -241,6 +241,7 @@ 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 ranges;
@@ -912,6 +913,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|_| Box::new(size_of_ref::SizeOfRef));
     store.register_late_pass(|_| Box::new(multiple_unsafe_ops_per_block::MultipleUnsafeOpsPerBlock));
     store.register_late_pass(|_| Box::new(extra_unused_type_parameters::ExtraUnusedTypeParameters));
+    store.register_early_pass(|| Box::new(pub_underscore_fields::PubUnderscoreFields));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

clippy_lints/src/pub_underscore_fields.rs

@@ -0,0 +1,57 @@
+use clippy_utils::diagnostics::span_lint_and_help;
+use rustc_ast::ast::{Item, ItemKind, VisibilityKind};
+use rustc_lint::{EarlyContext, EarlyLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+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,
+    /// }
+    /// ```
+    #[clippy::version = "1.69.0"]
+    pub PUB_UNDERSCORE_FIELDS,
+    pedantic,
+    "struct field prefixed with underscore and marked public"
+}
+declare_lint_pass!(PubUnderscoreFields => [PUB_UNDERSCORE_FIELDS]);
+
+impl EarlyLintPass for PubUnderscoreFields {
+    fn check_item(&mut self, cx: &EarlyContext<'_>, item: &Item) {
+        // this only pertains to structs
+        let ItemKind::Struct(ref st, _) = item.kind else {
+            return;
+        };
+
+        let msg = "field marked as public but also inferred as unused because it's prefixed with `_`";
+        for field in st.fields().iter() {
+            if let Some(ident) = field.ident.as_ref()
+            && ident.as_str().starts_with('_')
+            && !matches!(field.vis.kind, VisibilityKind::Inherited) {
+                span_lint_and_help(
+                    cx,
+                    PUB_UNDERSCORE_FIELDS,
+                    field.vis.span.to(ident.span),
+                    msg,
+                    None,
+                    "consider removing the underscore, or making the field private",
+                );
+            }
+        }
+    }
+}

tests/ui/pub_underscore_fields.rs

@@ -0,0 +1,51 @@
+#![allow(unused)]
+#![warn(clippy::pub_underscore_fields)]
+
+fn main() {
+    pub struct StructWithOneViolation {
+        pub _a: usize,
+    }
+
+    // should handle structs with multiple violations
+    pub struct StructWithMultipleViolations {
+        a: u8,
+        _b: usize,
+        pub _c: i64,
+        pub d: String,
+        pub _e: Option<u8>,
+    }
+
+    // shouldn't warn on anonymous fields
+    pub struct AnonymousFields(pub usize, i32);
+
+    // don't warn on empty structs
+    pub struct Empty1;
+    pub struct Empty2();
+    pub struct Empty3 {};
+
+    pub struct PubCrate {
+        pub(crate) a: String,
+        pub(crate) _b: Option<String>,
+    }
+
+    // shouldn't warn on fields named pub
+    pub struct NamedPub {
+        r#pub: bool,
+        _pub: String,
+    }
+
+    mod inner {
+        pub struct PubSuper {
+            pub(super) a: usize,
+            pub(super) _b: u8,
+            _c: i32,
+        }
+    }
+
+    mod inner2 {
+        pub struct PubModVisibility {
+            pub(in super::inner) e: bool,
+            pub(in super::inner) _f: Option<()>,
+        }
+    }
+}

tests/ui/pub_underscore_fields.stderr

@@ -0,0 +1,64 @@
+error[E0433]: failed to resolve: could not find `inner` in the crate root
+  --> $DIR/pub_underscore_fields.rs:47:27
+   |
+LL |             pub(in super::inner) e: bool,
+   |                           ^^^^^ could not find `inner` in the crate root
+
+error[E0433]: failed to resolve: could not find `inner` in the crate root
+  --> $DIR/pub_underscore_fields.rs:48:27
+   |
+LL |             pub(in super::inner) _f: Option<()>,
+   |                           ^^^^^ could not find `inner` in the crate root
+
+error: field marked as public but also inferred as unused because it's prefixed with `_`
+  --> $DIR/pub_underscore_fields.rs:6:9
+   |
+LL |         pub _a: usize,
+   |         ^^^^^^
+   |
+   = help: consider removing the underscore, or making the field private
+   = note: `-D clippy::pub-underscore-fields` implied by `-D warnings`
+
+error: field marked as public but also inferred as unused because it's prefixed with `_`
+  --> $DIR/pub_underscore_fields.rs:13:9
+   |
+LL |         pub _c: i64,
+   |         ^^^^^^
+   |
+   = help: consider removing the underscore, or making the field private
+
+error: field marked as public but also inferred as unused because it's prefixed with `_`
+  --> $DIR/pub_underscore_fields.rs:15:9
+   |
+LL |         pub _e: Option<u8>,
+   |         ^^^^^^
+   |
+   = help: consider removing the underscore, or making the field private
+
+error: field marked as public but also inferred as unused because it's prefixed with `_`
+  --> $DIR/pub_underscore_fields.rs:28:9
+   |
+LL |         pub(crate) _b: Option<String>,
+   |         ^^^^^^^^^^^^^
+   |
+   = help: consider removing the underscore, or making the field private
+
+error: field marked as public but also inferred as unused because it's prefixed with `_`
+  --> $DIR/pub_underscore_fields.rs:40:13
+   |
+LL |             pub(super) _b: u8,
+   |             ^^^^^^^^^^^^^
+   |
+   = help: consider removing the underscore, or making the field private
+
+error: field marked as public but also inferred as unused because it's prefixed with `_`
+  --> $DIR/pub_underscore_fields.rs:48:13
+   |
+LL |             pub(in super::inner) _f: Option<()>,
+   |             ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider removing the underscore, or making the field private
+
+error: aborting due to 8 previous errors
+
+For more information about this error, try `rustc --explain E0433`.