Add field_scoped_visibility_modifiers lint

clippy_lints/src/declared_lints.rs

@@ -178,6 +178,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::explicit_write::EXPLICIT_WRITE_INFO,
     crate::extra_unused_type_parameters::EXTRA_UNUSED_TYPE_PARAMETERS_INFO,
     crate::fallible_impl_from::FALLIBLE_IMPL_FROM_INFO,
+    crate::field_scoped_visibility_modifiers::FIELD_SCOPED_VISIBILITY_MODIFIERS_INFO,
     crate::float_literal::EXCESSIVE_PRECISION_INFO,
     crate::float_literal::LOSSY_FLOAT_LITERAL_INFO,
     crate::floating_point_arithmetic::IMPRECISE_FLOPS_INFO,

clippy_lints/src/field_scoped_visibility_modifiers.rs

@@ -0,0 +1,75 @@
+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_clippy_lint! {
+    /// ### What it does
+    /// Checks for usage of scoped visibility modifiers, like `pub(crate)`, on fields. These
+    /// make a field visible within a scope between public and private.
+    ///
+    /// ### Why restrict this?
+    /// Scoped visibility modifiers cause a field to be accessible within some scope between
+    /// public and private, potentially within an entire crate. This allows for fields to be
+    /// non-private while upholding internal invariants, but can be a code smell. Scoped visibility
+    /// requires checking a greater area, potentially an entire crate, to verify that an invariant
+    /// is upheld, and global analysis requires a lot of effort.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// pub mod public_module {
+    ///     struct MyStruct {
+    ///         pub(crate) first_field: bool,
+    ///         pub(super) second_field: bool
+    ///     }
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// pub mod public_module {
+    ///     struct MyStruct {
+    ///         first_field: bool,
+    ///         second_field: bool
+    ///     }
+    ///     impl MyStruct {
+    ///         pub(crate) fn get_first_field(&self) -> bool {
+    ///             self.first_field
+    ///         }
+    ///         pub(super) fn get_second_field(&self) -> bool {
+    ///             self.second_field
+    ///         }
+    ///     }
+    /// }
+    /// ```
+    #[clippy::version = "1.78.0"]
+    pub FIELD_SCOPED_VISIBILITY_MODIFIERS,
+    restriction,
+    "checks for usage of a scoped visibility modifier, like `pub(crate)`, on fields"
+}
+
+declare_lint_pass!(FieldScopedVisibilityModifiers => [FIELD_SCOPED_VISIBILITY_MODIFIERS]);
+
+impl EarlyLintPass for FieldScopedVisibilityModifiers {
+    fn check_item(&mut self, cx: &EarlyContext<'_>, item: &Item) {
+        let ItemKind::Struct(ref st, _) = item.kind else {
+            return;
+        };
+        for field in st.fields() {
+            let VisibilityKind::Restricted { path, .. } = &field.vis.kind else {
+                continue;
+            };
+            if !path.segments.is_empty() && path.segments[0].ident.name == rustc_span::symbol::kw::SelfLower {
+                // pub(self) is equivalent to not using pub at all, so we ignore it
+                continue;
+            }
+            span_lint_and_help(
+                cx,
+                FIELD_SCOPED_VISIBILITY_MODIFIERS,
+                field.vis.span,
+                "scoped visibility modifier on a field",
+                None,
+                "consider making the field private and adding a scoped visibility method for it",
+            );
+        }
+    }
+}

clippy_lints/src/lib.rs

@@ -136,6 +136,7 @@ mod exit;
 mod explicit_write;
 mod extra_unused_type_parameters;
 mod fallible_impl_from;
+mod field_scoped_visibility_modifiers;
 mod float_literal;
 mod floating_point_arithmetic;
 mod format;
@@ -1169,6 +1170,7 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
         })
     });
     store.register_late_pass(move |_| Box::new(string_patterns::StringPatterns::new(msrv())));
+    store.register_early_pass(|| Box::new(field_scoped_visibility_modifiers::FieldScopedVisibilityModifiers));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }