rustc_passes: lint unused #[path] attributes on inline modules

Author: ravlynd

compiler/rustc_attr_parsing/src/attributes/path.rs

@@ -19,6 +19,6 @@ impl SingleAttributeParser for PathParser {
         let nv = cx.expect_name_value(args, cx.attr_span, None)?;
         let path = cx.expect_string_literal(nv)?;
 
-        Some(AttributeKind::Path(path))
+        Some(AttributeKind::Path(path, cx.attr_span))
     }
 }

compiler/rustc_hir/src/attrs/data_structures.rs

@@ -1280,7 +1280,7 @@ pub enum AttributeKind {
     },
 
     /// Represents `#[path]`
-    Path(Symbol),
+    Path(Symbol, Span),
 
     /// Represents `#[pattern_complexity_limit]`
     PatternComplexityLimit {

compiler/rustc_passes/src/check_attr.rs

@@ -25,7 +25,7 @@ use rustc_hir::def_id::LocalModId;
 use rustc_hir::intravisit::{self, Visitor};
 use rustc_hir::{
     self as hir, Attribute, CRATE_HIR_ID, Constness, FnSig, ForeignItem, GenericParam,
-    GenericParamKind, HirId, Item, ItemKind, MethodKind, Node, ParamName, Target, TraitItem,
+    GenericParamKind, HirId, Item, ItemKind, MethodKind, Mod, Node, ParamName, Target, TraitItem,
     find_attr,
 };
 use rustc_macros::Diagnostic;
@@ -291,7 +291,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
             AttributeKind::Optimize(..) => (),
             AttributeKind::PanicRuntime => (),
             AttributeKind::PatchableFunctionEntry { .. } => (),
-            AttributeKind::Path(..) => (),
+            AttributeKind::Path(_, span) => self.check_path(*span, hir_id),
             AttributeKind::PatternComplexityLimit { .. } => (),
             AttributeKind::PinV2(..) => (),
             AttributeKind::PreludeImport => (),
@@ -411,6 +411,52 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
         }
     }
 
+    fn check_path(&self, span: Span, hir_id: HirId) {
+        let Node::Item(item) = self.tcx.hir_node(hir_id) else {
+            return;
+        };
+
+        let ItemKind::Mod(_, module) = &item.kind else {
+            return;
+        };
+
+        if !item.span.contains(module.spans.inner_span) {
+            return;
+        }
+
+        // Do not warn when a nested module uses `#[path]` or is out-of-line,
+        // because the attribute may affect nested module path resolution.
+        if self.has_nested_module_path_dependency(module) {
+            return;
+        }
+
+        self.tcx.emit_node_span_lint(
+            UNUSED_ATTRIBUTES,
+            hir_id,
+            span,
+            diagnostics::Unused {
+                attr_span: span,
+                note: diagnostics::UnusedNote::PathOnInlineModule,
+            },
+        );
+    }
+
+    fn has_nested_module_path_dependency(&self, module: &Mod<'tcx>) -> bool {
+        module.item_ids.iter().any(|item_id| {
+            let child = self.tcx.hir_item(*item_id);
+
+            let ItemKind::Mod(_, child_module) = &child.kind else {
+                return false;
+            };
+
+            let is_out_of_line = !child.span.contains(child_module.spans.inner_span);
+
+            let has_path_attr = find_attr!(self.tcx, child.hir_id(), Path(..));
+
+            is_out_of_line || has_path_attr || self.has_nested_module_path_dependency(child_module)
+        })
+    }
+
     fn check_rustc_must_implement_one_of(
         &self,
         attr_span: Span,

compiler/rustc_passes/src/diagnostics.rs

@@ -272,6 +272,8 @@ pub(crate) enum UnusedNote {
     LinkerMessagesBinaryCrateOnly,
     #[note("the `dead_code_pub_in_binary` lint has no effect in library crates")]
     NoEffectDeadCodePubInBinary,
+    #[note("`#[path]` is unused on this inline module")]
+    PathOnInlineModule,
 }
 
 #[derive(Diagnostic)]

tests/ui/attributes/auxiliary/foo.rs

@@ -0,0 +1 @@
+mod foo {}

tests/ui/attributes/auxiliary/sub/foo.rs

@@ -0,0 +1 @@
+mod foo {}

tests/ui/attributes/path-inline-module.rs

@@ -0,0 +1,74 @@
+//@ check-pass
+
+fn main() {}
+
+#[path = "foo.rs"] //~ WARN unused attribute [unused_attributes]
+mod inline_module {}
+
+mod inline_module_with_inner_path {
+    #![path = "foo.rs"] //~ WARN unused attribute [unused_attributes]
+}
+
+#[path = "auxiliary/foo.rs"]
+mod outline_module; // Should not warn
+
+mod inline_with_inner_path {
+    #![path = "auxiliary"]
+
+    #[path = "foo.rs"] // Should not warn
+    mod file_submodule;
+}
+
+mod inline_with_inline_child {
+    #![path = "auxiliary"]
+
+    #[path = "foo.rs"] //~ WARN unused attribute [unused_attributes]
+    mod nested_inline_module {}
+}
+
+#[path = "auxiliary"]
+mod inline_parent_with_file_child {
+    #[path = "foo.rs"] // Should not warn
+    mod file_submodule;
+}
+
+#[path = "auxiliary"]
+mod inline_parent_with_inline_child {
+    #[path = "foo.rs"] //~ WARN unused attribute [unused_attributes]
+    mod inline_submodule {}
+}
+
+#[path = "auxiliary"]
+mod inline_parent_with_nested_file_child {
+    mod sub {
+        #[path = "foo.rs"] // Should not warn
+        mod file_submodule;
+    }
+}
+
+mod inline_with_inner_path_and_nested_file_child {
+    #![path = "auxiliary"]
+
+    #[path = "sub"]
+    mod middle {
+        #[path = "foo.rs"] // Should not warn
+        mod file_submodule;
+    }
+}
+
+#[path = "auxiliary"] //~ WARN unused attribute [unused_attributes]
+mod inline_parent_with_nested_inline_child {
+    mod middle {
+        mod inline_submodule {}
+    }
+}
+
+#[path = "auxiliary"]
+mod inline_parent_with_nested_inline_path {
+
+    #[path = "sub"]
+    mod middle {
+        #[path = "foo.rs"] //~ WARN unused attribute [unused_attributes]
+        mod inline_submodule {}
+    }
+}

tests/ui/attributes/path-inline-module.stderr

@@ -0,0 +1,51 @@
+warning: unused attribute
+  --> $DIR/path-inline-module.rs:5:1
+   |
+LL | #[path = "foo.rs"]
+   | ^^^^^^^^^^^^^^^^^^ help: remove this attribute
+   |
+   = note: `#[path]` is unused on this inline module
+   = note: requested on the command line with `-W unused-attributes`
+
+warning: unused attribute
+  --> $DIR/path-inline-module.rs:9:5
+   |
+LL |     #![path = "foo.rs"]
+   |     ^^^^^^^^^^^^^^^^^^^ help: remove this attribute
+   |
+   = note: `#[path]` is unused on this inline module
+
+warning: unused attribute
+  --> $DIR/path-inline-module.rs:59:1
+   |
+LL | #[path = "auxiliary"]
+   | ^^^^^^^^^^^^^^^^^^^^^ help: remove this attribute
+   |
+   = note: `#[path]` is unused on this inline module
+
+warning: unused attribute
+  --> $DIR/path-inline-module.rs:25:5
+   |
+LL |     #[path = "foo.rs"]
+   |     ^^^^^^^^^^^^^^^^^^ help: remove this attribute
+   |
+   = note: `#[path]` is unused on this inline module
+
+warning: unused attribute
+  --> $DIR/path-inline-module.rs:37:5
+   |
+LL |     #[path = "foo.rs"]
+   |     ^^^^^^^^^^^^^^^^^^ help: remove this attribute
+   |
+   = note: `#[path]` is unused on this inline module
+
+warning: unused attribute
+  --> $DIR/path-inline-module.rs:71:9
+   |
+LL |         #[path = "foo.rs"]
+   |         ^^^^^^^^^^^^^^^^^^ help: remove this attribute
+   |
+   = note: `#[path]` is unused on this inline module
+
+warning: 6 warnings emitted
+

tests/ui/feature-gates/issue-43106-gating-of-builtin-attrs.rs

@@ -253,7 +253,12 @@ mod macro_export {
 
 #[path = "3800"]
 mod path {
-    mod inner { #![path="3800"] }
+    mod inner {
+        #![path="3800"]
+        //~^ WARN unused attribute
+        //~| NOTE `#[path]` is unused on this inline module
+        //~| HELP remove this attribute
+    }
 
     #[path = "3800"] fn f() { }
     //~^ WARN attribute cannot be used on