improve empty attribute diagnostic

Author: jdonszelmann

compiler/rustc_attr_parsing/messages.ftl

@@ -9,6 +9,7 @@ attr_parsing_deprecated_item_suggestion =
 attr_parsing_empty_attribute =
     unused attribute
     .suggestion = remove this attribute
+    .note = attribute `{$attr_path}` with an empty list has no effect
 
 attr_parsing_invalid_target = `#[{$name}]` attribute cannot be used on {$target}
     .help = `#[{$name}]` can {$only}be applied to {$applied}

compiler/rustc_attr_parsing/src/context.rs

@@ -591,7 +591,8 @@ impl<'f, 'sess: 'f, S: Stage> AcceptContext<'f, 'sess, S> {
     }
 
     pub(crate) fn warn_empty_attribute(&mut self, span: Span) {
-        self.emit_lint(AttributeLintKind::EmptyAttribute { first_span: span }, span);
+        let attr_path = self.attr_path.clone();
+        self.emit_lint(AttributeLintKind::EmptyAttribute { first_span: span, attr_path }, span);
     }
 }
 

compiler/rustc_attr_parsing/src/lints.rs

@@ -31,12 +31,16 @@ pub fn emit_attribute_lint<L: LintEmitter>(lint: &AttributeLint<L::Id>, lint_emi
                 },
             );
         }
-        AttributeLintKind::EmptyAttribute { first_span } => lint_emitter.emit_node_span_lint(
-            rustc_session::lint::builtin::UNUSED_ATTRIBUTES,
-            *id,
-            *first_span,
-            session_diagnostics::EmptyAttributeList { attr_span: *first_span },
-        ),
+        AttributeLintKind::EmptyAttribute { first_span, attr_path } => lint_emitter
+            .emit_node_span_lint(
+                rustc_session::lint::builtin::UNUSED_ATTRIBUTES,
+                *id,
+                *first_span,
+                session_diagnostics::EmptyAttributeList {
+                    attr_span: *first_span,
+                    attr_path: attr_path.clone(),
+                },
+            ),
         AttributeLintKind::InvalidTarget { name, target, applied, only } => lint_emitter
             .emit_node_span_lint(
                 // This check is here because `deprecated` had its own lint group and removing this would be a breaking change

compiler/rustc_attr_parsing/src/session_diagnostics.rs

@@ -475,9 +475,11 @@ pub(crate) struct EmptyConfusables {
 
 #[derive(LintDiagnostic)]
 #[diag(attr_parsing_empty_attribute)]
+#[note]
 pub(crate) struct EmptyAttributeList {
     #[suggestion(code = "", applicability = "machine-applicable")]
     pub attr_span: Span,
+    pub attr_path: AttrPath,
 }
 
 #[derive(LintDiagnostic)]

compiler/rustc_hir/src/lints.rs

@@ -33,7 +33,7 @@ pub struct AttributeLint<Id> {
 pub enum AttributeLintKind {
     UnusedDuplicate { this: Span, other: Span, warning: bool },
     IllFormedAttributeInput { suggestions: Vec<String> },
-    EmptyAttribute { first_span: Span },
+    EmptyAttribute { first_span: Span, attr_path: AttrPath },
     InvalidTarget { name: AttrPath, target: Target, applied: Vec<String>, only: &'static str },
     InvalidStyle { name: AttrPath, is_used_as_inner: bool, target: Target, target_span: Span },
 }

tests/ui/attributes/empty-repr.stderr

@@ -4,6 +4,7 @@ error: unused attribute
 LL | #[repr()]
    | ^^^^^^^^^ help: remove this attribute
    |
+   = note: attribute `repr` with an empty list has no effect
 note: the lint level is defined here
   --> $DIR/empty-repr.rs:4:9
    |

tests/ui/empty/empty-attributes.stderr

@@ -56,12 +56,16 @@ error: unused attribute
    |
 LL | #[repr()]
    | ^^^^^^^^^ help: remove this attribute
+   |
+   = note: attribute `repr` with an empty list has no effect
 
 error: unused attribute
   --> $DIR/empty-attributes.rs:12:1
    |
 LL | #[target_feature()]
    | ^^^^^^^^^^^^^^^^^^^ help: remove this attribute
+   |
+   = note: attribute `target_feature` with an empty list has no effect
 
 error: aborting due to 8 previous errors
 

tests/ui/macros/macro-use-all-and-none.stderr

@@ -4,6 +4,7 @@ warning: unused attribute
 LL | #[macro_use()]
    |            ^^ help: remove this attribute
    |
+   = note: attribute `macro_use` with an empty list has no effect
 note: the lint level is defined here
   --> $DIR/macro-use-all-and-none.rs:4:9
    |

tests/ui/repr/repr-empty-packed.stderr

@@ -15,6 +15,7 @@ error: unused attribute
 LL | #[repr()]
    | ^^^^^^^^^ help: remove this attribute
    |
+   = note: attribute `repr` with an empty list has no effect
 note: the lint level is defined here
   --> $DIR/repr-empty-packed.rs:2:9
    |