improve empty attribute diagnostic

compiler/rustc_attr_parsing/messages.ftl

@@ -8,7 +8,15 @@ attr_parsing_deprecated_item_suggestion =
 
 attr_parsing_empty_attribute =
     unused attribute
-    .suggestion = remove this attribute
+    .suggestion = {$valid_without_list ->
+        [true] remove these parentheses
+        *[other] remove this attribute
+    }
+    .note = {$valid_without_list ->
+        [true] using `{$attr_path}` with an empty list is equivalent to not using a list at all
+        *[other] using `{$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

@@ -597,7 +597,12 @@ 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();
+        let valid_without_list = self.template.word;
+        self.emit_lint(
+            AttributeLintKind::EmptyAttribute { first_span: span, attr_path, valid_without_list },
+            span,
+        );
     }
 }
 

compiler/rustc_attr_parsing/src/lints.rs

@@ -43,12 +43,18 @@ 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, valid_without_list } => {
+            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(),
+                    valid_without_list: *valid_without_list,
+                },
+            )
+        }
         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

@@ -503,9 +503,12 @@ 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,
+    pub valid_without_list: bool,
 }
 
 #[derive(LintDiagnostic)]

compiler/rustc_hir/src/lints.rs

@@ -31,6 +31,12 @@ pub struct AttributeLint<Id> {
 
 #[derive(Clone, Debug, HashStable_Generic)]
 pub enum AttributeLintKind {
+    /// Copy of `IllFormedAttributeInput`
+    /// specifically for the `invalid_macro_export_arguments` lint until that is removed,
+    /// see <https://github.com/rust-lang/rust/pull/143857#issuecomment-3079175663>
+    InvalidMacroExportArguments {
+        suggestions: Vec<String>,
+    },
     UnusedDuplicate {
         this: Span,
         other: Span,
@@ -41,13 +47,8 @@ pub enum AttributeLintKind {
     },
     EmptyAttribute {
         first_span: Span,
-    },
-
-    /// Copy of `IllFormedAttributeInput`
-    /// specifically for the `invalid_macro_export_arguments` lint until that is removed,
-    /// see <https://github.com/rust-lang/rust/pull/143857#issuecomment-3079175663>
-    InvalidMacroExportArguments {
-        suggestions: Vec<String>,
+        attr_path: AttrPath,
+        valid_without_list: bool,
     },
     InvalidTarget {
         name: AttrPath,

tests/ui/attributes/empty-repr.stderr

@@ -4,6 +4,7 @@ error: unused attribute
 LL | #[repr()]
    | ^^^^^^^^^ help: remove this attribute
    |
+   = note: using `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: using `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: using `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

@@ -2,8 +2,9 @@ warning: unused attribute
   --> $DIR/macro-use-all-and-none.rs:7:12
    |
 LL | #[macro_use()]
-   |            ^^ help: remove this attribute
+   |            ^^ help: remove these parentheses
    |
+   = note: using `macro_use` with an empty list is equivalent to not using a list at all
 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: using `repr` with an empty list has no effect
 note: the lint level is defined here
   --> $DIR/repr-empty-packed.rs:2:9
    |