fix(lints): Improve ill_formed_attribute_input with better help message

compiler/rustc_attr_parsing/src/attributes/doc.rs

@@ -665,7 +665,11 @@ impl DocParser {
                 let span = cx.attr_span;
                 cx.emit_lint(
                     rustc_session::lint::builtin::INVALID_DOC_ATTRIBUTES,
-                    AttributeLintKind::IllFormedAttributeInput { suggestions, docs: None },
+                    AttributeLintKind::IllFormedAttributeInput {
+                        suggestions,
+                        docs: None,
+                        help: None,
+                    },
                     span,
                 );
             }

compiler/rustc_attr_parsing/src/attributes/test_attrs.rs

@@ -27,8 +27,12 @@ impl<S: Stage> SingleAttributeParser<S> for IgnoreParser {
                     };
                     Some(str_value)
                 }
-                ArgParser::List(_) => {
-                    cx.adcx().warn_ill_formed_attribute_input(ILL_FORMED_ATTRIBUTE_INPUT);
+                ArgParser::List(list) => {
+                    let help = list.single().and_then(|item| item.meta_item()).and_then(|item| {
+                        item.args().no_args().ok()?;
+                        Some(item.path().to_string())
+                    });
+                    cx.warn_ill_formed_attribute_input_with_help(ILL_FORMED_ATTRIBUTE_INPUT, help);
                     return None;
                 }
             },

compiler/rustc_attr_parsing/src/context.rs

@@ -815,11 +815,19 @@ where
     }
 
     pub(crate) fn warn_ill_formed_attribute_input(&mut self, lint: &'static Lint) {
+        self.warn_ill_formed_attribute_input_with_help(lint, None);
+    }
+
+    pub(crate) fn warn_ill_formed_attribute_input_with_help(
+        &mut self,
+        lint: &'static Lint,
+        help: Option<String>,
+    ) {
         let suggestions = self.suggestions();
         let span = self.attr_span;
         self.emit_lint(
             lint,
-            AttributeLintKind::IllFormedAttributeInput { suggestions, docs: None },
+            AttributeLintKind::IllFormedAttributeInput { suggestions, docs: None, help },
             span,
         );
     }

compiler/rustc_attr_parsing/src/validate_attr.rs

@@ -211,6 +211,7 @@ fn emit_malformed_attribute(
             BuiltinLintDiag::AttributeLint(AttributeLintKind::IllFormedAttributeInput {
                 suggestions: suggestions.clone(),
                 docs: template.docs,
+                help: None,
             }),
         );
     } else {

compiler/rustc_lint/src/early/diagnostics.rs

@@ -175,14 +175,15 @@ impl<'a> Diagnostic<'a, ()> for DecorateAttrLint<'_, '_, '_> {
             &AttributeLintKind::UnusedDuplicate { this, other, warning } => {
                 lints::UnusedDuplicate { this, other, warning }.into_diag(dcx, level)
             }
-            AttributeLintKind::IllFormedAttributeInput { suggestions, docs } => {
+            AttributeLintKind::IllFormedAttributeInput { suggestions, docs, help } => {
                 lints::IllFormedAttributeInput {
                     num_suggestions: suggestions.len(),
                     suggestions: DiagArgValue::StrListSepByAnd(
                         suggestions.into_iter().map(|s| format!("`{s}`").into()).collect(),
                     ),
                     has_docs: docs.is_some(),
                     docs: docs.unwrap_or(""),
+                    help: help.clone().map(|h| lints::IllFormedAttributeInputHelp { lint: h }),
                 }
                 .into_diag(dcx, level)
             }

compiler/rustc_lint/src/lints.rs

@@ -3009,6 +3009,16 @@ pub(crate) struct IllFormedAttributeInput {
     #[note("for more information, visit <{$docs}>")]
     pub has_docs: bool,
     pub docs: &'static str,
+    #[subdiagnostic]
+    pub help: Option<IllFormedAttributeInputHelp>,
+}
+
+#[derive(Subdiagnostic)]
+#[help(
+    "if you meant to silence a warning, consider using #![allow({$lint})] or #![expect({$lint})]"
+)]
+pub(crate) struct IllFormedAttributeInputHelp {
+    pub lint: String,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_lint_defs/src/lib.rs

@@ -703,6 +703,7 @@ pub enum AttributeLintKind {
     IllFormedAttributeInput {
         suggestions: Vec<String>,
         docs: Option<&'static str>,
+        help: Option<String>,
     },
     EmptyAttribute {
         first_span: Span,

tests/ui/malformed/ignore-with-lint-name.rs

@@ -0,0 +1,6 @@
+#[ignore(clippy::single_match)]
+//~^ ERROR valid forms for the attribute are `#[ignore = "reason"]` and `#[ignore]`
+//~| HELP if you meant to silence a warning, consider using #![allow(clippy::single_match)] or #![expect(clippy::single_match)]
+//~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+
+fn main() {}

tests/ui/malformed/ignore-with-lint-name.stderr

@@ -0,0 +1,25 @@
+error: valid forms for the attribute are `#[ignore = "reason"]` and `#[ignore]`
+  --> $DIR/ignore-with-lint-name.rs:1:1
+   |
+LL | #[ignore(clippy::single_match)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: if you meant to silence a warning, consider using #![allow(clippy::single_match)] or #![expect(clippy::single_match)]
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #57571 <https://github.com/rust-lang/rust/issues/57571>
+   = note: `#[deny(ill_formed_attribute_input)]` (part of `#[deny(future_incompatible)]`) on by default
+
+error: aborting due to 1 previous error
+
+Future incompatibility report: Future breakage diagnostic:
+error: valid forms for the attribute are `#[ignore = "reason"]` and `#[ignore]`
+  --> $DIR/ignore-with-lint-name.rs:1:1
+   |
+LL | #[ignore(clippy::single_match)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: if you meant to silence a warning, consider using #![allow(clippy::single_match)] or #![expect(clippy::single_match)]
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #57571 <https://github.com/rust-lang/rust/issues/57571>
+   = note: `#[deny(ill_formed_attribute_input)]` (part of `#[deny(future_incompatible)]`) on by default
+