Improve emission of UnknownDiagnosticAttribute lint

compiler/rustc_attr_parsing/src/attributes/diagnostic/on_const.rs

@@ -17,6 +17,7 @@ impl<S: Stage> AttributeParser<S> for OnConstParser {
         template!(List: &[r#"/*opt*/ message = "...", /*opt*/ label = "...", /*opt*/ note = "...""#]),
         |this, cx, args| {
             if !cx.features().diagnostic_on_const() {
+                // `UnknownDiagnosticAttribute` is emitted in rustc_resolve/macros.rs
                 return;
             }
 

compiler/rustc_attr_parsing/src/attributes/diagnostic/on_move.rs

@@ -24,6 +24,7 @@ impl OnMoveParser {
         mode: Mode,
     ) {
         if !cx.features().diagnostic_on_move() {
+            // `UnknownDiagnosticAttribute` is emitted in rustc_resolve/macros.rs
             return;
         }
 

compiler/rustc_attr_parsing/src/attributes/diagnostic/on_unknown.rs

@@ -18,6 +18,7 @@ impl OnUnknownParser {
         mode: Mode,
     ) {
         if !cx.features().diagnostic_on_unknown() {
+            // `UnknownDiagnosticAttribute` is emitted in rustc_resolve/macros.rs
             return;
         }
         let span = cx.attr_span;

compiler/rustc_resolve/src/macros.rs

@@ -712,27 +712,30 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             feature_err(&self.tcx.sess, sym::custom_inner_attributes, path.span, msg).emit();
         }
 
-        let diagnostic_attributes: &[(Symbol, bool)] = &[
-            (sym::on_unimplemented, true),
-            (sym::do_not_recommend, true),
-            (sym::on_move, true),
-            (sym::on_const, self.tcx.features().diagnostic_on_const()),
-            (sym::on_unknown, self.tcx.features().diagnostic_on_unknown()),
+        const DIAGNOSTIC_ATTRIBUTES: &[(Symbol, Option<Symbol>)] = &[
+            (sym::on_unimplemented, None),
+            (sym::do_not_recommend, None),
+            (sym::on_move, Some(sym::diagnostic_on_move)),
+            (sym::on_const, Some(sym::diagnostic_on_const)),
+            (sym::on_unknown, Some(sym::diagnostic_on_unknown)),
         ];
 
         if res == Res::NonMacroAttr(NonMacroAttrKind::Tool)
             && let [namespace, attribute, ..] = &*path.segments
             && namespace.ident.name == sym::diagnostic
-            && !diagnostic_attributes
-                .iter()
-                .any(|(attr, stable)| *stable && attribute.ident.name == *attr)
+            && !DIAGNOSTIC_ATTRIBUTES.iter().any(|(attr, stable)| {
+                attribute.ident.name == *attr
+                    && stable.is_none_or(|f| self.tcx.features().enabled(f))
+            })
         {
             let span = attribute.span();
-            let candidates = diagnostic_attributes
+            let candidates = DIAGNOSTIC_ATTRIBUTES
                 .iter()
-                .filter_map(|(sym, stable)| stable.then_some(*sym))
+                .filter_map(|(sym, stable)| {
+                    stable.is_none_or(|f| self.tcx.features().enabled(f)).then_some(*sym)
+                })
                 .collect::<Vec<_>>();
-            let typo = find_best_match_for_name(&candidates, attribute.ident.name, Some(5))
+            let typo = find_best_match_for_name(&candidates, attribute.ident.name, None)
                 .map(|typo_name| errors::UnknownDiagnosticAttributeTypoSugg { span, typo_name });
 
             self.tcx.sess.psess.buffer_lint(

tests/ui/feature-gates/feature-gate-diagnostic-on-move.rs

@@ -2,9 +2,8 @@
 //! gate, but the fact that not adding the feature gate will cause the
 //! diagnostic to not emit the custom diagnostic message
 //!
-#[diagnostic::on_move(
-    message = "Foo"
-)]
+#[diagnostic::on_move(message = "Foo")]
+//~^ WARN unknown diagnostic attribute
 #[derive(Debug)]
 struct Foo;
 

tests/ui/feature-gates/feature-gate-diagnostic-on-move.stderr

@@ -1,5 +1,13 @@
+warning: unknown diagnostic attribute
+  --> $DIR/feature-gate-diagnostic-on-move.rs:5:15
+   |
+LL | #[diagnostic::on_move(message = "Foo")]
+   |               ^^^^^^^
+   |
+   = note: `#[warn(unknown_diagnostic_attributes)]` (part of `#[warn(unknown_or_malformed_diagnostic_attributes)]`) on by default
+
 error[E0382]: use of moved value: `foo`
-  --> $DIR/feature-gate-diagnostic-on-move.rs:16:15
+  --> $DIR/feature-gate-diagnostic-on-move.rs:15:15
    |
 LL |     let foo = Foo;
    |         --- move occurs because `foo` has type `Foo`, which does not implement the `Copy` trait
@@ -9,21 +17,21 @@ LL |     let bar = foo;
    |               ^^^ value used here after move
    |
 note: consider changing this parameter type in function `takes_foo` to borrow instead if owning the value isn't necessary
-  --> $DIR/feature-gate-diagnostic-on-move.rs:11:17
+  --> $DIR/feature-gate-diagnostic-on-move.rs:10:17
    |
 LL | fn takes_foo(_: Foo) {}
    |    ---------    ^^^ this parameter takes ownership of the value
    |    |
    |    in this function
 note: if `Foo` implemented `Clone`, you could clone the value
-  --> $DIR/feature-gate-diagnostic-on-move.rs:9:1
+  --> $DIR/feature-gate-diagnostic-on-move.rs:8:1
    |
 LL | struct Foo;
    | ^^^^^^^^^^ consider implementing `Clone` for this type
 ...
 LL |     takes_foo(foo);
    |               --- you could clone this value
 
-error: aborting due to 1 previous error
+error: aborting due to 1 previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0382`.