Add feature hint for unstable diagnostic attributes

compiler/rustc_resolve/src/errors.rs

@@ -1521,20 +1521,24 @@ pub(crate) struct RedundantImportVisibility {
 #[diag("unknown diagnostic attribute")]
 pub(crate) struct UnknownDiagnosticAttribute {
     #[subdiagnostic]
-    pub typo: Option<UnknownDiagnosticAttributeTypoSugg>,
+    pub help: Option<UnknownDiagnosticAttributeHelp>,
 }
 
 #[derive(Subdiagnostic)]
-#[suggestion(
-    "an attribute with a similar name exists",
-    style = "verbose",
-    code = "{typo_name}",
-    applicability = "machine-applicable"
-)]
-pub(crate) struct UnknownDiagnosticAttributeTypoSugg {
-    #[primary_span]
-    pub span: Span,
-    pub typo_name: Symbol,
+pub(crate) enum UnknownDiagnosticAttributeHelp {
+    #[suggestion(
+        "an attribute with a similar name exists",
+        style = "verbose",
+        code = "{typo_name}",
+        applicability = "machine-applicable"
+    )]
+    Typo {
+        #[primary_span]
+        span: Span,
+        typo_name: Symbol,
+    },
+    #[help("add `#![feature({$feature})]` to the crate attributes to enable")]
+    UseFeature { feature: Symbol },
 }
 
 // FIXME: Make this properly translatable.

compiler/rustc_resolve/src/macros.rs

@@ -723,26 +723,44 @@ fn smart_resolve_macro_path(
         if res == Res::NonMacroAttr(NonMacroAttrKind::Tool)
             && let [namespace, attribute, ..] = &*path.segments
             && namespace.ident.name == sym::diagnostic
-            && !DIAGNOSTIC_ATTRIBUTES.iter().any(|(attr, stable)| {
+            && !DIAGNOSTIC_ATTRIBUTES.iter().any(|(attr, feature)| {
                 attribute.ident.name == *attr
-                    && stable.is_none_or(|f| self.tcx.features().enabled(f))
+                    && feature.is_none_or(|f| self.tcx.features().enabled(f))
             })
         {
+            let name = attribute.ident.name;
             let span = attribute.span();
-            let candidates = DIAGNOSTIC_ATTRIBUTES
-                .iter()
-                .filter_map(|(sym, stable)| {
-                    stable.is_none_or(|f| self.tcx.features().enabled(f)).then_some(*sym)
+
+            let help = 'help: {
+                if self.tcx.sess.is_nightly_build() {
+                    for (attr, feature) in DIAGNOSTIC_ATTRIBUTES {
+                        if let Some(feature) = *feature
+                            && *attr == name
+                        {
+                            break 'help Some(errors::UnknownDiagnosticAttributeHelp::UseFeature {
+                                feature,
+                            });
+                        }
+                    }
+                }
+
+                let candidates = DIAGNOSTIC_ATTRIBUTES
+                    .iter()
+                    .filter_map(|(attr, feature)| {
+                        feature.is_none_or(|f| self.tcx.features().enabled(f)).then_some(*attr)
+                    })
+                    .collect::<Vec<_>>();
+
+                find_best_match_for_name(&candidates, name, None).map(|typo_name| {
+                    errors::UnknownDiagnosticAttributeHelp::Typo { span, typo_name }
                 })
-                .collect::<Vec<_>>();
-            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(
                 UNKNOWN_DIAGNOSTIC_ATTRIBUTES,
                 span,
                 node_id,
-                errors::UnknownDiagnosticAttribute { typo },
+                errors::UnknownDiagnosticAttribute { help },
             );
         }
 

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

@@ -4,6 +4,7 @@ warning: unknown diagnostic attribute
 LL | #[diagnostic::on_move(message = "Foo")]
    |               ^^^^^^^
    |
+   = help: add `#![feature(diagnostic_on_move)]` to the crate attributes to enable
    = note: `#[warn(unknown_diagnostic_attributes)]` (part of `#[warn(unknown_or_malformed_diagnostic_attributes)]`) on by default
 
 error[E0382]: use of moved value: `foo`

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

@@ -10,6 +10,7 @@ error: unknown diagnostic attribute
 LL | #[diagnostic::on_unknown(message = "Tada")]
    |               ^^^^^^^^^^
    |
+   = help: add `#![feature(diagnostic_on_unknown)]` to the crate attributes to enable
 note: the lint level is defined here
   --> $DIR/feature-gate-diagnostic-on-unknown.rs:1:9
    |