lang_items: add support for lang items on variants

This commit adds support for lang items (`#[lang = "..."]` attributes)
on enum variants.

Signed-off-by: David Wood <david@davidtw.co>

src/librustc_hir/target.rs

@@ -29,6 +29,7 @@ pub enum Target {
     TyAlias,
     OpaqueTy,
     Enum,
+    Variant,
     Struct,
     Union,
     Trait,
@@ -62,6 +63,7 @@ impl Display for Target {
                 Target::TyAlias => "type alias",
                 Target::OpaqueTy => "opaque type",
                 Target::Enum => "enum",
+                Target::Variant => "enum variant",
                 Target::Struct => "struct",
                 Target::Union => "union",
                 Target::Trait => "trait",

src/librustc_passes/lang_items.rs

@@ -30,7 +30,13 @@ struct LanguageItemCollector<'tcx> {
 
 impl ItemLikeVisitor<'v> for LanguageItemCollector<'tcx> {
     fn visit_item(&mut self, item: &hir::Item<'_>) {
-        self.check_for_lang(Target::from_item(item), item.hir_id, item.attrs)
+        self.check_for_lang(Target::from_item(item), item.hir_id, item.attrs);
+
+        if let hir::ItemKind::Enum(def, ..) = &item.kind {
+            for variant in def.variants {
+                self.check_for_lang(Target::Variant, variant.id, variant.attrs);
+            }
+        }
     }
 
     fn visit_trait_item(&mut self, trait_item: &hir::TraitItem<'_>) {