Lower extern type alias as foreign opaque type.

Author: crlf0710

crates/hir_def/src/data.rs

@@ -54,6 +54,7 @@ pub struct TypeAliasData {
     pub name: Name,
     pub type_ref: Option<TypeRef>,
     pub visibility: RawVisibility,
+    pub is_extern: bool,
     /// Bounds restricting the type alias itself (eg. `type Ty: Bound;` in a trait or impl).
     pub bounds: Vec<TypeBound>,
 }
@@ -71,6 +72,7 @@ impl TypeAliasData {
             name: typ.name.clone(),
             type_ref: typ.type_ref.clone(),
             visibility: item_tree[typ.visibility].clone(),
+            is_extern: typ.is_extern,
             bounds: typ.bounds.to_vec(),
         })
     }

crates/hir_ty/src/lower.rs

@@ -1101,9 +1101,13 @@ fn type_for_type_alias(db: &dyn HirDatabase, t: TypeAliasId) -> Binders<Ty> {
     let resolver = t.resolver(db.upcast());
     let ctx =
         TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
-    let type_ref = &db.type_alias_data(t).type_ref;
     let substs = Substs::bound_vars(&generics, DebruijnIndex::INNERMOST);
-    let inner = Ty::from_hir(&ctx, type_ref.as_ref().unwrap_or(&TypeRef::Error));
+    let inner = if db.type_alias_data(t).is_extern {
+        Ty::simple(TypeCtor::ForeignType(t))
+    } else {
+        let type_ref = &db.type_alias_data(t).type_ref;
+        Ty::from_hir(&ctx, type_ref.as_ref().unwrap_or(&TypeRef::Error))
+    };
     Binders::new(substs.len(), inner)
 }