extern-types: Lower to HIR::ExternalTypeItem properly

gcc/rust/ChangeLog:

	* hir/rust-ast-lower-extern.h: Lower to HIR::ExternalTypeItem nodes.
	* hir/tree/rust-hir-item.h (class ExternalTypeItem): Create private
	visibility by default as extern types have no visibility - add a comment
	about the correctness of this.

gcc/rust/hir/rust-ast-lower-extern.h

@@ -22,6 +22,7 @@
 #include "rust-ast-lower-base.h"
 #include "rust-ast-lower-type.h"
 #include "rust-ast-lower.h"
+#include "rust-hir-full-decls.h"
 
 namespace Rust {
 namespace HIR {
@@ -116,7 +117,13 @@ class ASTLoweringExternItem : public ASTLoweringBase
 
   void visit (AST::ExternalTypeItem &type) override
   {
-    rust_sorry_at (type.get_locus (), "extern types are not implemented yet");
+    auto crate_num = mappings->get_current_crate ();
+    Analysis::NodeMapping mapping (crate_num, type.get_node_id (),
+				   mappings->get_next_hir_id (crate_num),
+				   mappings->get_next_localdef_id (crate_num));
+
+    translated = new HIR::ExternalTypeItem (mapping, type.get_identifier (),
+					    type.get_locus ());
   }
 
 private:

gcc/rust/hir/tree/rust-hir-item.h

@@ -3152,16 +3152,20 @@ class ExternalFunctionItem : public ExternalItem
 
 class ExternalTypeItem : public ExternalItem
 {
+public:
   ExternalTypeItem (Analysis::NodeMapping mappings, Identifier item_name,
-		    Visibility vis, AST::AttrVec outer_attrs, location_t locus)
+		    location_t locus)
     : ExternalItem (std::move (mappings), std::move (item_name),
-		    std::move (vis), std::move (outer_attrs), locus)
+		    Visibility (Visibility::PRIVATE),
+		    /* FIXME: Is that correct? */
+		    {}, locus)
   {}
 
   ExternalTypeItem (ExternalTypeItem const &other) : ExternalItem (other) {}
 
   ExternalTypeItem (ExternalTypeItem &&other) = default;
   ExternalTypeItem &operator= (ExternalTypeItem &&other) = default;
+  ExternalTypeItem &operator= (ExternalTypeItem const &other) = default;
 
   std::string as_string () const override;
 
@@ -3171,6 +3175,8 @@ class ExternalTypeItem : public ExternalItem
   ExternKind get_extern_kind () override { return ExternKind::Type; }
 
 protected:
+  /* Use covariance to implement clone function as returning this object
+   * rather than base */
   ExternalTypeItem *clone_external_item_impl () const override
   {
     return new ExternalTypeItem (*this);