Add missing HIR lowering for SliceTypes

Addresses #849

Author: philberty

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

@@ -278,6 +278,24 @@ class ASTLoweringType : public ASTLoweringBase
 			       translated);
   }
 
+  void visit (AST::SliceType &type) override
+  {
+    HIR::Type *base_type
+      = ASTLoweringType::translate (type.get_elem_type ().get ());
+
+    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::SliceType (mapping, std::unique_ptr<HIR::Type> (base_type),
+			    type.get_locus ());
+
+    mappings->insert_hir_type (mapping.get_crate_num (), mapping.get_hirid (),
+			       translated);
+  }
+
   void visit (AST::InferredType &type) override
   {
     auto crate_num = mappings->get_current_crate ();