Reject empty cfg_attr

gcc/rust/ChangeLog:

	* ast/rust-ast.cc (Attribute::check_cfg_predicate): Emit an error with
	empty cfg_attr input.

gcc/testsuite/ChangeLog:

	* rust/compile/issue-3966.rs: New test.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>

Author: P-E-P

gcc/rust/ast/rust-ast.cc

@@ -4149,11 +4149,12 @@ AttrInputMetaItemContainer::separate_cfg_attrs () const
 bool
 Attribute::check_cfg_predicate (const Session &session) const
 {
+  auto string_path = path.as_string ();
   /* assume that cfg predicate actually can exist, i.e. attribute has cfg or
    * cfg_attr path */
   if (!has_attr_input ()
-      || (path.as_string () != Values::Attributes::CFG
-	  && path.as_string () != Values::Attributes::CFG_ATTR))
+      || (string_path != Values::Attributes::CFG
+	  && string_path != Values::Attributes::CFG_ATTR))
     {
       // DEBUG message
       rust_debug (
@@ -4169,6 +4170,13 @@ Attribute::check_cfg_predicate (const Session &session) const
     return false;
 
   auto &meta_item = static_cast<AttrInputMetaItemContainer &> (*attr_input);
+  if (meta_item.get_items ().empty ()
+      && string_path == Values::Attributes::CFG_ATTR)
+    {
+      rust_error_at (path.get_locus (), "malformed %qs attribute input",
+		     string_path.c_str ());
+      return false;
+    }
   return meta_item.get_items ().front ()->check_cfg_predicate (session);
 }
 

gcc/testsuite/rust/compile/issue-3966.rs

@@ -0,0 +1,5 @@
+struct S {
+    #[cfg_attr()]
+    field: u8,
+    // { dg-error "malformed .cfg_attr. attribute input" "" { target *-*-* } .-2 }
+}