gccrs: fix cfg attribute without parentheses error

gcc/rust/ChangeLog:

	* ast/rust-ast.cc (Attribute::check_cfg_predicate):

gcc/testsuite/ChangeLog:

	* rust/compile/issue-4262.rs: New test.

Signed-off-by: Lucas Ly Ba <lucas.ly-ba@outlook.com>

Author: lucasly-ba

gcc/rust/ast/rust-ast.cc

@@ -4160,16 +4160,10 @@ 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 ()
-      || (string_path != Values::Attributes::CFG
-	  && string_path != Values::Attributes::CFG_ATTR))
+  if (!has_attr_input ())
     {
-      // DEBUG message
-      rust_debug (
-	"tried to check cfg predicate on attr that either has no input "
-	"or invalid path. attr: '%s'",
-	as_string ().c_str ());
-
+      rust_error_at (path.get_locus (), "%<%s%> is not followed by parentheses",
+		     string_path.c_str ());
       return false;
     }
 

gcc/testsuite/rust/compile/issue-4262.rs

@@ -0,0 +1,3 @@
+#[cfg]
+// { dg-error ".cfg. is not followed by parentheses" "" { target *-*-* } .-1 }
+fn a() {}