Make foreverstack debug string const

Those function should not change anything within the foreverstack, it
can therefore be made const.

gcc/rust/ChangeLog:

	* resolve/rust-forever-stack.h: Make debug functions const.
	* resolve/rust-forever-stack.hxx: Likewise.

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

gcc/rust/resolve/rust-forever-stack.h

@@ -519,7 +519,7 @@ template <Namespace N> class ForeverStack
   tl::optional<Rib &> to_rib (NodeId rib_id);
   tl::optional<const Rib &> to_rib (NodeId rib_id) const;
 
-  std::string as_debug_string ();
+  std::string as_debug_string () const;
 
   /**
    * Used to check if a module is a descendant of another module
@@ -601,9 +601,9 @@ template <Namespace N> class ForeverStack
   std::reference_wrapper<Node> cursor_reference;
 
   void stream_rib (std::stringstream &stream, const Rib &rib,
-		   const std::string &next, const std::string &next_next);
+		   const std::string &next, const std::string &next_next) const;
   void stream_node (std::stringstream &stream, unsigned indentation,
-		    const Node &node);
+		    const Node &node) const;
 
   /* Helper types and functions for `resolve_path` */
 

gcc/rust/resolve/rust-forever-stack.hxx

@@ -699,7 +699,7 @@ template <Namespace N>
 void
 ForeverStack<N>::stream_rib (std::stringstream &stream, const Rib &rib,
 			     const std::string &next,
-			     const std::string &next_next)
+			     const std::string &next_next) const
 {
   if (rib.get_values ().empty ())
     {
@@ -718,7 +718,7 @@ ForeverStack<N>::stream_rib (std::stringstream &stream, const Rib &rib,
 template <Namespace N>
 void
 ForeverStack<N>::stream_node (std::stringstream &stream, unsigned indentation,
-			      const ForeverStack<N>::Node &node)
+			      const ForeverStack<N>::Node &node) const
 {
   auto indent = std::string (indentation, ' ');
   auto next = std::string (indentation + 4, ' ');
@@ -750,7 +750,7 @@ ForeverStack<N>::stream_node (std::stringstream &stream, unsigned indentation,
 
 template <Namespace N>
 std::string
-ForeverStack<N>::as_debug_string ()
+ForeverStack<N>::as_debug_string () const
 {
   std::stringstream stream;