gccrs: proc_macro: Add is_available callback

Add a callback from gcc to determine wether the bridge is available or
not.

gcc/rust/ChangeLog:

	* expand/rust-proc-macro.cc (available): Add symbol
	registration.
	(load_macros_array): Likewise.

libgrust/ChangeLog:

	* libproc_macro/proc_macro.cc (not_available): Add a
	function to express bridge unavailability.
	* libproc_macro/proc_macro.h (not_available): Likewise.
	* libproc_macro/registration.h: Add symbol type.

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

gcc/rust/expand/rust-proc-macro.cc

@@ -56,6 +56,12 @@ static_assert (
 	       ProcMacro::from_str_function_t>::value,
   "Registration callback signature not synced, check proc macro internals.");
 
+static bool
+available ()
+{
+  return true;
+}
+
 template <typename Symbol, typename Callback>
 bool
 register_callback (void *handle, Symbol, std::string symbol_name,
@@ -95,6 +101,9 @@ load_macros_array (std::string path)
   if (!REGISTER_CALLBACK (handle, __gccrs_proc_macro_from_str_fn,
 			  tokenstream_from_string))
     return nullptr;
+  if (!REGISTER_CALLBACK (handle, __gccrs_proc_macro_is_available_fn,
+			  available))
+    return nullptr;
 
   // FIXME: Add CrateStableId handling, right now all versions may be loaded,
   // even incompatible ones.

libgrust/libproc_macro/proc_macro.cc

@@ -49,6 +49,14 @@ Procmacro::make_bang (const char *name, BangMacro macro)
   return {BANG, payload};
 }
 
+bool
+not_available ()
+{
+  return false;
+}
+
 } // namespace ProcMacro
 
 ProcMacro::from_str_function_t __gccrs_proc_macro_from_str_fn = nullptr;
+ProcMacro::is_available_function_t __gccrs_proc_macro_is_available_fn
+  = ProcMacro::not_available;

libgrust/libproc_macro/proc_macro.h

@@ -99,6 +99,9 @@ struct ProcmacroArray
   Procmacro *macros;
 };
 
+bool
+not_available ();
+
 } // namespace ProcMacro
 
 #endif /* ! PROC_MACRO_H */

libgrust/libproc_macro/registration.h

@@ -29,9 +29,12 @@
 namespace ProcMacro {
 
 using from_str_function_t = ProcMacro::TokenStream (*) (std::string &, bool &);
+using is_available_function_t = bool (*) ();
 
 } // namespace ProcMacro
 
 extern "C" ProcMacro::from_str_function_t __gccrs_proc_macro_from_str_fn;
+extern "C" ProcMacro::is_available_function_t
+  __gccrs_proc_macro_is_available_fn;
 
 #endif /* !REGISTRATION_H */