ENHANCED: use push_prolog_flag/2 for xref-flag scoping

boot/autoload.pl exports/5, library/prolog_xref.pl with_xref/1 and
library/prolog_source.pl read_directives.  Drops the fallback for
"xref flag might not exist" and the state term threading OldXref,
both handled by push/pop.

Author: JanWielemaker

boot/autoload.pl

@@ -527,14 +527,10 @@
     exports(File, Module, Exports, _Meta, _Public).
 
 exports(File, Module, Exports, Meta, Public) :-
-    (   current_prolog_flag(xref, Old)
-    ->  true
-    ;   Old = false
-    ),
     setup_call_cleanup(
-        set_prolog_flag(xref, true),
+        push_prolog_flag(xref, true),
         snapshot(exports_(File, Module, Exports, Meta, Public)),
-        set_prolog_flag(xref, Old)).
+        pop_prolog_flag(xref)).
 
 exports_(File, Module, Exports, Meta, Public) :-
     State = state(true, _, [], [], []),

library/prolog_source.pl

@@ -651,15 +651,14 @@
         phrase(read_directives(In, Options, [true]), Directives),
         read_directives_cleanup(In, State)).
 
-read_directives_setup(TempModule, Path, In, state(OldM, OldXref)) :-
+read_directives_setup(TempModule, Path, In, OldM) :-
     prolog_open_source(Path, In),
     '$set_source_module'(OldM, TempModule),
-    current_prolog_flag(xref, OldXref),
-    set_prolog_flag(xref, true).
+    push_prolog_flag(xref, true).
 
-read_directives_cleanup(In, state(OldM, OldXref)) :-
+read_directives_cleanup(In, OldM) :-
     '$set_source_module'(OldM),
-    set_prolog_flag(xref, OldXref),
+    pop_prolog_flag(xref),
     prolog_close_source(In).
 
 read_directives(In, Options, State) -->

library/prolog_xref.pl

@@ -437,14 +437,10 @@
     with_xref(0).
 
 with_xref(Goal) :-
-    current_prolog_flag(xref, Xref),
-    (   Xref == true
-    ->  call(Goal)
-    ;   setup_call_cleanup(
-            set_prolog_flag(xref, true),
-            Goal,
-            set_prolog_flag(xref, Xref))
-    ).
+    setup_call_cleanup(
+        push_prolog_flag(xref, true),
+        Goal,
+        pop_prolog_flag(xref)).
 
 
 %!  set_initial_mode(+Stream, +Options) is det.