introduce instructiont::turn_into_assume

This method turns a GOTO or an ASSERT instruction into an ASSUME with the
same condition.  This is a common pattern in the code base, and introducing
the method enables doing so in a way that is guaranteed to preserve the
class invariants.

Author: kroening

src/goto-instrument/accelerate/disjunctive_polynomial_acceleration.cpp

@@ -845,9 +845,7 @@ void disjunctive_polynomial_accelerationt::build_fixed()
   for(auto &instruction : fixed.instructions)
   {
     if(instruction.is_assert())
-    {
-      instruction.type = ASSUME;
-    }
+      instruction.turn_into_assume();
   }
 
   // We're only interested in paths that loop back to the loop header.

src/goto-instrument/accelerate/sat_path_enumerator.cpp

@@ -196,7 +196,7 @@ void sat_path_enumeratort::build_fixed()
   for(auto &instruction : fixed.instructions)
   {
     if(instruction.is_assert())
-      instruction.type = ASSUME;
+      instruction.turn_into_assume();
   }
 
   // We're only interested in paths that loop back to the loop header.

src/goto-instrument/contracts/contracts.cpp

@@ -261,8 +261,8 @@ void code_contractst::check_apply_loop_contracts(
   insert_before_swap_and_advance(goto_function.body, loop_end, havoc_code);
 
   // change the back edge into assume(false) or assume(guard)
-  loop_end->targets.clear();
-  loop_end->type = ASSUME;
+  loop_end->turn_into_assume();
+
   if(loop_head->is_goto())
     loop_end->set_condition(false_exprt());
   else

src/goto-instrument/cover.cpp

@@ -311,7 +311,8 @@ static void instrument_cover_goals(
           {
             successor->turn_into_skip();
           }
-          i_it->type = goto_program_instruction_typet::ASSUME;
+
+          i_it->turn_into_assume();
         }
         else
         {

src/goto-instrument/k_induction.cpp

@@ -131,7 +131,7 @@ void k_inductiont::process_loop(
     {
       assert(t!=goto_function.body.instructions.end());
       if(t->is_assert())
-        t->type=ASSUME;
+        t->turn_into_assume();
     }
 
     // assume the loop condition has become false

src/goto-programs/goto_program.h

@@ -441,6 +441,16 @@ class goto_programt
       clear(SKIP);
     }
 
+    /// Transforms either an assertion or a GOTO instruction
+    /// into an assumption, with the same condition.
+    void turn_into_assume()
+    {
+      PRECONDITION(_type == GOTO || _type == ASSERT);
+      _type = ASSUME;
+      targets.clear();
+      code.make_nil();
+    }
+
     void complete_goto(targett _target)
     {
       PRECONDITION(type == INCOMPLETE_GOTO);