naive_interpreter: Change the contract lock mechanism

Signed-off-by: Delja <deljarry.florian@gmail.com>

Author: Delja

src/interpreter/naive_interpreter.nit

@@ -23,6 +23,7 @@ private import parser::tables
 import mixin
 private import model::serialize_model
 private import frontend::explain_assert_api
+private import contracts
 
 redef class ToolContext
 	# --discover-call-trace
@@ -74,6 +75,9 @@ class NaiveInterpreter
 	# Name of all supported functional names
 	var routine_types: Set[String] = new HashSet[String]
 
+	# Flag used to know if we are currently checking some assertions.
+	var in_assertion: Bool = false
+
 	init
 	do
 		if mainmodule.model.get_mclasses_by_name("Bool") != null then
@@ -1827,6 +1831,18 @@ redef class AIfexprExpr
 	end
 end
 
+
+redef class AIfInAssertion
+	redef fun stmt(v)
+	do
+		if not v.in_assertion then
+			v.in_assertion = true
+			v.stmt(self.n_body)
+			v.in_assertion = false
+		end
+	end
+end
+
 redef class ADoExpr
 	redef fun stmt(v)
 	do