Update theory rewriter ownership, add stats to strings (#4202)

This commit adds statistics for string rewrites. This is work towards proof     
support in the string solver. At a high level, this commit adds a pointer to a  
`SequenceStatistics` in the rewriters and modifies                              
`SequencesRewriter::returnRewrite()` to count the rewrites done. In practice,   
to make this work requires a couple of changes, some of them temporary:         
                                                                                
- We can't have a single `Rewriter` instance shared between different           
  `SmtEngine` instances anymore. Thus the `Rewriter` is now owned by the        
  `SmtEngine` and calling the rewriter retrieves the rewriter associated with   
  the current `SmtEngine`. This is a temporary workaround before we get rid of  
  singletons.                                                                   
- Methods in the `SequencesRewriter` and the `StringsRewriter` are made         
  non-`static` because they need access to the statistics instance.             
- `StringsEntail` now has non-`static` methods because it needs a reference to  
  the sequences rewriter that it can call.                                      
- The interaction between the `StringsRewriter` and the `SequencesRewriter`     
  changed: the `StringsRewriter` is now a proper `TheoryRewriter` that inherits 
  from `SequencesRewriter` and calls its `postRewrite()` before applying its    
  own rewrites (this is essentially a reversal of roles from before: the        
  `SequencesRewriter` used to call `static` methods in the `StringsRewriter`).  
- The theory rewriters are now owned by the individual theories. This design    
  mirrors the `EqualityEngine`s owned by the individual theories.

1 files changed, 5 insertions, 1 deletions

diff --git a/src/theory/quantifiers/theory_quantifiers.h b/src/theory/quantifiers/theory_quantifiers.h
index 7efe7419c..991a47ad0 100644
--- a/src/theory/quantifiers/theory_quantifiers.h
+++ b/src/theory/quantifiers/theory_quantifiers.h
@@ -23,6 +23,7 @@
 #include "context/context.h"
 #include "expr/node.h"
 #include "theory/output_channel.h"
+#include "theory/quantifiers/quantifiers_rewriter.h"
 #include "theory/theory.h"
 #include "theory/theory_engine.h"
 #include "theory/valuation.h"
@@ -39,7 +40,7 @@ class TheoryQuantifiers : public Theory {
                     const LogicInfo& logicInfo);
   ~TheoryQuantifiers();
 
-  std::unique_ptr<TheoryRewriter> mkTheoryRewriter() override;
+  TheoryRewriter* getTheoryRewriter() override { return &d_rewriter; }
 
   /** finish initialization */
   void finishInit() override;
@@ -58,6 +59,9 @@ class TheoryQuantifiers : public Theory {
                         std::vector<Node> node_values,
                         std::string str_value) override;
 
+ private:
+  /** The theory rewriter for this theory. */
+  QuantifiersRewriter d_rewriter;
 };/* class TheoryQuantifiers */
 
 }/* CVC4::theory::quantifiers namespace */