Use substitutions for implementing defined functions (#6437)

This eliminates explicit tracking of defined functions, and instead makes define-fun add to preprocessing substitutions.
In other words, the effect of:

(define-fun f X t)
is to add f -> (lambda X t) to the set of substitutions known by the preprocessor. This is essentially the same as when
(= f (lambda X t)) was an equality solved by non-clausal simplification

The motivation for this change is both uniformity and for performance, as fewer traversals of the input formula.

In this PR:

define-fun are now conceptually higher-order equalities provided to smt::Assertions. These assertions are always added as substitutions instead of being pushed to AssertionPipeline.
Top-level substitutions are moved from PreprocessingContext to Env, since they must be accessed by Assertions. Proofs for this class are enabled dynamically during SmtEngine::finishInit.
The expandDefinitions preprocessing step is replaced by apply-substs. The process assertions module no longer needs access to expand definitions.
The proof manager does not require a special case of using the define-function maps.
Define-function maps are eliminated from SmtEngine.
Further work will reorganize the relationship between the expand definitions module and the rewriter, after which global calls to SmtEngine::expandDefinitions can be cleaned up. There is also further work necessary to better integrate theory expand definitions and top-level substitutions, which will be done on a followup PR.

1 files changed, 4 insertions, 4 deletions

diff --git a/src/theory/trust_substitutions.h b/src/theory/trust_substitutions.h
index e8b627249..ec5b2ffb5 100644
--- a/src/theory/trust_substitutions.h
+++ b/src/theory/trust_substitutions.h
@@ -42,10 +42,12 @@ class TrustSubstitutionMap : public ProofGenerator
 
  public:
   TrustSubstitutionMap(context::Context* c,
-                       ProofNodeManager* pnm,
+                       ProofNodeManager* pnm = nullptr,
                        std::string name = "TrustSubstitutionMap",
                        PfRule trustId = PfRule::PREPROCESS_LEMMA,
                        MethodId ids = MethodId::SB_DEFAULT);
+  /** Set proof node manager */
+  void setProofNodeManager(ProofNodeManager* pnm);
   /** Gets a reference to the underlying substitution map */
   SubstitutionMap& get();
   /**
@@ -105,8 +107,6 @@ class TrustSubstitutionMap : public ProofGenerator
   context::Context* d_ctx;
   /** The substitution map */
   SubstitutionMap d_subs;
-  /** The proof node manager */
-  ProofNodeManager* d_pnm;
   /** A context-dependent list of trust nodes */
   context::CDList<TrustNode> d_tsubs;
   /** Theory proof step buffer */
@@ -118,7 +118,7 @@ class TrustSubstitutionMap : public ProofGenerator
   /**
    * A context-dependent list of LazyCDProof, allocated for internal steps.
    */
-  CDProofSet<LazyCDProof> d_helperPf;
+  std::unique_ptr<CDProofSet<LazyCDProof>> d_helperPf;
   /** Name for debugging */
   std::string d_name;
   /**