[ConstraintSystem] Add some counters helpful for understanding solver…

lib/Sema/CSSolver.cpp

@@ -1423,6 +1423,19 @@ bool ConstraintSystem::solve(Expr *const expr,
   // Solve the system.
   solveRec(solutions, allowFreeTypeVariables);
 
+  if (TC.getLangOpts().DebugConstraintSolver) {
+    auto &log = getASTContext().TypeCheckerDebug->getStream();
+    log << "---Solver statistics---\n";
+    log << "Total number of scopes explored: " << solverState->NumStatesExplored << "\n";
+    log << "Number of leaf scopes explored: " << solverState->leafScopes << "\n";
+    log << "Maximum depth reached while exploring solutions: " << solverState->maxDepth << "\n";
+    if (Timer) {
+      auto timeInMillis =
+        1000 * Timer->getElapsedProcessTimeInFractionalSeconds();
+      log << "Time: " << timeInMillis << "ms\n";
+    }
+  }
+
   // Filter deduced solutions, try to figure out if there is
   // a single best solution to use, if not explicitly disabled
   // by constraint system options.

lib/Sema/ConstraintSystem.h

@@ -1124,6 +1124,15 @@ class ConstraintSystem {
     /// \brief Depth of the solution stack.
     unsigned depth = 0;
 
+    /// \brief Maximum depth reached so far in exploring solutions.
+    unsigned maxDepth = 0;
+
+    /// \brief Count of the number of leaf scopes we've created. These
+    /// either result in a failure to solve, or in a solution, unlike
+    /// all the intermediate scopes. They are interesting to track as
+    /// part of a metric of whether an expression is too complex.
+    unsigned leafScopes = 0;
+
     /// \brief Whether to record failures or not.
     bool recordFixes = false;
 
@@ -1198,7 +1207,8 @@ class ConstraintSystem {
     /// \param scope The scope to associate with current solver state.
     void registerScope(SolverScope *scope) {
       ++depth;
-      ++NumStatesExplored;
+      maxDepth = std::max(maxDepth, depth);
+      scope->scopeNumber = NumStatesExplored++;
 
       CS.incrementScopeCounter();
       auto scopeInfo =
@@ -1217,6 +1227,10 @@ class ConstraintSystem {
     void rollback(SolverScope *scope) {
       --depth;
 
+      unsigned countScopesExplored = NumStatesExplored - scope->scopeNumber;
+      if (countScopesExplored == 1)
+        ++leafScopes;
+
       SolverScope *savedScope;
       // The position of last retired constraint before given scope.
       ConstraintList::iterator lastRetiredPos;
@@ -1445,6 +1459,9 @@ class ConstraintSystem {
     /// The previous score.
     Score PreviousScore;
 
+    /// The scope number of this scope. Set when the scope is registered.
+    unsigned scopeNumber = 0;
+
     /// Time in fractional seconds at which we entered this scope.
     double startTime;