[ConstraintSystem] Add skeleton of constraint optimizer

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -5691,6 +5691,11 @@ class ConstraintSystem {
   /// Collect the current inactive disjunction constraints.
   void collectDisjunctions(SmallVectorImpl<Constraint *> &disjunctions);
 
+  /// Based on the given set of disjunctions, attempt to determine
+  /// favored choices and perform other optimizations to help the
+  /// solver.
+  void optimizeDisjunctions(SmallVectorImpl<Constraint *> &disjunctions);
+
   /// Record a particular disjunction choice of
   void recordDisjunctionChoice(ConstraintLocator *disjunctionLocator,
                                unsigned index) {

lib/Sema/CMakeLists.txt

@@ -10,6 +10,7 @@ add_swift_host_library(swiftSema STATIC
   CSSolver.cpp
   CSStep.cpp
   CSFix.cpp
+  CSOptimizer.cpp
   CSDiagnostics.cpp
   CodeSynthesis.cpp
   CodeSynthesisDistributedActor.cpp

lib/Sema/CSOptimizer.cpp

@@ -0,0 +1,25 @@
+//===--- CSOptimizer.cpp - Constraint Optimizer ---------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2023 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements disjunction and other constraint optimizations.
+//
+//===----------------------------------------------------------------------===//
+
+#include "swift/Sema/ConstraintSystem.h"
+#include "llvm/ADT/SmallVector.h"
+
+using namespace swift;
+using namespace constraints;
+
+void ConstraintSystem::optimizeDisjunctions(
+    SmallVectorImpl<Constraint *> &disjunctions) {
+}

lib/Sema/CSSolver.cpp

@@ -1773,6 +1773,8 @@ Constraint *ConstraintSystem::selectDisjunction() {
   if (disjunctions.empty())
     return nullptr;
 
+  optimizeDisjunctions(disjunctions);
+
   if (auto *disjunction = selectBestBindingDisjunction(*this, disjunctions))
     return disjunction;