AST: Make ExpandChildTypeRefinementContextsRequest a side effectful request.

While returning the actual child vector from
`ExpandChildTypeRefinementContextsRequest` is a nice idea, it is both
inefficient (the vector gets copied in and out) and kind of inaccurate, since
the vector remains mutable after the initial expansion and may gain additional
children as macros are lazily expanded.

Author: tshortli

include/swift/AST/TypeCheckRequests.h

@@ -4852,23 +4852,22 @@ class InitAccessorReferencedVariablesRequest
 /// Expand the children of the given type refinement context.
 class ExpandChildTypeRefinementContextsRequest
     : public SimpleRequest<ExpandChildTypeRefinementContextsRequest,
-                           std::vector<TypeRefinementContext *>(
-                               TypeRefinementContext *),
+                           evaluator::SideEffect(TypeRefinementContext *),
                            RequestFlags::SeparatelyCached> {
 public:
   using SimpleRequest::SimpleRequest;
 
 private:
   friend SimpleRequest;
 
-  std::vector<TypeRefinementContext *>
-  evaluate(Evaluator &evaluator, TypeRefinementContext *parentTRC) const;
+  evaluator::SideEffect evaluate(Evaluator &evaluator,
+                                 TypeRefinementContext *parentTRC) const;
 
 public:
   // Separate caching.
   bool isCached() const { return true; }
-  std::optional<std::vector<TypeRefinementContext *>> getCachedResult() const;
-  void cacheResult(std::vector<TypeRefinementContext *> children) const;
+  std::optional<evaluator::SideEffect> getCachedResult() const;
+  void cacheResult(evaluator::SideEffect) const;
 };
 
 class SerializeAttrGenericSignatureRequest

lib/AST/TypeRefinementContext.cpp

@@ -208,20 +208,21 @@ TypeRefinementContext::findMostRefinedSubContext(SourceLoc Loc,
   if (SrcRange.isValid() && !Ctx.SourceMgr.containsTokenLoc(SrcRange, Loc))
     return nullptr;
 
-  auto expandedChildren = evaluateOrDefault(
-      Ctx.evaluator, ExpandChildTypeRefinementContextsRequest{this}, {});
+  (void)evaluateOrDefault(Ctx.evaluator,
+                          ExpandChildTypeRefinementContextsRequest{this}, {});
+  assert(!getNeedsExpansion());
 
   // Do a binary search to find the first child with a source range that
   // ends after the given location.
   auto iter = std::lower_bound(
-      expandedChildren.begin(), expandedChildren.end(), Loc,
+      Children.begin(), Children.end(), Loc,
       [&Ctx](TypeRefinementContext *context, SourceLoc loc) {
         return Ctx.SourceMgr.isBefore(context->getSourceRange().End, loc);
       });
 
   // Check whether the matching child or any of its descendants contain
   // the given location.
-  if (iter != expandedChildren.end()) {
+  if (iter != Children.end()) {
     if (auto found = (*iter)->findMostRefinedSubContext(Loc, Ctx))
       return found;
   }
@@ -478,18 +479,17 @@ void swift::simple_display(
   out << "TRC @" << trc;
 }
 
-std::optional<std::vector<TypeRefinementContext *>>
+std::optional<evaluator::SideEffect>
 ExpandChildTypeRefinementContextsRequest::getCachedResult() const {
   auto *TRC = std::get<0>(getStorage());
   if (TRC->getNeedsExpansion())
     return std::nullopt;
-  return TRC->Children;
+  return evaluator::SideEffect();
 }
 
 void ExpandChildTypeRefinementContextsRequest::cacheResult(
-    std::vector<TypeRefinementContext *> children) const {
+    evaluator::SideEffect sideEffect) const {
   auto *TRC = std::get<0>(getStorage());
-  TRC->Children = children;
   TRC->setNeedsExpansion(false);
 }
 

lib/Sema/TypeCheckAvailability.cpp

@@ -1384,8 +1384,7 @@ TypeChecker::getOrBuildTypeRefinementContext(SourceFile *SF) {
   return TRC;
 }
 
-std::vector<TypeRefinementContext *>
-ExpandChildTypeRefinementContextsRequest::evaluate(
+evaluator::SideEffect ExpandChildTypeRefinementContextsRequest::evaluate(
     Evaluator &evaluator, TypeRefinementContext *parentTRC) const {
   assert(parentTRC->getNeedsExpansion());
   if (auto decl = parentTRC->getDeclOrNull()) {
@@ -1394,7 +1393,7 @@ ExpandChildTypeRefinementContextsRequest::evaluate(
     builder.prepareDeclForLazyExpansion(decl);
     builder.build(decl);
   }
-  return parentTRC->Children;
+  return evaluator::SideEffect();
 }
 
 AvailabilityRange TypeChecker::overApproximateAvailabilityAtLocation(