[AST] Use cached deserialized decl in getOpaqueResultTypeDecl

Mangling and looking up the opaque result type decl
for serialized decls is a fairly expensive
operation. Instead, fallthrough to the request
which will have a cached value set by deserialization.
This shaves ~30ms off the cached completion for:

```swift
import SwiftUI

struct V: View {
  var body: some View {
    Table(#^CC^#
  }
}
```

include/swift/AST/Decl.h

@@ -2758,6 +2758,10 @@ private:
     /// output a null pointer.
     unsigned noDynamicallyReplacedDecl : 1;
 
+    /// Whether the OpaqueResultTypeRequest request was evaluated and produced
+    /// a null pointer.
+    unsigned noOpaqueResultType : 1;
+
     /// Whether the "isFinal" bit has been computed yet.
     unsigned isFinalComputed : 1;
 
@@ -2785,7 +2789,7 @@ private:
   friend class InterfaceTypeRequest;
   friend class CheckRedeclarationRequest;
   friend class ActorIsolationRequest;
-  friend class DynamicallyReplacedDeclRequest;
+  friend class OpaqueResultTypeRequest;
   friend class ApplyAccessNoteRequest;
 
   friend class Decl;

include/swift/AST/TypeCheckRequests.h

@@ -2295,21 +2295,22 @@ public:
 
 /// Builds an opaque result type for a declaration.
 class OpaqueResultTypeRequest
-    : public SimpleRequest<OpaqueResultTypeRequest,
-                           OpaqueTypeDecl *(ValueDecl *),
-                           RequestFlags::Cached> {
+    : public SimpleRequest<
+          OpaqueResultTypeRequest, OpaqueTypeDecl *(ValueDecl *),
+          RequestFlags::SeparatelyCached | RequestFlags::SplitCached> {
 public:
   using SimpleRequest::SimpleRequest;
 
 private:
   friend SimpleRequest;
 
-  OpaqueTypeDecl *
-  evaluate(Evaluator &evaluator, ValueDecl *VD) const;
+  OpaqueTypeDecl *evaluate(Evaluator &evaluator, ValueDecl *VD) const;
 
 public:
-  // Caching.
+  // Split caching.
   bool isCached() const { return true; }
+  std::optional<OpaqueTypeDecl *> getCachedResult() const;
+  void cacheResult(OpaqueTypeDecl *result) const;
 };
 
 /// Determines if a function declaration is 'static'.

include/swift/AST/TypeCheckerTypeIDZone.def

@@ -262,7 +262,7 @@ SWIFT_REQUEST(TypeChecker, OpaqueReadOwnershipRequest,
               NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, OpaqueResultTypeRequest,
               OpaqueTypeDecl *(ValueDecl *),
-              Cached, NoLocationInfo)
+              SeparatelyCached | SplitCached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, OperatorPrecedenceGroupRequest,
               PrecedenceGroupDecl *(PrecedenceGroupDecl *),
               Cached, NoLocationInfo)

lib/AST/Decl.cpp

@@ -3887,21 +3887,6 @@ TypeRepr *ValueDecl::getOpaqueResultTypeRepr() const {
 }
 
 OpaqueTypeDecl *ValueDecl::getOpaqueResultTypeDecl() const {
-  if (getOpaqueResultTypeRepr() == nullptr) {
-    if (!isa<VarDecl>(this) &&
-        !isa<FuncDecl>(this) &&
-        !isa<SubscriptDecl>(this))
-      return nullptr;
-    auto file = cast<FileUnit>(getDeclContext()->getModuleScopeContext());
-    // Don't look up when the decl is from source, otherwise a cycle will happen.
-    if (file->getKind() == FileUnitKind::SerializedAST) {
-      Mangle::ASTMangler mangler;
-      auto name = mangler.mangleOpaqueTypeDecl(this);
-      return file->lookupOpaqueResultType(name);
-    }
-    return nullptr;
-  }
-
   return evaluateOrDefault(getASTContext().evaluator,
     OpaqueResultTypeRequest{const_cast<ValueDecl *>(this)},
     nullptr);

lib/AST/TypeCheckRequests.cpp

@@ -385,6 +385,28 @@ void DynamicallyReplacedDeclRequest::cacheResult(ValueDecl *result) const {
   decl->getASTContext().evaluator.cacheNonEmptyOutput(*this, std::move(result));
 }
 
+//----------------------------------------------------------------------------//
+// OpaqueResultTypeRequest caching.
+//----------------------------------------------------------------------------//
+
+std::optional<OpaqueTypeDecl *>
+OpaqueResultTypeRequest::getCachedResult() const {
+  auto *decl = std::get<0>(getStorage());
+  if (decl->LazySemanticInfo.noOpaqueResultType)
+    return std::optional(nullptr);
+
+  return decl->getASTContext().evaluator.getCachedNonEmptyOutput(*this);
+}
+
+void OpaqueResultTypeRequest::cacheResult(OpaqueTypeDecl *result) const {
+  auto *decl = std::get<0>(getStorage());
+  if (!result) {
+    decl->LazySemanticInfo.noOpaqueResultType = 1;
+    return;
+  }
+  decl->getASTContext().evaluator.cacheNonEmptyOutput(*this, std::move(result));
+}
+
 //----------------------------------------------------------------------------//
 // ApplyAccessNoteRequest computation.
 //----------------------------------------------------------------------------//

lib/Sema/TypeCheckGeneric.cpp

@@ -42,7 +42,9 @@ OpaqueTypeDecl *
 OpaqueResultTypeRequest::evaluate(Evaluator &evaluator,
                                   ValueDecl *originatingDecl) const {
   auto *repr = originatingDecl->getOpaqueResultTypeRepr();
-  assert(repr && "Declaration does not have an opaque result type");
+  if (!repr)
+    return nullptr;
+
   auto *dc = originatingDecl->getInnermostDeclContext();
   auto &ctx = dc->getASTContext();
 

lib/Serialization/Deserialization.cpp

@@ -3936,14 +3936,17 @@ public:
     if (var->hasStorage())
       AddAttribute(new (ctx) HasStorageAttr(/*isImplicit:*/true));
 
-    if (opaqueReturnTypeID) {
-      auto opaqueReturnType = MF.getDeclChecked(opaqueReturnTypeID);
-      if (!opaqueReturnType)
-        return opaqueReturnType.takeError();
+    {
+      OpaqueTypeDecl *opaqueDecl = nullptr;
+      if (opaqueReturnTypeID) {
+        auto opaqueReturnType = MF.getDeclChecked(opaqueReturnTypeID);
+        if (!opaqueReturnType)
+          return opaqueReturnType.takeError();
 
-      ctx.evaluator.cacheOutput(
-          OpaqueResultTypeRequest{var},
-          cast<OpaqueTypeDecl>(opaqueReturnType.get()));
+        opaqueDecl = cast<OpaqueTypeDecl>(opaqueReturnType.get());
+      }
+      ctx.evaluator.cacheOutput(OpaqueResultTypeRequest{var},
+                                std::move(opaqueDecl));
     }
 
     // If this is a lazy property, record its backing storage.
@@ -4357,14 +4360,17 @@ public:
     fn->setIsObjC(isObjC);
     fn->setForcedStaticDispatch(hasForcedStaticDispatch);
 
-    if (opaqueReturnTypeID) {
-      auto declOrError = MF.getDeclChecked(opaqueReturnTypeID);
-      if (!declOrError)
-        return declOrError.takeError();
+    {
+      OpaqueTypeDecl *opaqueDecl = nullptr;
+      if (opaqueReturnTypeID) {
+        auto declOrError = MF.getDeclChecked(opaqueReturnTypeID);
+        if (!declOrError)
+          return declOrError.takeError();
 
-      ctx.evaluator.cacheOutput(
-          OpaqueResultTypeRequest{fn},
-          cast<OpaqueTypeDecl>(declOrError.get()));
+        opaqueDecl = cast<OpaqueTypeDecl>(declOrError.get());
+      }
+      ctx.evaluator.cacheOutput(OpaqueResultTypeRequest{fn},
+                                std::move(opaqueDecl));
     }
 
     if (!isAccessor)
@@ -5204,16 +5210,19 @@ public:
     subscript->setOverriddenDecl(cast_or_null<SubscriptDecl>(overridden.get()));
     if (subscript->getOverriddenDecl())
       AddAttribute(new (ctx) OverrideAttr(SourceLoc()));
-    
-    if (opaqueReturnTypeID) {
-      Decl *opaqueReturnType;
-      UNWRAP(MF.getDeclChecked(opaqueReturnTypeID), opaqueReturnType);
 
-      ctx.evaluator.cacheOutput(
-          OpaqueResultTypeRequest{subscript},
-          cast<OpaqueTypeDecl>(opaqueReturnType));
+    {
+      OpaqueTypeDecl *opaqueDecl = nullptr;
+      if (opaqueReturnTypeID) {
+        Decl *opaqueReturnType;
+        UNWRAP(MF.getDeclChecked(opaqueReturnTypeID), opaqueReturnType);
+
+        opaqueDecl = cast<OpaqueTypeDecl>(opaqueReturnType);
+      }
+      ctx.evaluator.cacheOutput(OpaqueResultTypeRequest{subscript},
+                                std::move(opaqueDecl));
     }
-    
+
     return subscript;
   }