Requestify FallthroughStmt source and destination lookup

Follow a similar pattern to BreakTargetRequest
and ContinueTargetRequest.

include/swift/AST/ASTBridging.h

@@ -1491,10 +1491,10 @@ BridgedDoCatchStmt BridgedDoCatchStmt_createParsed(
     BridgedNullableTypeRepr cThrownType, BridgedStmt cBody,
     BridgedArrayRef cCatches);
 
-SWIFT_NAME("BridgedFallthroughStmt.createParsed(_:loc:)")
+SWIFT_NAME("BridgedFallthroughStmt.createParsed(loc:declContext:)")
 BridgedFallthroughStmt
-BridgedFallthroughStmt_createParsed(BridgedASTContext cContext,
-                                    BridgedSourceLoc cLoc);
+BridgedFallthroughStmt_createParsed(BridgedSourceLoc cLoc,
+                                    BridgedDeclContext cDC);
 
 SWIFT_NAME("BridgedForEachStmt.createParsed(_:labelInfo:forLoc:tryLoc:awaitLoc:"
            "pattern:inLoc:sequence:whereLoc:whereExpr:body:)")

include/swift/AST/Stmt.h

@@ -1159,36 +1159,29 @@ public:
 /// FallthroughStmt - The keyword "fallthrough".
 class FallthroughStmt : public Stmt {
   SourceLoc Loc;
-  CaseStmt *FallthroughSource;
-  CaseStmt *FallthroughDest;
+  DeclContext *DC;
 
-public:
-  FallthroughStmt(SourceLoc Loc, std::optional<bool> implicit = std::nullopt)
+  FallthroughStmt(SourceLoc Loc, DeclContext *DC,
+                  std::optional<bool> implicit = std::nullopt)
       : Stmt(StmtKind::Fallthrough, getDefaultImplicitFlag(implicit, Loc)),
-        Loc(Loc), FallthroughSource(nullptr), FallthroughDest(nullptr) {}
+        Loc(Loc), DC(DC) {}
+public:
+  static FallthroughStmt *createParsed(SourceLoc Loc, DeclContext *DC);
 
   SourceLoc getLoc() const { return Loc; }
 
   SourceRange getSourceRange() const { return Loc; }
 
+  DeclContext *getDeclContext() const { return DC; }
+  void setDeclContext(DeclContext *newDC) { DC = newDC; }
+
   /// Get the CaseStmt block from which the fallthrough transfers control.
-  /// Set during Sema. (May stay null if fallthrough is invalid.)
-  CaseStmt *getFallthroughSource() const { return FallthroughSource; }
-  void setFallthroughSource(CaseStmt *C) {
-    assert(!FallthroughSource && "fallthrough source already set?!");
-    FallthroughSource = C;
-  }
+  /// Returns \c nullptr if the fallthrough is invalid.
+  CaseStmt *getFallthroughSource() const;
 
   /// Get the CaseStmt block to which the fallthrough transfers control.
-  /// Set during Sema.
-  CaseStmt *getFallthroughDest() const {
-    assert(FallthroughDest && "fallthrough dest is not set until Sema");
-    return FallthroughDest;
-  }
-  void setFallthroughDest(CaseStmt *C) {
-    assert(!FallthroughDest && "fallthrough dest already set?!");
-    FallthroughDest = C;
-  }
+  /// Returns \c nullptr if the fallthrough is invalid.
+  CaseStmt *getFallthroughDest() const;
 
   static bool classof(const Stmt *S) {
     return S->getKind() == StmtKind::Fallthrough;

include/swift/AST/TypeCheckRequests.h

@@ -4162,6 +4162,29 @@ public:
   bool isCached() const { return true; }
 };
 
+struct FallthroughSourceAndDest {
+  CaseStmt *Source;
+  CaseStmt *Dest;
+};
+
+/// Lookup the source and destination of a 'fallthrough'.
+class FallthroughSourceAndDestRequest
+    : public SimpleRequest<FallthroughSourceAndDestRequest,
+                           FallthroughSourceAndDest(const FallthroughStmt *),
+                           RequestFlags::Cached> {
+public:
+  using SimpleRequest::SimpleRequest;
+
+private:
+  friend SimpleRequest;
+
+  FallthroughSourceAndDest evaluate(Evaluator &evaluator,
+                                    const FallthroughStmt *FS) const;
+
+public:
+  bool isCached() const { return true; }
+};
+
 /// Precheck a ReturnStmt, which involves some initial validation, as well as
 /// applying a conversion to a FailStmt if needed.
 class PreCheckReturnStmtRequest

include/swift/AST/TypeCheckerTypeIDZone.def

@@ -482,6 +482,9 @@ SWIFT_REQUEST(TypeChecker, BreakTargetRequest,
 SWIFT_REQUEST(TypeChecker, ContinueTargetRequest,
               LabeledStmt *(const ContinueStmt *),
               Cached, NoLocationInfo)
+SWIFT_REQUEST(TypeChecker, FallthroughSourceAndDestRequest,
+              FallthroughSourceAndDest(const FallthroughStmt *),
+              Cached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, PreCheckReturnStmtRequest,
               Stmt *(ReturnStmt *, DeclContext *),
               Cached, NoLocationInfo)