[clangd] Refactor computation of extracted expr in ExtractVariable tweak. NFC

[sammccall]

Summary

This takes this logic out of the Tweak class, and simplifies the signature of
the function where the main logic is.

The goal is to make it easier to turn into a loop like:

for (current = N; current and current->parent are both expr; current = current->parent)
  if (suitable(current))
    return current;
return null;

[sammccall]

rebase

clang-tools-extra/clangd/refactor/tweaks/ExtractVariable.cpp

SourceRange BinaryOperatorRange =
       getBinaryOperatorRange(*ExprNode, SM, Ctx.getLangOpts());
   if (BinaryOperatorRange.isValid())
     return BinaryOperatorRange;
 
   // Usual case: we're extracting the whole expression.
   return *toHalfOpenFileRange(SM, Ctx.getLangOpts(), Expr->getSourceRange());
 }
 
-/// Extracts an expression to the variable dummy
-/// Before:
-/// int x = 5 + 4 * 3;
-///         ^^^^^
-/// After:
-/// auto dummy = 5 + 4;
-/// int x = dummy * 3;
-class ExtractVariable : public Tweak {
-public:
-  const char *id() const override final;
-  bool prepare(const Selection &Inputs) override;
-  Expected<Effect> apply(const Selection &Inputs) override;
-  std::string title() const override {
-    return "Extract subexpression to variable";
-  }
-  Intent intent() const override { return Refactor; }
-  // Compute the extraction context for the Selection
-  bool computeExtractionContext(const SelectionTree::Node *N,
-                                const SourceManager &SM, const ASTContext &Ctx);
-
-private:
-  // the expression to extract
-  std::unique_ptr<ExtractionContext> Target;
-};
-REGISTER_TWEAK(ExtractVariable)
-bool ExtractVariable::prepare(const Selection &Inputs) {
-  // we don't trigger on empty selections for now
-  if (Inputs.SelectionBegin == Inputs.SelectionEnd)
-    return false;
-  const ASTContext &Ctx = Inputs.AST.getASTContext();
-  const SourceManager &SM = Inputs.AST.getSourceManager();
-  const SelectionTree::Node *N = Inputs.ASTSelection.commonAncestor();
-  return computeExtractionContext(N, SM, Ctx);
-}
-
-Expected<Tweak::Effect> ExtractVariable::apply(const Selection &Inputs) {
-  tooling::Replacements Result;
-  // FIXME: get variable name from user or suggest based on type
-  std::string VarName = "dummy";
-  SourceRange Range = Target->getExtractionChars();
-  // insert new variable declaration
-  if (auto Err = Result.add(Target->insertDeclaration(VarName, Range)))
-    return std::move(Err);
-  // replace expression with variable name
-  if (auto Err = Result.add(Target->replaceWithVar(Range, VarName)))
-    return std::move(Err);
-  return Effect::applyEdit(Result);
-}
-
 // Find the CallExpr whose callee is the (possibly wrapped) DeclRef
 const SelectionTree::Node *getCallExpr(const SelectionTree::Node *DeclRef) {
   const SelectionTree::Node &MaybeCallee = DeclRef->outerImplicit();
   const SelectionTree::Node *MaybeCall = MaybeCallee.Parent;
   if (!MaybeCall)
     return nullptr;
   const CallExpr *CE =
       llvm::dyn_cast_or_null<CallExpr>(MaybeCall->ASTNode.get<Expr>());
if (!OuterImplicit.Parent ||
       childExprIsStmt(OuterImplicit.Parent->ASTNode.get<Stmt>(),
                       OuterImplicit.ASTNode.get<Expr>()))
     return false;
 
   // FIXME: ban extracting the RHS of an assignment: `a = [[foo()]]`
   return true;
 }
 
-// Find the node that will form our ExtractionContext.
+// Find the Expr node that we're going to extract.
 // We don't want to trigger for assignment expressions and variable/field
 // DeclRefs. For function/member function, we want to extract the entire
 // function call.
-bool ExtractVariable::computeExtractionContext(const SelectionTree::Node *N,
-                                               const SourceManager &SM,
-                                               const ASTContext &Ctx) {
+const SelectionTree::Node *computeExtractedExpr(const SelectionTree::Node *N) {
   if (!N)
-    return false;
+    return nullptr;
   const SelectionTree::Node *TargetNode = N;
+  const clang::Expr *SelectedExpr = N->ASTNode.get<clang::Expr>();
+  if (!SelectedExpr)
+    return nullptr;
   // For function and member function DeclRefs, extract the whole call.
-  if (const Expr *E = N->ASTNode.get<clang::Expr>())
-    if (llvm::isa<DeclRefExpr>(E) || llvm::isa<MemberExpr>(E))
-      if (const SelectionTree::Node *Call = getCallExpr(N))
-        TargetNode = Call;
-  if (!eligibleForExtraction(TargetNode))
+  if (llvm::isa<DeclRefExpr>(SelectedExpr) ||
+      llvm::isa<MemberExpr>(SelectedExpr))
+  // Extracting Exprs like a = 1 gives dummy = a = 1 which isn't useful.
+  if (const BinaryOperator *BinOpExpr =
+          dyn_cast_or_null<BinaryOperator>(SelectedExpr)) {
+    if (BinOpExpr->getOpcode() == BinaryOperatorKind::BO_Assign)
+      return nullptr;
+  }
+  if (!TargetNode || !eligibleForExtraction(TargetNode))
+    return nullptr;
+  return TargetNode;
+}
+
+/// Extracts an expression to the variable dummy
+/// Before:
+/// int x = 5 + 4 * 3;
+///         ^^^^^
+/// After:
+/// auto dummy = 5 + 4;
+/// int x = dummy * 3;
+class ExtractVariable : public Tweak {
+public:
+  const char *id() const override final;
+  bool prepare(const Selection &Inputs) override;
+  Expected<Effect> apply(const Selection &Inputs) override;
+  std::string title() const override {
+    return "Extract subexpression to variable";
+  }
+  Intent intent() const override { return Refactor; }
+
+private:
+  // the expression to extract
+  std::unique_ptr<ExtractionContext> Target;
+};
+REGISTER_TWEAK(ExtractVariable)
+bool ExtractVariable::prepare(const Selection &Inputs) {
+  // we don't trigger on empty selections for now
+  if (Inputs.SelectionBegin == Inputs.SelectionEnd)
     return false;
-  Target = llvm::make_unique<ExtractionContext>(TargetNode, SM, Ctx);
-  return Target->isExtractable();
+  const ASTContext &Ctx = Inputs.AST.getASTContext();
+  const SourceManager &SM = Inputs.AST.getSourceManager();
+  if (const SelectionTree::Node *N =
+          computeExtractedExpr(Inputs.ASTSelection.commonAncestor()))
+    Target = llvm::make_unique<ExtractionContext>(N, SM, Ctx);
+  return Target && Target->isExtractable();
+}
+
+Expected<Tweak::Effect> ExtractVariable::apply(const Selection &Inputs) {
+  tooling::Replacements Result;
+  // FIXME: get variable name from user or suggest based on type
+  std::string VarName = "dummy";
+  SourceRange Range = Target->getExtractionChars();
+  // insert new variable declaration
+  if (auto Err = Result.add(Target->insertDeclaration(VarName, Range)))
+    return std::move(Err);
+  // replace expression with variable name
+  if (auto Err = Result.add(Target->replaceWithVar(Range, VarName)))
+    return std::move(Err);
+  return Effect::applyEdit(Result);
 }
 
 } // namespace
 } // namespace clangd
 } // namespace clang