[LibTooling] Add Stencil library for format-string style codegen.

[ymandel]

Summary

This file defines the *Stencil* abstraction: a code-generating object, parameterized by named references to (bound) AST nodes. Given a match result, a stencil can be evaluated to a string of source code.

A stencil is similar in spirit to a format string: it is composed of a series of raw text strings, references to nodes (the parameters) and helper code-generation operations.

See thread on cfe-dev list with subject "[RFC] Easier source-to-source transformations with clang tooling" for background.

[ymandel]

Build fixes.

[ymandel]

gentle ping...

[ymandel]

Sever dependency on NodeId and some general cleanup.

[ymandel]

Remove noisy default-defined constructors/operators

[ymandel]

Adjust to use SourceCode instead of FixIt.

[ymandel]

Add operator() to Stencil for compatability with TextGenerator.

Stencil is technically independent of Transformer. However, Transformers
replacements and explanations take a generic std::function (there abbreviated as
TextGenerator). This addition let's Stencils act as TextGenerators.

[sbenza]

clang/include/clang/Tooling/Refactoring/Stencil.h

46

Why not use llvm::Expected<std::string> as the return type?

76

The interface API is all const. Why not keep a shared_ptr instead?
That way we don't have to have users implement a clone function.

95

Parens around Other.Impl are not necessary.

111

I don't like naming the variable the same as the type.
You could just use S as the variable name. That is ok with llvm style for small functions.

128

"asserts" as it only fails in debug mode?

131

I usually prefer free functions for binary operators (friends is fine).
It makes them symmetric.

156

sNode ?
ie camelCase

clang/lib/Tooling/Refactoring/Stencil.cpp

46

maybe this anon namespace should cover the functions above?

77

If you define ==, imo you should define != too.
Otherwise just call it isEqual

95

alignment.
(below too)

[ymandel]

Responded to comments and added wrapper for Stencil::cat in stencil namespace.

[ymandel]

deleted some stray comments.

[ymandel]

tweaks.

[ymandel]

Thanks for the review. I've changed most of the things with only a few open questions. PTAL.

clang/include/clang/Tooling/Refactoring/Stencil.h

46

so that each stencil part can append to a single string that we construct when evaluating the whole stencil. this was an (attempted?) optimization. if concatenating is about as efficient, I'd prefer that approach. What do you advise?

76

Excellent! I guess I'm allergic to shared_ptr because of its atomics but its clearly a win here. Thanks!

128

I thought assert() also fails in normal mode, based on my reading of the llvm docs, but it's not explicit about this: http://llvm.org/docs/ProgrammersManual.html#programmatic-errors

FWIW, I'm on the fence whether eval() should actually have the same signature and similarly just crash the program on errors. Its not clear there's any value to propogating the errors up the stack via Expected.

clang/lib/Tooling/Refactoring/Stencil.cpp

46

the style guide advises against. I've been told to only put type definitions inside anon namespaces.

77

Since I don't have a general need for these as operators, just went w/ the latter. Used the same style as evalData().

[sbenza]

clang/include/clang/Tooling/Refactoring/Stencil.h

46

I think its fine as it is then.
I don't really think the performance would matter that much, but I don't see a reason to add all those copies unnecessarily.
Mention that Result is in an unspecified state in case of error.

102

We could simplify this code if you change void push(T) to instead StencilPart wrap(T).
Then this could be:

Stencil S;
S.Parts = {wrap(std::forward<Ts>(Parts))...};
return S;

128

assert follows NDEBUG, which is not explicitly related to -Ox, but they usually go together.

Don't make it crash, please.
Otherwise it would be hard to have a refactoring service or alike.

clang/lib/Tooling/Refactoring/Stencil.cpp

79

Any particular reason to use function template specialization instead of function overloading?
The former is rare and thus harder to reason about.

(same for evalData below)

[ymandel]

Responded to comments, including changing call operator to return Expected<string> instead of string.

[ymandel]

clang/include/clang/Tooling/Refactoring/Stencil.h

102

Much better. I believe the current design came about when StencilPart was move only. Thanks!

128

Right, you've convinced me. For some background: TextGenerator (in Transformer) originally return an Expected<std::string>, but Ilya suggested that I simplify to just a string and use assertions. The logic being that these are compiler-style tools. However, I agree that this simply doesn't work for running these in servers, which is a very real concern. It's also trivial to imagine a user mistyping a stencil (say, in web UI) which results in a failure in stencil eval. We want to return a nice error to the user, not crash the server.

So, I've adjusted the signature. I will (separately) change the definition of TextGenerator in Transformer.

clang/lib/Tooling/Refactoring/Stencil.cpp

79

Stylistic -- it forces you to explicitly declare the "overload set" so to speak. However, i'm not particularly attached -- if you think this a bad idea given its rarity, I'm happy to use overload sets. WDYT?

[ymandel]

Convert template specialization to overload sets.

[ymandel]

clang/lib/Tooling/Refactoring/Stencil.cpp

79

Converted to normal overload sets (as per off-thread chat).

clang/include/clang/Tooling/Refactoring/Stencil.h

+//===--- Stencil.h - Stencil class ------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// /file
+/// This file defines the *Stencil* abstraction: a code-generating object,
+/// parameterized by named references to (bound) AST nodes.  Given a match
+/// result, a stencil can be evaluated to a string of source code.
+///
+/// A stencil is similar in spirit to a format string: it is composed of a
+/// series of raw text strings, references to nodes (the parameters) and helper
+/// code-generation operations.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLING_REFACTOR_STENCIL_H_
+#define LLVM_CLANG_TOOLING_REFACTOR_STENCIL_H_
+
+#include "clang/AST/ASTContext.h"
+#include "clang/AST/ASTTypeTraits.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Error.h"
+#include <string>
+#include <vector>
+
+namespace clang {
+namespace tooling {
+
+/// A stencil is represented as a sequence of "parts" that can each individually
+/// generate a code string based on a match result.  The different kinds of
+/// parts include (raw) text, references to bound nodes and assorted operations
+/// on bound nodes.
+///
+/// Users can create custom Stencil operations by implementing this interface.
+class StencilPartInterface {
+public:
+  virtual ~StencilPartInterface() = default;
+
+  /// Evaluates this part to a string and appends it to \c Result.  \c Result is
+  /// undefined in the case of an error.
+  virtual llvm::Error eval(const ast_matchers::MatchFinder::MatchResult &Match,
+                           std::string *Result) const = 0;
+
+  virtual bool isEqual(const StencilPartInterface &other) const = 0;
+
+  const void *typeId() const { return TypeId; }
+
+protected:
+  StencilPartInterface(const void *DerivedId) : TypeId(DerivedId) {}
+
+  // Since this is an abstract class, copying/assigning only make sense for
+  // derived classes implementing `clone()`.
+  StencilPartInterface(const StencilPartInterface &) = default;
+  StencilPartInterface &operator=(const StencilPartInterface &) = default;
+
+  /// Unique identifier of the concrete type of this instance.  Supports safe
+  /// downcasting.
+  const void *TypeId;
+};
+
+/// A copyable facade for a std::unique_ptr<StencilPartInterface>. Copies result
+/// in a copy of the underlying pointee object.
+class StencilPart {
+public:
+  explicit StencilPart(std::shared_ptr<StencilPartInterface> Impl)
+      : Impl(std::move(Impl)) {}
+
+  /// See `StencilPartInterface::eval()`.
+  llvm::Error eval(const ast_matchers::MatchFinder::MatchResult &Match,
+                   std::string *Result) const {
+    return Impl->eval(Match, Result);
+  }
+
+  bool operator==(const StencilPart &Other) const {
+    if (Impl == Other.Impl)
+      return true;
+    if (Impl == nullptr || Other.Impl == nullptr)
+      return false;
+    return Impl->isEqual(*Other.Impl);
+  }
+
+private:
+  std::shared_ptr<StencilPartInterface> Impl;
+};
+
+/// A sequence of code fragments, references to parameters and code-generation
+/// operations that together can be evaluated to (a fragment of) source code,
+/// given a match result.
+class Stencil {
+public:
+  Stencil() = default;
+
+  /// Composes a stencil from a series of parts.
+  template <typename... Ts> static Stencil cat(Ts &&... Parts) {
+    Stencil S;
+    S.Parts = {wrap(std::forward<Ts>(Parts))...};
+    return S;
+  }
+
+  /// Appends data from a \p OtherStencil to this stencil.
+  void append(Stencil OtherStencil);
+
+  // Evaluates the stencil given a match result. Requires that the nodes in the
+  // result includes any ids referenced in the stencil. References to missing
+  // nodes will result in an invalid_argument error.
+  llvm::Expected<std::string>
+  eval(const ast_matchers::MatchFinder::MatchResult &Match) const;
+
+  // Allow Stencils to operate as std::function, for compatibility with
+  // Transformer's TextGenerator.
+  llvm::Expected<std::string>
+  operator()(const ast_matchers::MatchFinder::MatchResult &Result) const {
+    return eval(Result);
+  }
+
+private:
+  friend bool operator==(const Stencil &A, const Stencil &B);
+  static StencilPart wrap(llvm::StringRef Text);
+  static StencilPart wrap(StencilPart Part) { return Part; }
+
+  std::vector<StencilPart> Parts;
+};
+
+inline bool operator==(const Stencil &A, const Stencil &B) {
+  return A.Parts == B.Parts;
+}
+
+inline bool operator!=(const Stencil &A, const Stencil &B) { return !(A == B); }
+
+// Functions for conveniently building stencils.
+namespace stencil {
+/// Convenience wrapper for Stencil::cat that can be imported with a using decl.
+template <typename... Ts> Stencil cat(Ts &&... Parts) {
+  return Stencil::cat(std::forward<Ts>(Parts)...);
+}
+
+/// \returns exactly the text provided.
+StencilPart text(llvm::StringRef Text);
+
+/// \returns the source corresponding to the identified node.
+StencilPart node(llvm::StringRef Id);
+/// Variant of \c node() that identifies the node as a statement, for purposes
+/// of deciding whether to include any trailing semicolon.  Only relevant for
+/// Expr nodes, which, by default, are *not* considered as statements.
+/// \returns the source corresponding to the identified node, considered as a
+/// statement.
+StencilPart sNode(llvm::StringRef Id);
+
+/// For debug use only; semantics are not guaranteed.
+///
+/// \returns the string resulting from calling the node's print() method.
+StencilPart dPrint(llvm::StringRef Id);
+} // namespace stencil
+} // namespace tooling
+} // namespace clang
+#endif // LLVM_CLANG_TOOLING_REFACTOR_STENCIL_H_

clang/lib/Tooling/Refactoring/CMakeLists.txt

 set(LLVM_LINK_COMPONENTS Support)
 
 add_clang_library(clangToolingRefactor
   ASTSelection.cpp
   ASTSelectionRequirements.cpp
   AtomicChange.cpp
   Extract/Extract.cpp
   Extract/SourceExtraction.cpp
   RefactoringActions.cpp
   Rename/RenamingAction.cpp
   Rename/SymbolOccurrences.cpp
   Rename/USRFinder.cpp
   Rename/USRFindingAction.cpp
   Rename/USRLocFinder.cpp
   SourceCode.cpp
+  Stencil.cpp
   Transformer.cpp
 
   LINK_LIBS
   clangAST
   clangASTMatchers
   clangBasic
   clangFormat
   clangIndex
   clangLex
   clangRewrite
   clangToolingCore
   )

clang/lib/Tooling/Refactoring/Stencil.cpp

+//===--- Stencil.cpp - Stencil implementation -------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "clang/Tooling/Refactoring/Stencil.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/AST/ASTTypeTraits.h"
+#include "clang/AST/Expr.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
+#include "clang/Lex/Lexer.h"
+#include "clang/Tooling/Refactoring/SourceCode.h"
+#include "llvm/Support/Errc.h"
+#include <atomic>
+#include <string>
+
+using namespace clang;
+using namespace tooling;
+
+using ast_matchers::MatchFinder;
+using llvm::Error;
+
+// A down_cast function to safely down cast a StencilPartInterface to a subclass
+// D. Returns nullptr if P is not an instance of D.
+template <typename D> const D *down_cast(const StencilPartInterface *P) {
+  if (P == nullptr || D::typeId() != P->typeId())
+    return nullptr;
+  return static_cast<const D *>(P);
+}
+
+static llvm::Expected<ast_type_traits::DynTypedNode>
+getNode(const ast_matchers::BoundNodes &Nodes, StringRef Id) {
+  auto &NodesMap = Nodes.getMap();
+  auto It = NodesMap.find(Id);
+  if (It == NodesMap.end())
+    return llvm::make_error<llvm::StringError>(llvm::errc::invalid_argument,
+                                               "Id not bound: " + Id);
+  return It->second;
+}
+
+namespace {
+// An arbitrary fragment of code within a stencil.
+struct RawTextData {
+  explicit RawTextData(std::string T) : Text(std::move(T)) {}
+  std::string Text;
+};
+
+// A debugging operation to dump the AST for a particular (bound) AST node.
+struct DebugPrintNodeOpData {
+  explicit DebugPrintNodeOpData(std::string S) : Id(std::move(S)) {}
+  std::string Id;
+};
+// Whether to associate a trailing semicolon with a node when identifying it's
+// text.  This flag is needed for expressions (clang::Expr), because their role
+// is ambiguous when they are also complete statements.  When this flag is
+// `Always`, an expression node will be treated like a statement, and will
+// therefore be associated with any trailing semicolon.
+enum class SemiAssociation : bool {
+  Always,
+  Inferred,
+};
+
+// A reference to a particular (bound) AST node.
+struct NodeRefData {
+  explicit NodeRefData(std::string S, SemiAssociation SA)
+      : Id(std::move(S)), SemiAssoc(SA) {}
+  std::string Id;
+  SemiAssociation SemiAssoc;
+};
+} // namespace
+
+bool isEqualData(const RawTextData &A, const RawTextData &B) {
+  return A.Text == B.Text;
+}
+
+bool isEqualData(const DebugPrintNodeOpData &A, const DebugPrintNodeOpData &B) {
+  return A.Id == B.Id;
+}
+
+bool isEqualData(const NodeRefData &A, const NodeRefData &B) {
+  return A.Id == B.Id && A.SemiAssoc == B.SemiAssoc;
+}
+
+// The `evalData()` overloads evaluate the given stencil data to a string, given
+// the match result, and append it to `Result`. We define an overload for each
+// type of stencil data.
+
+Error evalData(const RawTextData &Data, const MatchFinder::MatchResult &,
+               std::string *Result) {
+  Result->append(Data.Text);
+  return Error::success();
+}
+
+Error evalData(const DebugPrintNodeOpData &Data,
+               const MatchFinder::MatchResult &Match, std::string *Result) {
+  std::string Output;
+  llvm::raw_string_ostream Os(Output);
+  auto NodeOrErr = getNode(Match.Nodes, Data.Id);
+  if (auto Err = NodeOrErr.takeError())
+    return Err;
+  NodeOrErr->print(Os, PrintingPolicy(Match.Context->getLangOpts()));
+  *Result += Os.str();
+  return Error::success();
+}
+
+Error evalData(const NodeRefData &Data, const MatchFinder::MatchResult &Match,
+               std::string *Result) {
+  auto NodeOrErr = getNode(Match.Nodes, Data.Id);
+  if (auto Err = NodeOrErr.takeError())
+    return Err;
+  auto &Node = *NodeOrErr;
+  switch (Data.SemiAssoc) {
+  case SemiAssociation::Inferred:
+    // Include the semicolon for non-expression statements:
+    *Result += Node.get<Stmt>() != nullptr && Node.get<Expr>() == nullptr
+                   ? getExtendedText(NodeOrErr.get(), tok::TokenKind::semi,
+                                     *Match.Context)
+                   : getText(NodeOrErr.get(), *Match.Context);
+    break;
+  case SemiAssociation::Always:
+    *Result +=
+        getExtendedText(NodeOrErr.get(), tok::TokenKind::semi, *Match.Context);
+    break;
+  }
+  return Error::success();
+}
+
+template <typename T>
+class StencilPartImpl : public StencilPartInterface {
+  T Data;
+
+public:
+  template <typename... Ps>
+  explicit StencilPartImpl(Ps &&... Args)
+      : StencilPartInterface(StencilPartImpl::typeId()),
+        Data(std::forward<Ps>(Args)...) {}
+
+  // Generates a unique identifier for this class (specifically, one per
+  // instantiation of the template).
+  static const void* typeId() {
+    static bool b;
+    return &b;
+  }
+
+  Error eval(const MatchFinder::MatchResult &Match,
+             std::string *Result) const override {
+    return evalData(Data, Match, Result);
+  }
+
+  bool isEqual(const StencilPartInterface &Other) const override {
+    if (const auto *OtherPtr = down_cast<StencilPartImpl>(&Other))
+      return isEqualData(Data, OtherPtr->Data);
+    return false;
+  }
+};
+
+namespace {
+using RawText = StencilPartImpl<RawTextData>;
+using DebugPrintNodeOp = StencilPartImpl<DebugPrintNodeOpData>;
+using NodeRef = StencilPartImpl<NodeRefData>;
+} // namespace
+
+StencilPart Stencil::wrap(StringRef Text) {
+  return stencil::text(Text);
+}
+
+void Stencil::append(Stencil OtherStencil) {
+  for (auto &Part : OtherStencil.Parts)
+    Parts.push_back(std::move(Part));
+}
+
+llvm::Expected<std::string>
+Stencil::eval(const MatchFinder::MatchResult &Match) const {
+  std::string Result;
+  for (const auto &Part : Parts)
+    if (auto Err = Part.eval(Match, &Result))
+      return std::move(Err);
+  return Result;
+}
+
+StencilPart stencil::text(StringRef Text) {
+  return StencilPart(llvm::make_unique<RawText>(Text));
+}
+
+StencilPart stencil::node(StringRef Id) {
+  return StencilPart(llvm::make_unique<NodeRef>(Id, SemiAssociation::Inferred));
+}
+
+StencilPart stencil::sNode(StringRef Id) {
+  return StencilPart(llvm::make_unique<NodeRef>(Id, SemiAssociation::Always));
+}
+
+StencilPart stencil::dPrint(StringRef Id) {
+  return StencilPart(llvm::make_unique<DebugPrintNodeOp>(Id));
+}

clang/unittests/Tooling/CMakeLists.txt

add_clang_unittest(ToolingTests
   RecursiveASTVisitorTestPostOrderVisitor.cpp
   RecursiveASTVisitorTestTypeLocVisitor.cpp
   RefactoringActionRulesTest.cpp
   RefactoringCallbacksTest.cpp
   RefactoringTest.cpp
   ReplacementsYamlTest.cpp
   RewriterTest.cpp
   SourceCodeTest.cpp
+  StencilTest.cpp
   ToolingTest.cpp
   TransformerTest.cpp
   )
 
 target_link_libraries(ToolingTests
   PRIVATE
   clangAST
   clangASTMatchers

clang/unittests/Tooling/StencilTest.cpp

+//===- unittest/Tooling/StencilTest.cpp -----------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "clang/Tooling/Refactoring/Stencil.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
+#include "clang/Tooling/FixIt.h"
+#include "clang/Tooling/Tooling.h"
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+using namespace clang;
+using namespace tooling;
+using namespace ast_matchers;
+
+namespace {
+using ::testing::AllOf;
+using ::testing::Eq;
+using ::testing::HasSubstr;
+using MatchResult = MatchFinder::MatchResult;
+using tooling::stencil::node;
+using tooling::stencil::sNode;
+using tooling::stencil::text;
+
+// In tests, we can't directly match on llvm::Expected since its accessors
+// mutate the object. So, we collapse it to an Optional.
+static llvm::Optional<std::string> toOptional(llvm::Expected<std::string> V) {
+  if (V)
+    return *V;
+  ADD_FAILURE() << "Losing error in conversion to IsSomething: "
+                << llvm::toString(V.takeError());
+  return llvm::None;
+}
+
+// A very simple matcher for llvm::Optional values.
+MATCHER_P(IsSomething, ValueMatcher, "") {
+  if (!arg)
+    return false;
+  return ::testing::ExplainMatchResult(ValueMatcher, *arg, result_listener);
+}
+
+// Create a valid translation-unit from a statement.
+static std::string wrapSnippet(llvm::Twine StatementCode) {
+  return ("auto stencil_test_snippet = []{" + StatementCode + "};").str();
+}
+
+static DeclarationMatcher wrapMatcher(const StatementMatcher &Matcher) {
+  return varDecl(hasName("stencil_test_snippet"),
+                 hasDescendant(compoundStmt(hasAnySubstatement(Matcher))));
+}
+
+struct TestMatch {
+  // The AST unit from which `result` is built. We bundle it because it backs
+  // the result. Users are not expected to access it.
+  std::unique_ptr<ASTUnit> AstUnit;
+  // The result to use in the test. References `ast_unit`.
+  MatchResult Result;
+};
+
+// Matches `Matcher` against the statement `StatementCode` and returns the
+// result. Handles putting the statement inside a function and modifying the
+// matcher correspondingly. `Matcher` should match `StatementCode` exactly --
+// that is, produce exactly one match.
+static llvm::Optional<TestMatch> matchStmt(llvm::Twine StatementCode,
+                                           StatementMatcher Matcher) {
+  auto AstUnit = buildASTFromCode(wrapSnippet(StatementCode));
+  if (AstUnit == nullptr) {
+    ADD_FAILURE() << "AST construction failed";
+    return llvm::None;
+  }
+  ASTContext &Context = AstUnit->getASTContext();
+  auto Matches = ast_matchers::match(wrapMatcher(Matcher), Context);
+  // We expect a single, exact match for the statement.
+  if (Matches.size() != 1) {
+    ADD_FAILURE() << "Wrong number of matches: " << Matches.size();
+    return llvm::None;
+  }
+  return TestMatch{std::move(AstUnit), MatchResult(Matches[0], &Context)};
+}
+
+class StencilTest : public ::testing::Test {
+protected:
+  // Verifies that the given stencil fails when evaluated on a valid match
+  // result. Binds a statement to "stmt", a (non-member) ctor-initializer to
+  // "init", an expression to "expr" and a (nameless) declaration to "decl".
+  void testError(const Stencil &Stencil,
+                 ::testing::Matcher<std::string> Matcher) {
+    const std::string Snippet = R"cc(
+      struct A {};
+      class F : public A {
+       public:
+        F(int) {}
+      };
+      F(1);
+    )cc";
+    auto StmtMatch = matchStmt(
+        Snippet,
+        stmt(hasDescendant(
+                 cxxConstructExpr(
+                     hasDeclaration(decl(hasDescendant(cxxCtorInitializer(
+                                                           isBaseInitializer())
+                                                           .bind("init")))
+                                        .bind("decl")))
+                     .bind("expr")))
+            .bind("stmt"));
+    ASSERT_TRUE(StmtMatch);
+    if (auto ResultOrErr = Stencil.eval(StmtMatch->Result)) {
+      ADD_FAILURE() << "Expected failure but succeeded: " << *ResultOrErr;
+    } else {
+      auto Err = llvm::handleErrors(ResultOrErr.takeError(),
+                                    [&Matcher](const llvm::StringError &Err) {
+                                      EXPECT_THAT(Err.getMessage(), Matcher);
+                                    });
+      if (Err) {
+        ADD_FAILURE() << "Unhandled error: " << llvm::toString(std::move(Err));
+      }
+    }
+  }
+
+  // Tests failures caused by references to unbound nodes. `unbound_id` is the
+  // id that will cause the failure.
+  void testUnboundNodeError(const Stencil &Stencil, llvm::StringRef UnboundId) {
+    testError(Stencil, AllOf(HasSubstr(UnboundId), HasSubstr("not bound")));
+  }
+};
+
+TEST_F(StencilTest, SingleStatement) {
+  StringRef Condition("C"), Then("T"), Else("E");
+  const std::string Snippet = R"cc(
+    if (true)
+      return 1;
+    else
+      return 0;
+  )cc";
+  auto StmtMatch = matchStmt(
+      Snippet, ifStmt(hasCondition(expr().bind(Condition)),
+                      hasThen(stmt().bind(Then)), hasElse(stmt().bind(Else))));
+  ASSERT_TRUE(StmtMatch);
+  // Invert the if-then-else.
+  auto Stencil = Stencil::cat("if (!", node(Condition), ") ", sNode(Else),
+                              " else ", sNode(Then));
+  EXPECT_THAT(toOptional(Stencil.eval(StmtMatch->Result)),
+              IsSomething(Eq("if (!true) return 0; else return 1;")));
+}
+
+TEST_F(StencilTest, SingleStatementCallOperator) {
+  StringRef Condition("C"), Then("T"), Else("E");
+  const std::string Snippet = R"cc(
+    if (true)
+      return 1;
+    else
+      return 0;
+  )cc";
+  auto StmtMatch = matchStmt(
+      Snippet, ifStmt(hasCondition(expr().bind(Condition)),
+                      hasThen(stmt().bind(Then)), hasElse(stmt().bind(Else))));
+  ASSERT_TRUE(StmtMatch);
+  // Invert the if-then-else.
+  Stencil S = Stencil::cat("if (!", node(Condition), ") ", sNode(Else),
+                              " else ", sNode(Then));
+  EXPECT_THAT(toOptional(S(StmtMatch->Result)),
+              IsSomething(Eq("if (!true) return 0; else return 1;")));
+}
+
+TEST_F(StencilTest, UnboundNode) {
+  const std::string Snippet = R"cc(
+    if (true)
+      return 1;
+    else
+      return 0;
+  )cc";
+  auto StmtMatch = matchStmt(Snippet, ifStmt(hasCondition(stmt().bind("a1")),
+                                             hasThen(stmt().bind("a2"))));
+  ASSERT_TRUE(StmtMatch);
+  auto Stencil = Stencil::cat("if(!", sNode("a1"), ") ", node("UNBOUND"), ";");
+  auto ResultOrErr = Stencil.eval(StmtMatch->Result);
+  EXPECT_TRUE(llvm::errorToBool(ResultOrErr.takeError()))
+      << "Expected unbound node, got " << *ResultOrErr;
+}
+
+// Tests that a stencil with a single parameter (`Id`) evaluates to the expected
+// string, when `Id` is bound to the expression-statement in `Snippet`.
+void testExpr(StringRef Id, StringRef Snippet, const Stencil &Stencil,
+              StringRef Expected) {
+  auto StmtMatch = matchStmt(Snippet, expr().bind(Id));
+  ASSERT_TRUE(StmtMatch);
+  EXPECT_THAT(toOptional(Stencil.eval(StmtMatch->Result)),
+              IsSomething(Expected));
+}
+
+TEST_F(StencilTest, NodeOp) {
+  StringRef Id = "id";
+  testExpr(Id, "3;", Stencil::cat(node(Id)), "3");
+}
+
+TEST_F(StencilTest, SNodeOp) {
+  StringRef Id = "id";
+  testExpr(Id, "3;", Stencil::cat(sNode(Id)), "3;");
+}
+
+TEST(StencilEqualityTest, Equality) {
+  using stencil::dPrint;
+  auto Lhs = Stencil::cat("foo", node("node"), dPrint("dprint_id"));
+  auto Rhs = Lhs;
+  EXPECT_EQ(Lhs, Rhs);
+}
+
+TEST(StencilEqualityTest, InEqualityDifferentOrdering) {
+  auto Lhs = Stencil::cat("foo", node("node"));
+  auto Rhs = Stencil::cat(node("node"), "foo");
+  EXPECT_NE(Lhs, Rhs);
+}
+
+TEST(StencilEqualityTest, InEqualityDifferentSizes) {
+  auto Lhs = Stencil::cat("foo", node("node"), "bar", "baz");
+  auto Rhs = Stencil::cat("foo", node("node"), "bar");
+  EXPECT_NE(Lhs, Rhs);
+}
+} // namespace