[clang-tidy][NFC] Add test for Transformer-based checks with diagnostics.

[ymandel]

Summary

Adds a test that checks the diagnostic output of a tidy built from TransformerClangTidyCheck.

[ymandel]

Discarded API changes, reduced to new clang-tidy lib test.

[ymandel]

Reworked, per our discussion. PTAL.

clang-tools-extra/unittests/clang-tidy/TransformerClangTidyCheckTest.cpp

 //===---- TransformerClangTidyCheckTest.cpp - clang-tidy ------------------===//
 //
 // 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-tidy/utils/TransformerClangTidyCheck.h"
 #include "ClangTidyTest.h"
 #include "clang/ASTMatchers/ASTMatchers.h"
 #include "clang/Tooling/Transformer/RangeSelector.h"
+#include "clang/Tooling/Transformer/RewriteRule.h"
 #include "clang/Tooling/Transformer/Stencil.h"
 #include "clang/Tooling/Transformer/Transformer.h"
+#include "gmock/gmock.h"
 #include "gtest/gtest.h"
 
 namespace clang {
 namespace tidy {
 namespace utils {
 namespace {
 using namespace ::clang::ast_matchers;
 
 using transformer::cat;
 using transformer::change;
 using transformer::IncludeFormat;
 using transformer::makeRule;
 using transformer::node;
+using transformer::noopEdit;
 using transformer::RewriteRule;
+using transformer::RootID;
 using transformer::statement;
 
 // Invert the code of an if-statement, while maintaining its semantics.
 RewriteRule invertIf() {
   StringRef C = "C", T = "T", E = "E";
-  RewriteRule Rule =
-      makeRule(ifStmt(hasCondition(expr().bind(C)), hasThen(stmt().bind(T)),
-                      hasElse(stmt().bind(E))),
-               change(statement(std::string(RewriteRule::RootID)),
-                      cat("if(!(", node(std::string(C)), ")) ",
+  RewriteRule Rule = makeRule(
+      ifStmt(hasCondition(expr().bind(C)), hasThen(stmt().bind(T)),
+      change(statement(RootID), cat("if(!(", node(std::string(C)), ")) ",
                                     statement(std::string(E)), " else ",
                                     statement(std::string(T)))),
-               cat("negate condition and reverse `then` and `else` branches"));
   return Rule;
 }
 
 class IfInverterCheck : public TransformerClangTidyCheck {
 public:
   IfInverterCheck(StringRef Name, ClangTidyContext *Context)
       : TransformerClangTidyCheck(invertIf(), Name, Context) {}
 };
const std::string Expected = R"(
     void log(const char* msg);
     void foo() {
       if(!(10 > 1.0)) log("ok"); else log("oh no!");
     }
   )";
   EXPECT_EQ(Expected, test::runCheckOnCode<IfInverterCheck>(Input));
 }
 
+TEST(TransformerClangTidyCheckTest, DiagnosticsCorrectlyGenerated) {
+  class DiagOnlyCheck : public TransformerClangTidyCheck {
+  public:
+    DiagOnlyCheck(StringRef Name, ClangTidyContext *Context)
+        : TransformerClangTidyCheck(
+              makeRule(returnStmt(), noopEdit(node(RootID)), cat("message")),
+              Name, Context) {}
+  };
+  std::string Input = "int h() { return 5; }";
+  std::vector<ClangTidyError> Errors;
+  EXPECT_EQ(Input, test::runCheckOnCode<DiagOnlyCheck>(Input, &Errors));
+  EXPECT_EQ(Errors.size(), 1U);
+  EXPECT_EQ(Errors[0].Message.Message, "message");
+  EXPECT_THAT(Errors[0].Ranges, testing::IsEmpty());
+
+  // The diagnostic is anchored to the match, "return 5".
+  EXPECT_EQ(Errors[0].Message.FileOffset, 10U);
+}
+
 class IntLitCheck : public TransformerClangTidyCheck {
 public:
   IntLitCheck(StringRef Name, ClangTidyContext *Context)
       : TransformerClangTidyCheck(
             makeRule(integerLiteral(), change(cat("LIT")), cat("no message")),
             Name, Context) {}
 };