[clang-tidy] Add fix descriptions to clang-tidy checks.

Summary:
Motivation/Context: in the code review system integrating with clang-tidy,
clang-tidy doesn't provide a human-readable description of the fix. Usually
developers have to preview a code diff (before vs after apply the fix) to
understand what the fix does before applying a fix.

This patch proposes that each clang-tidy check provides a short and
actional fix description that can be shown in the UI, so that users can know
what the fix does without previewing diff.

This patch extends clang-tidy framework to support fix descriptions (will add implementations for
existing checks in the future). Fix descriptions and fixes are emitted via diagnostic::Note (rather than
attaching the main warning diagnostic).

Before this patch:

```
void MyCheck::check(...) {
   ...
   diag(loc, "my check warning") <<  FixtItHint::CreateReplacement(...);
}
```

After:

```
void MyCheck::check(...) {
   ...
   diag(loc, "my check warning"); // Emit a check warning
   diag(loc, "fix description", DiagnosticIDs::Note) << FixtItHint::CreateReplacement(...); // Emit a diagnostic note and a fix
}
```

Reviewers: sammccall, alexfh

Reviewed By: alexfh

Subscribers: MyDeveloperDay, Eugene.Zelenko, aaron.ballman, JonasToth, xazax.hun, jdoerfert, cfe-commits

Tags: #clang-tools-extra, #clang

Differential Revision: https://reviews.llvm.org/D59932

llvm-svn: 358576

1 files changed, 11 insertions, 8 deletions

diff --git a/clang-tools-extra/unittests/clang-tidy/ClangTidyTest.h b/clang-tools-extra/unittests/clang-tidy/ClangTidyTest.h
index 33139d72c29..560b0ff893c 100644
--- a/clang-tools-extra/unittests/clang-tidy/ClangTidyTest.h
+++ b/clang-tools-extra/unittests/clang-tidy/ClangTidyTest.h
@@ -14,6 +14,8 @@
 #include "clang/ASTMatchers/ASTMatchFinder.h"
 #include "clang/Frontend/CompilerInstance.h"
 #include "clang/Frontend/FrontendActions.h"
+#include "clang/Tooling/Core/Diagnostic.h"
+#include "clang/Tooling/Core/Replacement.h"
 #include "clang/Tooling/Refactoring.h"
 #include "clang/Tooling/Tooling.h"
 #include "llvm/ADT/Optional.h"
@@ -130,16 +132,17 @@ runCheckOnCode(StringRef Code, std::vector<ClangTidyError> *Errors = nullptr,
   tooling::Replacements Fixes;
   std::vector<ClangTidyError> Diags = DiagConsumer.take();
   for (const ClangTidyError &Error : Diags) {
-    for (const auto &FileAndFixes : Error.Fix) {
-      for (const auto &Fix : FileAndFixes.second) {
-        auto Err = Fixes.add(Fix);
-        // FIXME: better error handling. Keep the behavior for now.
-        if (Err) {
-          llvm::errs() << llvm::toString(std::move(Err)) << "\n";
-          return "";
+    if (const auto *ChosenFix = tooling::selectFirstFix(Error))
+      for (const auto &FileAndFixes : *ChosenFix) {
+        for (const auto &Fix : FileAndFixes.second) {
+          auto Err = Fixes.add(Fix);
+          // FIXME: better error handling. Keep the behavior for now.
+          if (Err) {
+            llvm::errs() << llvm::toString(std::move(Err)) << "\n";
+            return "";
+          }
         }
       }
-    }
   }
   if (Errors)
     *Errors = std::move(Diags);