[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, 13 insertions, 2 deletions

diff --git a/clang/lib/Tooling/Core/Diagnostic.cpp b/clang/lib/Tooling/Core/Diagnostic.cpp
index 8351f3e000c..235bd7fc143 100644
--- a/clang/lib/Tooling/Core/Diagnostic.cpp
+++ b/clang/lib/Tooling/Core/Diagnostic.cpp
@@ -12,6 +12,7 @@
 
 #include "clang/Tooling/Core/Diagnostic.h"
 #include "clang/Basic/SourceManager.h"
+#include "llvm/ADT/STLExtras.h"
 
 namespace clang {
 namespace tooling {
@@ -40,11 +41,21 @@ Diagnostic::Diagnostic(llvm::StringRef DiagnosticName,
 
 Diagnostic::Diagnostic(llvm::StringRef DiagnosticName,
                        const DiagnosticMessage &Message,
-                       const llvm::StringMap<Replacements> &Fix,
                        const SmallVector<DiagnosticMessage, 1> &Notes,
                        Level DiagLevel, llvm::StringRef BuildDirectory)
-    : DiagnosticName(DiagnosticName), Message(Message), Fix(Fix), Notes(Notes),
+    : DiagnosticName(DiagnosticName), Message(Message), Notes(Notes),
       DiagLevel(DiagLevel), BuildDirectory(BuildDirectory) {}
 
+const llvm::StringMap<Replacements> *selectFirstFix(const Diagnostic& D) {
+   if (!D.Message.Fix.empty())
+    return &D.Message.Fix;
+  auto Iter = llvm::find_if(D.Notes, [](const tooling::DiagnosticMessage &D) {
+    return !D.Fix.empty();
+  });
+  if (Iter != D.Notes.end())
+    return &Iter->Fix;
+  return nullptr;
+}
+
 } // end namespace tooling
 } // end namespace clang