indexer: log the number of errors and the first diagnostic

Example log:

```
15:47:45 indexer1         pipeline.cc:379 I parse /tmp/d/a.c error:1 use of undeclared identifier 'arg'
 clang /tmp/d/a.c --gcc-toolchain=/usr -working-directory=/tmp/d/
```

src/indexer.cc

@@ -1203,6 +1203,17 @@ public:
     return std::make_unique<MultiplexConsumer>(std::move(consumers));
   }
 };
+
+class IndexDiags : public DiagnosticConsumer {
+public:
+  llvm::SmallString<64> message;
+  void HandleDiagnostic(DiagnosticsEngine::Level level,
+    const clang::Diagnostic &info) override {
+    DiagnosticConsumer::HandleDiagnostic(level, info);
+    if (message.empty())
+      info.FormatDiagnostic(message);
+  }
+};
 } // namespace
 
 const int IndexFile::kMajorVersion = 21;
@@ -1252,7 +1263,7 @@ void init() {
                                        g_config->index.multiVersionBlacklist);
 }
 
-std::vector<std::unique_ptr<IndexFile>>
+IndexResult
 index(SemaManager *manager, WorkingFiles *wfiles, VFS *vfs,
       const std::string &opt_wdir, const std::string &main,
       const std::vector<const char *> &args,
@@ -1281,7 +1292,7 @@ index(SemaManager *manager, WorkingFiles *wfiles, VFS *vfs,
       ci->getPreprocessorOpts().addRemappedFile(filename, bufs.back().get());
     }
 
-  DiagnosticConsumer dc;
+  IndexDiags dc;
   auto clang = std::make_unique<CompilerInstance>(pch);
   clang->setInvocation(std::move(ci));
   clang->createDiagnostics(&dc, false);
@@ -1355,7 +1366,10 @@ index(SemaManager *manager, WorkingFiles *wfiles, VFS *vfs,
     return {};
   }
 
-  std::vector<std::unique_ptr<IndexFile>> result;
+  IndexResult result;
+  result.n_errs = (int)dc.getNumErrors();
+  // clang 7 does not implement operator std::string.
+  result.first_error = std::string(dc.message.data(), dc.message.size());
   for (auto &it : param.uid2file) {
     if (!it.second.db)
       continue;
@@ -1392,7 +1406,7 @@ index(SemaManager *manager, WorkingFiles *wfiles, VFS *vfs,
         entry->dependencies[llvm::CachedHashStringRef(intern(path))] =
             file.mtime;
     }
-    result.push_back(std::move(entry));
+    result.indexes.push_back(std::move(entry));
   }
 
   return result;

src/indexer.hh

@@ -322,13 +322,19 @@ struct IndexFile {
   std::string toString();
 };
 
+struct IndexResult {
+  std::vector<std::unique_ptr<IndexFile>> indexes;
+  int n_errs = 0;
+  std::string first_error;
+};
+
 struct SemaManager;
 struct WorkingFiles;
 struct VFS;
 
 namespace idx {
 void init();
-std::vector<std::unique_ptr<IndexFile>>
+IndexResult
 index(SemaManager *complete, WorkingFiles *wfiles, VFS *vfs,
       const std::string &opt_wdir, const std::string &file,
       const std::vector<const char *> &args,

src/pipeline.cc

@@ -330,17 +330,9 @@ bool indexer_Parse(SemaManager *completion, WorkingFiles *wfiles,
       return true;
     } while (0);
 
-  if (loud) {
-    std::string line;
-    if (LOG_V_ENABLED(1)) {
-      line = "\n ";
-      for (auto &arg : entry.args)
-        (line += ' ') += arg;
-    }
-    LOG_S(INFO) << (deleted ? "delete " : "parse ") << path_to_index << line;
-  }
-
   std::vector<std::unique_ptr<IndexFile>> indexes;
+  int n_errs = 0;
+  std::string first_error;
   if (deleted) {
     indexes.push_back(std::make_unique<IndexFile>(request.path, "", false));
     if (request.path != path_to_index)
@@ -353,8 +345,12 @@ bool indexer_Parse(SemaManager *completion, WorkingFiles *wfiles,
         remapped.emplace_back(path_to_index, content);
     }
     bool ok;
-    indexes = idx::index(completion, wfiles, vfs, entry.directory,
+    auto result =
-                         path_to_index, entry.args, remapped, no_linkage, ok);
+        idx::index(completion, wfiles, vfs, entry.directory, path_to_index,
+                   entry.args, remapped, no_linkage, ok);
+    indexes = std::move(result.indexes);
+    n_errs = result.n_errs;
+    first_error = std::move(result.first_error);
 
     if (!ok) {
       if (request.id.valid()) {
@@ -367,6 +363,21 @@ bool indexer_Parse(SemaManager *completion, WorkingFiles *wfiles,
     }
   }
 
+  if (loud || n_errs) {
+    std::string line;
+    SmallString<64> tmp;
+    SmallString<256> msg;
+    (Twine(deleted ? "delete " : "parse ") + path_to_index).toVector(msg);
+    if (n_errs)
+      msg += (" error:" + Twine(n_errs) + " " + first_error).toStringRef(tmp);
+    if (LOG_V_ENABLED(1)) {
+      msg += "\n ";
+      for (const char *arg : entry.args)
+        (msg += ' ') += arg;
+    }
+    LOG_S(INFO) << std::string_view(msg.data(), msg.size());
+  }
+
   for (std::unique_ptr<IndexFile> &curr : indexes) {
     std::string path = curr->path;
     if (!matcher.matches(path)) {

src/test.cc

@@ -315,15 +315,15 @@ bool runIndexTests(const std::string &filter_path, bool enable_update) {
         for (auto &arg : flags)
           cargs.push_back(arg.c_str());
         bool ok;
-        auto dbs = ccls::idx::index(&completion, &wfiles, &vfs, "", path, cargs,
+        auto result = ccls::idx::index(&completion, &wfiles, &vfs, "", path,
-                                    {}, true, ok);
+                                       cargs, {}, true, ok);
 
         for (const auto &entry : all_expected_output) {
           const std::string &expected_path = entry.first;
           std::string expected_output = text_replacer.apply(entry.second);
 
           // Get output from index operation.
-          IndexFile *db = findDbForPathEnding(expected_path, dbs);
+          IndexFile *db = findDbForPathEnding(expected_path, result.indexes);
           std::string actual_output = "{}";
           if (db) {
             verifySerializeToFrom(db);