[clangd] Log dropped diagnostics.

[ilya-biryukov]

Summary

clangd drops diagnostics coming outside the main file, but it is still
useful to see that something went wrong in the logs.

[sammccall]

clangd/ClangdUnit.cpp

182

does Message include file:line? we probably want that in the log line.

186

Agree this is useful.

Have we turned off the diagnostics limit (or will we?) a misconfig of e.g. C vs C++ parsing of a header seems like it could produce 1000+ log lines from preamble that we should probably avoid somehow.
(Maybe even avoid paying to format)

[ilya-biryukov]

• Show location of dropped diagnostic.

clangd/ClangdUnit.cpp

182

No, it didn't include the line before. Now dumping the location too.

186

I don't think we have changed default clang behavior.
This look like a good use-case for introducing log levels.
This information is definitely too verbose most of the time, but very useful when debugging failures

clangd/ClangdUnit.cpp

TextEdit toTextEdit(const FixItHint &FixIt, const SourceManager &M,
   Result.range = toRange(Lexer::makeFileCharRange(FixIt.RemoveRange, M, L), M);
   Result.newText = FixIt.CodeToInsert;
   return Result;
 }
 
 llvm::Optional<DiagWithFixIts> toClangdDiag(const clang::Diagnostic &D,
                                             DiagnosticsEngine::Level Level,
                                             const LangOptions &LangOpts) {
+  SmallString<64> Message;
+  D.FormatDiagnostic(Message);
+
   if (!D.hasSourceManager() || !D.getLocation().isValid() ||
-      !D.getSourceManager().isInMainFile(D.getLocation()))
+      !D.getSourceManager().isInMainFile(D.getLocation())) {
+    log("Ignored diagnostic outside main file: " + Twine(Message));
     return llvm::None;
+  }
 
   DiagWithFixIts Result;
   Result.Diag.range = diagnosticRange(D, LangOpts);
   Result.Diag.severity = getSeverity(Level);
-  SmallString<64> Message;
-  D.FormatDiagnostic(Message);
   Result.Diag.message = Message.str();
   for (const FixItHint &Fix : D.getFixItHints())
     Result.FixIts.push_back(toTextEdit(Fix, D.getSourceManager(), LangOpts));
   return std::move(Result);
 }
 
 class StoreDiagsConsumer : public DiagnosticConsumer {
 public: