[clangd] Print .clang-tidy configuration parsing errors using [ev]?log.

[njames93]

Summary

Currently warnings when parsing .clang-tidy are printed directly to errs.
This is less than ideal as there is no synchronisation printing to errs, leading to potential races.

[sammccall]

Nice! From here it seems like mostly a question of plumbing/refactoring on the clangd side to be able to surface these diagnostics, similar to how you did it with config files.

At some point, particularly if we also support .clang-format files, it might even be worth reconsidering if there's a lightweight way to become a language-server for such files. Less in order to provide quickfixes and code completion (which would be huge effort), and more to be able to surface diagnostics for files that are opened in the editor (rather than waiting until clangd tries to parse them for some other reason)

clang-tools-extra/clangd/TidyProvider.cpp

 #include "support/ThreadsafeFS.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringSet.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/Process.h"
+#include "llvm/Support/SourceMgr.h"
 #include "llvm/Support/VirtualFileSystem.h"
 #include <memory>
 
 namespace clang {
 namespace clangd {
 namespace {
 
 // Access to config from a .clang-tidy file, caching IO and parsing.
public:
   get(const ThreadsafeFS &TFS,
       std::chrono::steady_clock::time_point FreshTime) const {
     std::shared_ptr<const tidy::ClangTidyOptions> Result;
     read(
         TFS, FreshTime,
         [this](llvm::Optional<llvm::StringRef> Data) {
           Value.reset();
           if (Data && !Data->empty()) {
-            if (auto Parsed = tidy::parseConfiguration(
-                    llvm::MemoryBufferRef(*Data, path())))
+            tidy::DiagCallback Diagnostics = [](const llvm::SMDiagnostic &D) {
+              switch (D.getKind()) {
+              case llvm::SourceMgr::DK_Error:
+                elog("tidy-config error at {0}:{1}:{2}: {3}", D.getFilename(),
+                     D.getLineNo(), D.getColumnNo(), D.getMessage());
+                break;
+              case llvm::SourceMgr::DK_Warning:
+                log("tidy-config warning at {0}:{1}:{2}: {3}", D.getFilename(),
+                    D.getLineNo(), D.getColumnNo(), D.getMessage());
+                break;
+              case llvm::SourceMgr::DK_Note:
+              case llvm::SourceMgr::DK_Remark:
+                vlog("tidy-config note at {0}:{1}:{2}: {3}", D.getFilename(),
+                     D.getLineNo(), D.getColumnNo(), D.getMessage());
+                break;
+              }
+            };
+            if (auto Parsed = tidy::parseConfigurationWithDiags(
+                    llvm::MemoryBufferRef(*Data, path()), Diagnostics))
               Value = std::make_shared<const tidy::ClangTidyOptions>(
                   std::move(*Parsed));
             else
               elog("Error parsing clang-tidy configuration in {0}: {1}", path(),
                    Parsed.getError().message());
           }
         },
         [&]() { Result = Value; });