[clangd] Correct setting ignoreWarnings in CodeCompletion.

Summary:
We should set the flag before creating ComplierInstance -- when
CopmilerInstance gets initialized, it also initializes the DiagnosticsEngine
using the DiagnosticOptions.

This was hidden deeply -- as clang suppresses all diagnostics when we
hit the code-completion (but internally it does do unnecessary analysis stuff).

As a bonus point, this fix will optmize the completion speed -- clang won't do
any analysis (e.g. -Wunreachable-code, -Wthread-safety-analysisi) at all internally.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: klimek, jkorous-apple, ioeric, cfe-commits

Differential Revision: https://reviews.llvm.org/D43569

llvm-svn: 325779

1 files changed, 2 insertions, 2 deletions

diff --git a/clang-tools-extra/clangd/Headers.cpp b/clang-tools-extra/clangd/Headers.cpp
index db8029b2ac0..3cdacffc452 100644
--- a/clang-tools-extra/clangd/Headers.cpp
+++ b/clang-tools-extra/clangd/Headers.cpp
@@ -79,12 +79,12 @@ calculateIncludePath(llvm::StringRef File, llvm::StringRef Code,
   // added more than once.
   CI->getPreprocessorOpts().SingleFileParseMode = true;
 
+  // The diagnostic options must be set before creating a CompilerInstance.
+  CI->getDiagnosticOpts().IgnoreWarnings = true;
   auto Clang = prepareCompilerInstance(
       std::move(CI), /*Preamble=*/nullptr,
       llvm::MemoryBuffer::getMemBuffer(Code, File),
       std::make_shared<PCHContainerOperations>(), FS, IgnoreDiags);
-  auto &DiagOpts = Clang->getDiagnosticOpts();
-  DiagOpts.IgnoreWarnings = true;
 
   if (Clang->getFrontendOpts().Inputs.empty())
     return llvm::make_error<llvm::StringError>(