[clangd] Pass Context implicitly using TLS.

Summary:
Instead of passing Context explicitly around, we now have a thread-local
Context object `Context::current()` which is an implicit argument to
every function.
Most manipulation of this should use the WithContextValue helper, which
augments the current Context to add a single KV pair, and restores the
old context on destruction.

Advantages are:
- less boilerplate in functions that just propagate contexts
- reading most code doesn't require understanding context at all, and
  using context as values in fewer places still
- fewer options to pass the "wrong" context when it changes within a
  scope (e.g. when using Span)
- contexts pass through interfaces we can't modify, such as VFS
- propagating contexts across threads was slightly tricky (e.g.
  copy vs move, no move-init in lambdas), and is now encapsulated in
  the threadpool

Disadvantages are all the usual TLS stuff - hidden magic, and
potential for higher memory usage on threads that don't use the
context. (In practice, it's just one pointer)

Reviewers: ilya-biryukov

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

Differential Revision: https://reviews.llvm.org/D42517

llvm-svn: 323872

1 files changed, 6 insertions, 7 deletions

diff --git a/clang-tools-extra/clangd/TUScheduler.cpp b/clang-tools-extra/clangd/TUScheduler.cpp
index 96b29f12fdd..4c18dcdab6b 100644
--- a/clang-tools-extra/clangd/TUScheduler.cpp
+++ b/clang-tools-extra/clangd/TUScheduler.cpp
@@ -23,9 +23,8 @@ TUScheduler::TUScheduler(unsigned AsyncThreadsCount,
       Threads(AsyncThreadsCount) {}
 
 void TUScheduler::update(
-    Context Ctx, PathRef File, ParseInputs Inputs,
-    UniqueFunction<void(Context Ctx,
-                        llvm::Optional<std::vector<DiagWithFixIts>>)>
+    PathRef File, ParseInputs Inputs,
+    UniqueFunction<void(llvm::Optional<std::vector<DiagWithFixIts>>)>
         OnUpdated) {
   CachedInputs[File] = Inputs;
 
@@ -33,12 +32,12 @@ void TUScheduler::update(
   auto DeferredRebuild = Resources->deferRebuild(std::move(Inputs));
 
   Threads.addToFront(
-      [](Context Ctx, decltype(OnUpdated) OnUpdated,
+      [](decltype(OnUpdated) OnUpdated,
          decltype(DeferredRebuild) DeferredRebuild) {
-        auto Diags = DeferredRebuild(Ctx);
-        OnUpdated(std::move(Ctx), Diags);
+        auto Diags = DeferredRebuild();
+        OnUpdated(Diags);
       },
-      std::move(Ctx), std::move(OnUpdated), std::move(DeferredRebuild));
+      std::move(OnUpdated), std::move(DeferredRebuild));
 }
 
 void TUScheduler::remove(PathRef File,