[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, 14 insertions, 0 deletions

diff --git a/clang-tools-extra/clangd/Context.cpp b/clang-tools-extra/clangd/Context.cpp
index d7a1a04b035..5a161216a25 100644
--- a/clang-tools-extra/clangd/Context.cpp
+++ b/clang-tools-extra/clangd/Context.cpp
@@ -20,5 +20,19 @@ Context::Context(std::shared_ptr<const Data> DataPtr)
 
 Context Context::clone() const { return Context(DataPtr); }
 
+// The thread-local Context is scoped in a function to avoid
+// initialization-order issues. It's created when first needed.
+static Context &currentContext() {
+  static thread_local Context C = Context::empty();
+  return C;
+}
+
+const Context &Context::current() { return currentContext(); }
+
+Context Context::swapCurrent(Context Replacement) {
+  std::swap(Replacement, currentContext());
+  return Replacement;
+}
+
 } // namespace clangd
 } // namespace clang