[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, 2 insertions, 3 deletions

diff --git a/clang-tools-extra/clangd/Logger.h b/clang-tools-extra/clangd/Logger.h
index b6d7a1b4caf..7811111a2f6 100644
--- a/clang-tools-extra/clangd/Logger.h
+++ b/clang-tools-extra/clangd/Logger.h
@@ -10,7 +10,6 @@
 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_LOGGER_H
 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_LOGGER_H
 
-#include "Context.h"
 #include "llvm/ADT/Twine.h"
 
 namespace clang {
@@ -19,7 +18,7 @@ namespace clangd {
 /// Main logging function.
 /// Logs messages to a global logger, which can be set up by LoggingSesssion.
 /// If no logger is registered, writes to llvm::errs().
-void log(const Context &Ctx, const llvm::Twine &Message);
+void log(const llvm::Twine &Message);
 
 /// Interface to allow custom logging in clangd.
 class Logger {
@@ -27,7 +26,7 @@ public:
   virtual ~Logger() = default;
 
   /// Implementations of this method must be thread-safe.
-  virtual void log(const Context &Ctx, const llvm::Twine &Message) = 0;
+  virtual void log(const llvm::Twine &Message) = 0;
 };
 
 /// Only one LoggingSession can be active at a time.