Index: clangd/CMakeLists.txt
===================================================================
--- clangd/CMakeLists.txt
+++ clangd/CMakeLists.txt
@@ -8,6 +8,7 @@
   ClangdUnit.cpp
   ClangdUnitStore.cpp
   CodeComplete.cpp
+  Context.cpp
   Compiler.cpp
   DraftStore.cpp
   FuzzyMatch.cpp
Index: clangd/Context.h
===================================================================
--- clangd/Context.h
+++ clangd/Context.h
@@ -0,0 +1,186 @@
+//===--- Context.h - Mechanism for passing implicit data --------*- C++-*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// Context for storing and retrieving implicit data. Useful for passing implicit
+// parameters on a per-request basis.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_CONTEXT_H_
+#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_CONTEXT_H_
+
+#include "llvm/ADT/STLExtras.h"
+#include <memory>
+#include <type_traits>
+
+namespace clang {
+namespace clangd {
+
+/// A key for a value of type \p Type, stored inside a context. Keys are
+/// non-movable and non-copyable. See documentation of the Context class for
+/// more details and usage examples.
+template <class Type> class Key {
+public:
+  static_assert(!std::is_reference<Type>::value,
+                "Reference arguments to Key<> are not allowed");
+
+  Key() = default;
+
+  Key(Key const &) = delete;
+  Key &operator=(Key const &) = delete;
+  Key(Key &&) = delete;
+  Key &operator=(Key &&) = delete;
+};
+
+/// A context is an immutable container for per-request data that must be
+/// propagated through layers that don't care about it. An example is a request
+/// ID that we may want to use when logging.
+///
+/// Conceptually, a context is a heterogeneous map<Key<T>, T>. Each key has
+/// an associated value type, which allows the map to be typesafe.
+///
+/// You can't add data to an existing context, instead you create a new
+/// immutable context derived from it with extra data added. When you retrieve
+/// data, the context will walk up the parent chain until the key is found.
+///
+/// Contexts should be:
+///  - passed by reference when calling synchronous functions
+///  - passed by value (move) when calling asynchronous functions. The result
+///    callback of async operations will receive the context again.
+///  - cloned only when 'forking' an asynchronous computation that we don't wait
+///    for.
+///
+/// Copy operations for this class are deleted, use an explicit clone() method
+/// when you need a copy of the context instead.
+///
+/// To derive a child context use derive() function, e.g.
+///     Context ChildCtx = ParentCtx.derive(RequestIdKey, 123);
+///
+/// To create a new root context, derive() from empty Context.
+/// e.g.:
+///     Context Ctx = Context::empty().derive(RequestIdKey, 123);
+///
+/// Values in the context are indexed by typed keys (instances of Key<T> class).
+/// Key<T> serves two purposes:
+///   - it provides a lookup key for the context (each instance of a key is
+///   unique),
+///   - it keeps the type information about the value stored in the context map
+///   in the template arguments.
+/// This provides a type-safe interface to store and access values of multiple
+/// types inside a single context.
+/// For example,
+///    Key<int> RequestID;
+///    Key<int> Version;
+///
+///    Context Ctx = Context::empty().derive(RequestID, 10).derive(Version, 3);
+///    assert(*Ctx.get(RequestID) == 10);
+///    assert(*Ctx.get(Version) == 3);
+///
+/// Keys are typically used across multiple functions, so most of the time you
+/// would want to make them static class members or global variables.
+class Context {
+public:
+  /// Returns an empty context that contains no data. Useful for calling
+  /// functions that require a context when no explicit context is available.
+  static Context empty();
+
+private:
+  struct Data;
+  Context(std::shared_ptr<const Data> DataPtr);
+
+public:
+  /// Move-only.
+  Context(Context const &) = delete;
+  Context &operator=(const Context &) = delete;
+
+  Context(Context &&) = default;
+  Context &operator=(Context &&) = default;
+
+  /// Get data stored for a typed \p Key. If values are not found
+  /// \returns Pointer to the data associated with \p Key. If no data is
+  /// specified for \p Key, return null.
+  template <class Type> const Type *get(const Key<Type> &Key) const {
+    for (const Data *DataPtr = this->DataPtr.get(); DataPtr != nullptr;
+         DataPtr = DataPtr->Parent.get()) {
+      if (DataPtr->KeyPtr == &Key)
+        return static_cast<const Type *>(DataPtr->Value->getValuePtr());
+    }
+    return nullptr;
+  }
+
+  /// A helper to get a reference to a \p Key that must exist in the map.
+  /// Must not be called for keys that are not in the map.
+  template <class Type> const Type &getExisting(const Key<Type> &Key) const {
+    auto Val = get(Key);
+    assert(Val && "Key does not exist");
+    return *Val;
+  }
+
+  /// Derives a child context
+  /// It is safe to move or destroy a parent context after calling derive() from
+  /// it. The child context will continue to have access to the data stored in
+  /// the parent context.
+  template <class Type>
+  Context derive(const Key<Type> &Key,
+                 typename std::decay<Type>::type Value) const & {
+    return Context(std::make_shared<Data>(Data{
+        /*Parent=*/DataPtr, &Key,
+        llvm::make_unique<TypedAnyStorage<typename std::decay<Type>::type>>(
+            std::move(Value))}));
+  }
+
+  template <class Type>
+  Context
+  derive(const Key<Type> &Key,
+         typename std::decay<Type>::type Value) && /* takes ownership */ {
+    return Context(std::make_shared<Data>(Data{
+        /*Parent=*/std::move(DataPtr), &Key,
+        llvm::make_unique<TypedAnyStorage<typename std::decay<Type>::type>>(
+            std::move(Value))}));
+  }
+
+  /// Clone this context object.
+  Context clone() const;
+
+private:
+  class AnyStorage {
+  public:
+    virtual ~AnyStorage() = default;
+    virtual void *getValuePtr() = 0;
+  };
+
+  template <class T> class TypedAnyStorage : public Context::AnyStorage {
+    static_assert(std::is_same<typename std::decay<T>::type, T>::value,
+                  "Argument to TypedAnyStorage must be decayed");
+
+  public:
+    TypedAnyStorage(T &&Value) : Value(std::move(Value)) {}
+
+    void *getValuePtr() override { return &Value; }
+
+  private:
+    T Value;
+  };
+
+  struct Data {
+    // We need to make sure Parent outlives the Value, so the order of members
+    // is important. We do that to allow classes stored in Context's child
+    // layers to store references to the data in the parent layers.
+    std::shared_ptr<const Data> Parent;
+    const void *KeyPtr;
+    std::unique_ptr<AnyStorage> Value;
+  };
+
+  std::shared_ptr<const Data> DataPtr;
+}; // namespace clangd
+
+} // namespace clangd
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANGD_CONTEXT_H_
Index: clangd/Context.cpp
===================================================================
--- clangd/Context.cpp
+++ clangd/Context.cpp
@@ -0,0 +1,24 @@
+//===--- Context.cpp -----------------------------------------*- C++-*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===---------------------------------------------------------------------===//
+
+#include "Context.h"
+#include <cassert>
+
+namespace clang {
+namespace clangd {
+
+Context Context::empty() { return Context(/*Data=*/nullptr); }
+
+Context::Context(std::shared_ptr<const Data> DataPtr)
+    : DataPtr(std::move(DataPtr)) {}
+
+Context Context::clone() const { return Context(DataPtr); }
+
+} // namespace clangd
+} // namespace clang
Index: unittests/clangd/CMakeLists.txt
===================================================================
--- unittests/clangd/CMakeLists.txt
+++ unittests/clangd/CMakeLists.txt
@@ -11,6 +11,7 @@
 add_extra_unittest(ClangdTests
   ClangdTests.cpp
   CodeCompleteTests.cpp
+  ContextTests.cpp
   FuzzyMatchTests.cpp
   JSONExprTests.cpp
   TestFS.cpp
Index: unittests/clangd/ContextTests.cpp
===================================================================
--- unittests/clangd/ContextTests.cpp
+++ unittests/clangd/ContextTests.cpp
@@ -0,0 +1,57 @@
+//===-- ContextTests.cpp - Context tests ------------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "Context.h"
+
+#include "gtest/gtest.h"
+
+namespace clang {
+namespace clangd {
+
+TEST(ContextTests, Simple) {
+  Key<int> IntParam;
+  Key<int> ExtraIntParam;
+
+  Context Ctx = Context::empty().derive(IntParam, 10).derive(ExtraIntParam, 20);
+
+  EXPECT_EQ(*Ctx.get(IntParam), 10);
+  EXPECT_EQ(*Ctx.get(ExtraIntParam), 20);
+}
+
+TEST(ContextTests, MoveOps) {
+  Key<std::unique_ptr<int>> Param;
+
+  Context Ctx = Context::empty().derive(Param, llvm::make_unique<int>(10));
+  EXPECT_EQ(**Ctx.get(Param), 10);
+
+  Context NewCtx = std::move(Ctx);
+  EXPECT_EQ(**NewCtx.get(Param), 10);
+}
+
+TEST(ContextTests, Builders) {
+  Key<int> ParentParam;
+  Key<int> ParentAndChildParam;
+  Key<int> ChildParam;
+
+  Context ParentCtx =
+      Context::empty().derive(ParentParam, 10).derive(ParentAndChildParam, 20);
+  Context ChildCtx =
+      ParentCtx.derive(ParentAndChildParam, 30).derive(ChildParam, 40);
+
+  EXPECT_EQ(*ParentCtx.get(ParentParam), 10);
+  EXPECT_EQ(*ParentCtx.get(ParentAndChildParam), 20);
+  EXPECT_EQ(ParentCtx.get(ChildParam), nullptr);
+
+  EXPECT_EQ(*ChildCtx.get(ParentParam), 10);
+  EXPECT_EQ(*ChildCtx.get(ParentAndChildParam), 30);
+  EXPECT_EQ(*ChildCtx.get(ChildParam), 40);
+}
+
+} // namespace clangd
+} // namespace clang