--- /dev/null
+//===--- 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_
--- /dev/null
+//===-- 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
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