Add Cleanup class.

[jlebar]

Summary

Like absl::Cleanup, llvm::Cleanup runs an arbitrary function right
before the scope containing the Cleanup exits.

You can use this to do some work at the end of a function, right before
it returns.

This is used in a later patch.

[nikic]

Unless I'm missing something, this is already covered by make_scope_exit().

[jlebar]

In D134008#3794569, @nikic wrote:

Unless I'm missing something, this is already covered by make_scope_exit().

You are definitely not missing something. :) I looked through ADT before writing this patch, but clearly I did not look closely enough.

Thank you.

llvm/include/llvm/ADT/Cleanup.h

+//===- Cleanup.h - Run a lambda when a fn exits -----------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+///  Cleanup is an RAII type whose destructor runs an arbitrary function or
+///  lambda.  You can use this to do some work at the end of a function, right
+///  before it returns.
+///
+///  Prior art: absl::Cleanup.
+///
+///  Example::
+///
+///    Cleanup cleanup = [&] { close(fd); };
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ADT_CLEANUP_H
+#define LLVM_ADT_CLEANUP_H
+
+#include <algorithm>
+namespace llvm {
+
+template <typename FnT> class Cleanup {
+public:
+  Cleanup(FnT Fn) : Fn(std::move(Fn)) {}
+  ~Cleanup() { Fn(); }
+
+private:
+  static_assert(std::is_same_v<decltype(std::declval<FnT>()()), void>,
+                "Callback function must return void.");
+
+  FnT Fn;
+};
+
+template <typename FnT> Cleanup(FnT Fn) -> Cleanup<FnT>;
+
+} // end namespace llvm
+
+#endif // LLVM_ADT_CLEANUP_H

llvm/unittests/ADT/CMakeLists.txt

add_llvm_unittest(ADTTests
   APSIntTest.cpp
   ArrayRefTest.cpp
   BitFieldsTest.cpp
   BitmaskEnumTest.cpp
   BitTest.cpp
   BitVectorTest.cpp
   BreadthFirstIteratorTest.cpp
   BumpPtrListTest.cpp
+  CleanupTest.cpp
   CoalescingBitVectorTest.cpp
   CombinationGeneratorTest.cpp
   DAGDeltaAlgorithmTest.cpp
   DeltaAlgorithmTest.cpp
   DenseMapTest.cpp
   DenseSetTest.cpp
   DepthFirstIteratorTest.cpp
   DirectedGraphTest.cpp

llvm/unittests/ADT/CleanupTest.cpp

+//===- llvm/unittest/Support/CleanupTest.cpp - Cleanup tests ---===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/Cleanup.h"
+#include "gtest/gtest.h"
+#include <cstdlib>
+
+using namespace llvm;
+
+namespace {
+
+TEST(CleanupTest, Simple) {
+  bool ran = false;
+  {
+    Cleanup c = [&] { ran = true; };
+  }
+  EXPECT_TRUE(ran);
+}
+
+} // anonymous namespace