utils: Add TINT_SCOPED_ASSIGNMENT()

A helper class and macro used to simplify scope-based assignment of
variables.

Change-Id: I02b3a05240a2c4628f813de931c40d8fba3cb07b
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/51480
Auto-Submit: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/utils/scoped_assignment.h

@@ -0,0 +1,64 @@
+// Copyright 2021 The Tint Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef SRC_UTILS_SCOPED_ASSIGNMENT_H_
+#define SRC_UTILS_SCOPED_ASSIGNMENT_H_
+
+#include <type_traits>
+
+namespace tint {
+namespace utils {
+
+/// Helper class that temporarily assigns a value to a variable for the lifetime
+/// of the ScopedAssignment object. Once the ScopedAssignment is destructed, the
+/// original value is restored.
+template <typename T>
+class ScopedAssignment {
+ public:
+  /// Constructor
+  /// @param var the variable to temporarily assign a new value to
+  /// @param val the value to assign to `ref` for the lifetime of this
+  /// ScopedAssignment.
+  ScopedAssignment(T& var, T val) : ref_(var) {
+    old_value_ = var;
+    var = val;
+  }
+
+  /// Destructor
+  /// Restores the original value of the variable.
+  ~ScopedAssignment() { ref_ = old_value_; }
+
+ private:
+  ScopedAssignment(const ScopedAssignment&) = delete;
+  ScopedAssignment& operator=(const ScopedAssignment&) = delete;
+
+  T& ref_;
+  T old_value_;
+};
+
+}  // namespace utils
+}  // namespace tint
+
+#define TINT_CONCAT_2(a, b) a##b
+#define TINT_CONCAT(a, b) TINT_CONCAT_2(a, b)
+
+/// TINT_SCOPED_ASSIGNMENT(var, val) assigns `val` to `var`, and automatically
+/// restores the original value of `var` when exiting the current lexical scope.
+#define TINT_SCOPED_ASSIGNMENT(var, val)                                  \
+  ::tint::utils::ScopedAssignment<std::remove_reference_t<decltype(var)>> \
+  TINT_CONCAT(tint_scoped_assignment_, __COUNTER__) {                     \
+    var, val                                                              \
+  }
+
+#endif  //  SRC_UTILS_SCOPED_ASSIGNMENT_H_

src/utils/scoped_assignment_test.cc

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+// Copyright 2021 The Tint Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "src/utils/scoped_assignment.h"
+
+#include "gtest/gtest.h"
+
+namespace tint {
+namespace utils {
+namespace {
+
+TEST(ScopedAssignmentTest, Scopes) {
+  int i = 0;
+  EXPECT_EQ(i, 0);
+  {
+    EXPECT_EQ(i, 0);
+    TINT_SCOPED_ASSIGNMENT(i, 1);
+    EXPECT_EQ(i, 1);
+    {
+      EXPECT_EQ(i, 1);
+      TINT_SCOPED_ASSIGNMENT(i, 2);
+      EXPECT_EQ(i, 2);
+    }
+    {
+      EXPECT_EQ(i, 1);
+      TINT_SCOPED_ASSIGNMENT(i, 3);
+      EXPECT_EQ(i, 3);
+    }
+    EXPECT_EQ(i, 1);
+  }
+  EXPECT_EQ(i, 0);
+}
+
+}  // namespace
+}  // namespace utils
+}  // namespace tint