Resolver: Traverse expressions without recursion

This CL changes the way that the resolver traverses expressions to avoid stack overflows for deeply nested expressions.

Instead of having the expression resolver methods call back into
Expression(), add a TraverseExpressions() method that collects all the
expression nodes with a simple DFS.

This currently only changes the way that Expressions are traversed. We
may need to do the same for statements.

Bug: chromium:1246375
Change-Id: Ie81905da1b790b6dd1df9f1ac42e06593d397c21
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/63700
Auto-Submit: Ben Clayton <bclayton@google.com>
Reviewed-by: David Neto <dneto@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/utils/reverse.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_REVERSE_H_
+#define SRC_UTILS_REVERSE_H_
+
+#include <iterator>
+
+namespace tint {
+namespace utils {
+
+namespace detail {
+/// Used by utils::Reverse to hold the underlying iterable.
+/// begin(ReverseIterable<T>) and end(ReverseIterable<T>) are automatically
+/// called for range-for loops, via argument-dependent lookup.
+/// See https://en.cppreference.com/w/cpp/language/range-for
+template <typename T>
+struct ReverseIterable {
+  /// The wrapped iterable object.
+  T& iterable;
+};
+
+template <typename T>
+auto begin(ReverseIterable<T> r_it) {
+  return std::rbegin(r_it.iterable);
+}
+
+template <typename T>
+auto end(ReverseIterable<T> r_it) {
+  return std::rend(r_it.iterable);
+}
+}  // namespace detail
+
+/// Reverse returns an iterable wrapper that when used in range-for loops,
+/// performs a reverse iteration over the object `iterable`.
+/// Example:
+/// ```
+/// /* Equivalent to:
+///  * for (auto it = vec.rbegin(); i != vec.rend(); ++it) {
+///  *   auto v = *it;
+///  */
+/// for (auto v : utils::Reverse(vec)) {
+/// }
+/// ```
+template <typename T>
+detail::ReverseIterable<T> Reverse(T&& iterable) {
+  return {iterable};
+}
+
+}  // namespace utils
+}  // namespace tint
+
+#endif  // SRC_UTILS_REVERSE_H_

src/utils/reverse_test.cc

@@ -0,0 +1,36 @@
+// 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/reverse.h"
+
+#include <vector>
+
+#include "gmock/gmock.h"
+
+namespace tint {
+namespace utils {
+namespace {
+
+TEST(ReverseTest, Vector) {
+  std::vector<int> vec{1, 3, 5, 7, 9};
+  std::vector<int> rev;
+  for (auto v : Reverse(vec)) {
+    rev.emplace_back(v);
+  }
+  ASSERT_THAT(rev, testing::ElementsAre(9, 7, 5, 3, 1));
+}
+
+}  // namespace
+}  // namespace utils
+}  // namespace tint