type::Manager: Add Wrap()

Wrap returns a new Manager created with the types of `inner`.

The Manager returned by Wrap is intended to temporarily extend the types of an existing immutable Manager.

Bug: tint:390
Change-Id: I46bebf8b83cf7987ddcf2513c54f9f885a028c60
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/38551
Reviewed-by: dan sinclair <dsinclair@chromium.org>

src/type/type_manager.h

@@ -63,6 +63,19 @@ class Manager {
     return type;
   }
 
+  /// Wrap returns a new Manager created with the types of `inner`.
+  /// The Manager returned by Wrap is intended to temporarily extend the types
+  /// of an existing immutable Manager.
+  /// As the copied types are owned by `inner`, `inner` must not be destructed
+  /// or assigned while using the returned Manager.
+  /// @param inner the immutable Manager to extend
+  /// @return the Manager that wraps `inner`
+  static Manager Wrap(const Manager& inner) {
+    Manager out;
+    out.by_name_ = inner.by_name_;
+    return out;
+  }
+
   /// Returns the type map
   /// @returns the mapping from name string to type.
   const std::unordered_map<std::string, type::Type*>& types() const {

src/type/type_manager_test.cc

@@ -22,6 +22,16 @@ namespace tint {
 namespace type {
 namespace {
 
+template <typename T>
+size_t count(const T& range_loopable) {
+  size_t n = 0;
+  for (auto it : range_loopable) {
+    (void)it;
+    n++;
+  }
+  return n;
+}
+
 using TypeManagerTest = testing::Test;
 
 TEST_F(TypeManagerTest, GetUnregistered) {
@@ -53,6 +63,21 @@ TEST_F(TypeManagerTest, GetDifferentTypeReturnsDifferentPtr) {
   EXPECT_TRUE(t2->Is<U32>());
 }
 
+TEST_F(TypeManagerTest, WrapDoesntAffectInner) {
+  Manager inner;
+  Manager outer = Manager::Wrap(inner);
+
+  inner.Get<I32>();
+
+  EXPECT_EQ(count(inner), 1u);
+  EXPECT_EQ(count(outer), 0u);
+
+  outer.Get<U32>();
+
+  EXPECT_EQ(count(inner), 1u);
+  EXPECT_EQ(count(outer), 1u);
+}
+
 }  // namespace
 }  // namespace type
 }  // namespace tint