Bug 1345842 - Add Ignore to allow ignore elements in Tie. r=froydnj

Differential Revision: https://phabricator.services.mozilla.com/D68742

--HG--
extra : moz-landing-system : lando

mfbt/Tuple.h

@@ -67,6 +67,19 @@ struct CheckConvertibility<Group<SourceTypes...>, Group<TargetTypes...>>
                               sizeof...(SourceTypes) ==
                                   sizeof...(TargetTypes)> {};
 
+/*
+ * Helper type for Tie(args...) to allow ignoring specific elements
+ * during Tie unpacking.  Supports assignment from any type.
+ *
+ * Not for direct usage; instead, use mozilla::Ignore in calls to Tie.
+ */
+struct IgnoreImpl {
+  template <typename T>
+  constexpr const IgnoreImpl& operator=(const T&) const {
+    return *this;
+  }
+};
+
 /*
  * TupleImpl is a helper class used to implement mozilla::Tuple.
  * It represents one node in a recursive inheritance hierarchy.
@@ -463,6 +476,22 @@ inline Tuple<std::decay_t<Elements>...> MakeTuple(Elements&&... aElements) {
   return Tuple<std::decay_t<Elements>...>(std::forward<Elements>(aElements)...);
 }
 
+/**
+ * A helper placholder to allow ignoring specific elements during Tie unpacking.
+ * Can be used with any type and any number of elements in a call to Tie.
+ *
+ * Usage of Ignore with Tie is equivalent to using std::ignore with
+ * std::tie.
+ *
+ * Example:
+ *
+ * int i;
+ * float f;
+ * char c;
+ * Tie(i, Ignore, f, c, Ignore) = FunctionThatReturnsATuple();
+ */
+constexpr detail::IgnoreImpl Ignore;
+
 /**
  * A convenience function for constructing a tuple of references to a
  * sequence of variables. Since assignments to the elements of the tuple

mfbt/tests/TestTuple.cpp

@@ -281,6 +281,19 @@ static bool TestTie() {
   return true;
 }
 
+static bool TestTieIgnore() {
+  int i;
+  char c;
+  Tuple<int, float, char> rhs1(42, 0.5f, 'c');
+
+  Tie(i, mozilla::Ignore, c) = rhs1;
+
+  CHECK(i == Get<0>(rhs1));
+  CHECK(c == Get<2>(rhs1));
+
+  return true;
+}
+
 int main() {
   TestConstruction();
   TestConstructionFromMozPair();
@@ -291,6 +304,7 @@ int main() {
   TestGet();
   TestMakeTuple();
   TestTie();
+  TestTieIgnore();
   TestTieMozPair();
   return 0;
 }