gccrs: Implement the inline visitor

gcc/rust/ChangeLog:

	* util/rust-inline-visitor.h: New file.

gcc/rust/util/rust-inline-visitor.h

+// Copyright (C) 2021-2022 Free Software Foundation, Inc.
+
+// This file is part of GCC.
+
+// GCC is free software; you can redistribute it and/or modify it under
+// the terms of the GNU General Public License as published by the Free
+// Software Foundation; either version 3, or (at your option) any later
+// version.
+
+// GCC is distributed in the hope that it will be useful, but WITHOUT ANY
+// WARRANTY; without even the implied warranty of MERCHANTABILITY or
+// FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+// for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with GCC; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// An improved implementation of the inline visitor.
+// Original idea from https://members.accu.org/index.php/articles/2021
+
+#ifndef RUST_INLINE_VISITOR
+#define RUST_INLINE_VISITOR
+
+#include <utility>
+#include <type_traits>
+
+namespace Rust {
+
+// Wrapper for the target Visitor we're matching against.
+// Consumes the final nullptr of the _args linked list.
+template <typename TargetVisitor> struct EmptyVisitor : TargetVisitor
+{
+  EmptyVisitor (std::nullptr_t ptr) {}
+
+  using TargetVisitor::visit;
+};
+
+// Wrapper for a (possibly incomplete) Visitor.
+template <typename BaseVisitor, typename Args> struct VisitorWrapper
+{
+  // Lambdas are stored in _args as a linked list and passed to the actual
+  // visitor when end_visitor() is called.
+  Args _args;
+
+  // The actual visitor being created.
+  // Each visitor inherits from the last one and implements one more visit().
+  template <typename T, typename F> struct Visitor : BaseVisitor
+  {
+    F _f;
+
+    Visitor (std::pair<F, Args> &&args)
+      : BaseVisitor (std::move (args.second)), _f (std::move (args.first))
+    {}
+
+    using BaseVisitor::visit;
+    virtual void visit (T &t) final override { _f (t); }
+  };
+
+  VisitorWrapper (Args &&args) : _args (std::move (args)) {}
+
+  // Add another visit() method to the visitor.
+  // _args will be moved over, so don't keep the old wrapper around.
+  template <typename T, typename F>
+  VisitorWrapper<Visitor<T, F>, std::pair<F, Args>> on (F &&f)
+  {
+    return VisitorWrapper<Visitor<T, F>, std::pair<F, Args>> (
+      std::make_pair (std::move (f), std::move (_args)));
+  }
+
+  // Returns the finished visitor.
+  // NOTE: The reference implementation has a bug that exposes this method even
+  // when BaseVisitor is still an abstract class. The C++11 standard states that
+  // "An abstract class shall not be used [...] as a function return type". GCC
+  // rejects the buggy code as expected, but Clang accepts the code as long as
+  // the method is not actually called. Maybe this is a bug in Clang?
+  template <typename T = BaseVisitor>
+  typename std::enable_if<std::is_constructible<T, Args>::value, T>::type
+  end_visitor ()
+  {
+    return T (std::move (_args));
+  }
+};
+
+// The entry point.
+template <typename TargetVisitor>
+VisitorWrapper<EmptyVisitor<TargetVisitor>, std::nullptr_t>
+begin_visitor ()
+{
+  return VisitorWrapper<EmptyVisitor<TargetVisitor>, std::nullptr_t> (nullptr);
+}
+
+} // namespace Rust
+
+#endif