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

   1 // Copyright (C) 2021-2024 Free Software Foundation, Inc.
   2
   3 // This file is part of GCC.
   4
   5 // GCC is free software; you can redistribute it and/or modify it under
   6 // the terms of the GNU General Public License as published by the Free
   7 // Software Foundation; either version 3, or (at your option) any later
   8 // version.
   9
  10 // GCC is distributed in the hope that it will be useful, but WITHOUT ANY
  11 // WARRANTY; without even the implied warranty of MERCHANTABILITY or
  12 // FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  13 // for more details.
  14
  15 // You should have received a copy of the GNU General Public License
  16 // along with GCC; see the file COPYING3.  If not see
  17 // <http://www.gnu.org/licenses/>.
  18
  19 // An improved implementation of the inline visitor.
  20 // Original idea from https://members.accu.org/index.php/articles/2021
  21
  22 #ifndef RUST_INLINE_VISITOR
  23 #define RUST_INLINE_VISITOR
  24
  25 namespace Rust {
  26
  27 // Wrapper for the target Visitor we're matching against.
  28 // Consumes the final nullptr of the _args linked list.
  29 template <typename TargetVisitor> struct EmptyVisitor : TargetVisitor
  30 {
  31   EmptyVisitor (std::nullptr_t ptr) {}
  32
  33   using TargetVisitor::visit;
  34 };
  35
  36 // Wrapper for a (possibly incomplete) Visitor.
  37 template <typename BaseVisitor, typename Args> struct VisitorWrapper
  38 {
  39   // Lambdas are stored in _args as a linked list and passed to the actual
  40   // visitor when end_visitor() is called.
  41   Args _args;
  42
  43   // The actual visitor being created.
  44   // Each visitor inherits from the last one and implements one more visit().
  45   template <typename T, typename F> struct Visitor : BaseVisitor
  46   {
  47     F _f;
  48
  49     Visitor (std::pair<F, Args> &&args)
  50       : BaseVisitor (std::move (args.second)), _f (std::move (args.first))
  51     {}
  52
  53     using BaseVisitor::visit;
  54     virtual void visit (T &t) final override { _f (t); }
  55   };
  56
  57   VisitorWrapper (Args &&args) : _args (std::move (args)) {}
  58
  59   // Add another visit() method to the visitor.
  60   // _args will be moved over, so don't keep the old wrapper around.
  61   template <typename T, typename F>
  62   VisitorWrapper<Visitor<T, F>, std::pair<F, Args>> on (F &&f)
  63   {
  64     return VisitorWrapper<Visitor<T, F>, std::pair<F, Args>> (
  65       std::make_pair (std::move (f), std::move (_args)));
  66   }
  67
  68   // Returns the finished visitor.
  69   // NOTE: The reference implementation has a bug that exposes this method even
  70   // when BaseVisitor is still an abstract class. The C++11 standard states that
  71   // "An abstract class shall not be used [...] as a function return type". GCC
  72   // rejects the buggy code as expected, but Clang accepts the code as long as
  73   // the method is not actually called. Maybe this is a bug in Clang?
  74   template <typename T = BaseVisitor>
  75   typename std::enable_if<std::is_constructible<T, Args>::value, T>::type
  76   end_visitor ()
  77   {
  78     return T (std::move (_args));
  79   }
  80 };
  81
  82 // The entry point.
  83 template <typename TargetVisitor>
  84 VisitorWrapper<EmptyVisitor<TargetVisitor>, std::nullptr_t>
  85 begin_visitor ()
  86 {
  87   return VisitorWrapper<EmptyVisitor<TargetVisitor>, std::nullptr_t> (nullptr);
  88 }
  89
  90 } // namespace Rust
  91
  92 #endif