gcc/rust/util/rust-stacked-contexts.h

   1 // Copyright (C) 2020-2024 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 #ifndef RUST_CONTEXT_STACK_H
  20 #define RUST_CONTEXT_STACK_H
  21
  22 #include "rust-system.h"
  23
  24 namespace Rust {
  25
  26 /**
  27  * Context stack util class. This class is useful for situations where you can
  28  * enter the same kind of context multiple times. For example, when dealing with
  29  * unsafe contexts, you might be tempted to simply keep a boolean value.
  30  *
  31  * ```rust
  32  * let a = 15;
  33  * unsafe { // we set the boolean to true
  34  *     // Now unsafe operations are allowed!
  35  *     let b = *(&a as *const i32);
  36  *     let c = std::mem::transmute<i32, f32>(b); // Urgh!
  37  * } // we set it to false
  38  * ```
  39  *
  40  * However, since the language allows nested unsafe blocks, you may run into
  41  * this situation:
  42  *
  43  * ```rust
  44  * unsafe { // we set the boolean to true
  45  *     unsafe { // we set the boolean to true
  46  *     } // we set it to false
  47  *
  48  *     // Now unsafe operations are forbidden again, the boolean is false
  49  *     let f = std::mem::transmute<i32, f32>(15); // Error!
  50  * } // we set it to false
  51  * ```
  52  */
  53 template <typename T> class StackedContexts
  54 {
  55 public:
  56   /**
  57    * Enter a special context
  58    */
  59   void enter (T value) { stack.emplace_back (value); }
  60
  61   /**
  62    * Exit a special context
  63    */
  64   T exit ()
  65   {
  66     rust_assert (!stack.empty ());
  67
  68     auto last = stack.back ();
  69     stack.pop_back ();
  70
  71     return last;
  72   }
  73
  74   /**
  75    * Are we currently inside of a special context?
  76    */
  77   bool is_in_context () const { return !stack.empty (); }
  78
  79 private:
  80   /* Actual data */
  81   std::vector<T> stack;
  82 };
  83
  84 } // namespace Rust
  85
  86 #endif /* !RUST_CONTEXT_STACK_H */