[Rust,front-end,v4,24/46] gccrs: Add implementation of Optional

diff --git a/gcc/rust/util/rust-optional-test.cc b/gcc/rust/util/rust-optional-test.cc
new file mode 100644
index 00000000000..43fa175fa40
--- /dev/null
+++ b/gcc/rust/util/rust-optional-test.cc
@@ -0,0 +1,110 @@ 
+// Copyright (C) 2020-2022 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/>.
+
+#include "rust-system.h"
+#include "rust-optional.h"
+#include "selftest.h"
+
+#if CHECKING_P
+
+static void
+rust_optional_create ()
+{
+  auto opt = Rust::Optional<int>::some (15);
+
+  ASSERT_TRUE (opt.is_some ());
+  ASSERT_EQ (opt.get (), 15);
+
+  Rust::Optional<int> const_opt = Rust::Optional<int>::some (15);
+  const int &value = const_opt.get ();
+
+  ASSERT_EQ (value, 15);
+}
+
+static void
+rust_optional_operators ()
+{
+  auto opt = Rust::Optional<int>::some (15);
+
+  // as bool
+  ASSERT_TRUE (opt);
+
+  // deref
+  ASSERT_EQ (*opt, 15);
+
+  class Methodable
+  {
+  public:
+    int method () { return 15; }
+  };
+
+  auto m_opt = Rust::Optional<Methodable>::some (Methodable ());
+  ASSERT_EQ (m_opt->method (), 15);
+}
+
+static void
+rust_optional_take ()
+{
+  auto opt = Rust::Optional<int>::some (15);
+  auto value = opt.take ();
+
+  ASSERT_EQ (value, 15);
+  ASSERT_TRUE (opt.is_none ());
+}
+
+static void
+rust_optional_map ()
+{
+  auto opt = Rust::Optional<int>::some (15);
+  auto twice = opt.map<int> ([] (int value) { return value * 2; });
+
+  ASSERT_FALSE (opt);
+  ASSERT_TRUE (twice);
+  ASSERT_EQ (*twice, 30);
+}
+
+static void
+rust_optional_reference ()
+{
+  auto value = std::vector<std::string> ();
+  value.emplace_back ("rust");
+  value.emplace_back ("+");
+  value.emplace_back ("gcc");
+  value.emplace_back ("=");
+  value.emplace_back ("<3");
+
+  auto opt = Rust::Optional<std::vector<std::string> &>::some (value);
+
+  ASSERT_EQ (opt->at (0), "rust");
+  ASSERT_EQ (opt->at (2), "gcc");
+}
+
+#endif /* #if CHECKING_P */
+
+void
+rust_optional_test ()
+{
+#if CHECKING_P
+  rust_optional_create ();
+  rust_optional_operators ();
+  rust_optional_take ();
+  rust_optional_map ();
+  rust_optional_reference ();
+
+#endif /* #if CHECKING_P */
+}
diff --git a/gcc/rust/util/rust-optional.h b/gcc/rust/util/rust-optional.h
new file mode 100644
index 00000000000..56465400250
--- /dev/null
+++ b/gcc/rust/util/rust-optional.h
@@ -0,0 +1,278 @@ 
+// Copyright (C) 2020-2022 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/>.
+
+#ifndef RUST_OPTIONAL_H
+#define RUST_OPTIONAL_H
+
+#include "config.h"
+#include "rust-system.h"
+
+#include "selftest.h"
+
+namespace Rust {
+
+/**
+ * Tagged union to try and simulate a sum type. This is safer and more ergonomic
+ * than one of the two alternatives we're currently using in the compiler:
+ *
+ * 1. Storing a raw pointer, which can be `nullptr` or valid
+ *
+ * This is wildly unsafe, and usable in conjunction with local references, stack
+ * variables, or pointers managed elsewhere, which can cause crashes, hard to
+ * debug issues or undefined behavior. Likewise, if you do not check for the
+ * pointer's validity, this will cause a crash.
+ *
+ * 2. Storing an extra boolean alongside the object
+ *
+ * This causes implementors to use a "dummy object": Either an empty version or
+ * an error version. But what happens if what you really wanted to store was
+ * the empty or error version? You can also easily incorporate logic bugs if you
+ * forget to check for the associated boolean.
+ *
+ * The `Optional<T>` type has the same "ergonomic" cost: You need to check
+ * whether your option is valid or not. However, the main advantage is that it
+ * is more restrictive: You can only acess the member it contains "safely".
+ * It is similar to storing a value + an associated boolean, but has the
+ * advantage of making up only one member in your class.
+ * You also benefit from some helper methods such as `map()`.
+ *
+ * You also get helper functions and operator overloading to "seamlessly"
+ * replace raw pointer alternatives.
+ *
+ * ```c++
+ * MyType *raw_pointer = something_that_can_fail();
+ * if (raw_pointer)
+ *     raw_pointer->method();
+ *
+ * // or
+ *
+ * Optional<MyType> opt = something_that_can_fail2();
+ * if (opt)
+ *     opt->method();
+ *
+ * // equivalent to
+ *
+ * if (opt.is_some())
+ *     opt.get().method();
+ * ```
+ */
+template <typename T> class Optional
+{
+private:
+  struct Empty
+  {
+  };
+
+  enum Kind
+  {
+    Some,
+    None
+  } kind;
+
+  union Content
+  {
+    Empty empty;
+    T value;
+
+    Content () = default;
+  } content;
+
+  Optional<T> (Kind kind, Content content) : kind (kind), content (content) {}
+
+public:
+  Optional (const Optional &other) = default;
+  Optional &operator= (const Optional &other) = default;
+  Optional (Optional &&other) = default;
+
+  static Optional<T> some (T value)
+  {
+    Content content;
+    content.value = value;
+
+    return Optional (Kind::Some, content);
+  }
+
+  static Optional<T> none ()
+  {
+    Content content;
+    content.empty = Empty ();
+
+    return Optional (Kind::None, content);
+  }
+
+  bool is_some () const { return kind == Kind::Some; }
+  bool is_none () const { return !is_some (); }
+
+  /**
+   * Enable boolean-like comparisons.
+   */
+  operator bool () { return is_some (); }
+
+  /**
+   * Enables dereferencing to access the contained value
+   */
+  T &operator* () { return get (); }
+  const T &operator* () const { return get (); }
+  T *operator-> () { return &get (); }
+  const T *operator-> () const { return &get (); }
+
+  const T &get () const
+  {
+    rust_assert (is_some ());
+
+    return content.value;
+  }
+
+  T &get ()
+  {
+    rust_assert (is_some ());
+
+    return content.value;
+  }
+
+  T take ()
+  {
+    rust_assert (is_some ());
+
+    auto to_return = std::move (content.value);
+
+    content.empty = Empty ();
+    kind = Kind::None;
+
+    return to_return;
+  }
+
+  template <typename U> Optional<U> map (std::function<U (T)> functor)
+  {
+    if (is_none ())
+      return Optional::none ();
+
+    auto value = functor (take ());
+
+    return Optional::some (value);
+  }
+};
+
+template <typename T> class Optional<T &>
+{
+private:
+  struct Empty
+  {
+  };
+
+  enum Kind
+  {
+    Some,
+    None
+  } kind;
+
+  union Content
+  {
+    Empty empty;
+    T *value;
+
+    Content () = default;
+  } content;
+
+  Optional<T &> (Kind kind, Content content) : kind (kind), content (content) {}
+
+public:
+  Optional (const Optional &other) = default;
+  Optional (Optional &&other) = default;
+
+  static Optional<T &> some (T &value)
+  {
+    Content content;
+    content.value = &value;
+
+    return Optional (Kind::Some, content);
+  }
+
+  static Optional<T &> none ()
+  {
+    Content content;
+    content.empty = Empty ();
+
+    return Optional (Kind::None, content);
+  }
+
+  bool is_some () const { return kind == Kind::Some; }
+  bool is_none () const { return !is_some (); }
+
+  // FIXME: Can we factor this in a single class?
+
+  /**
+   * Enable boolean-like comparisons.
+   */
+  operator bool () { return is_some (); }
+
+  /**
+   * Enables dereferencing to access the contained value
+   */
+  T &operator* () { return get (); }
+  const T &operator* () const { return get (); }
+  T *operator-> () { return &get (); }
+  const T *operator-> () const { return &get (); }
+
+  const T &get () const
+  {
+    rust_assert (is_some ());
+
+    return *content.value;
+  }
+
+  T &get ()
+  {
+    rust_assert (is_some ());
+
+    return *content.value;
+  }
+
+  T &take ()
+  {
+    rust_assert (is_some ());
+
+    auto to_return = std::move (content.value);
+
+    content.empty = Empty ();
+    kind = Kind::None;
+
+    return *to_return;
+  }
+
+  template <typename U> Optional<U &> map (std::function<U &(T &)> functor)
+  {
+    if (is_none ())
+      return Optional::none ();
+
+    auto value = functor (take ());
+
+    return Optional::some (value);
+  }
+};
+
+} // namespace Rust
+
+#ifdef CHECKING_P
+
+void
+rust_optional_test ();
+
+#endif // !CHECKING_P
+
+#endif // !RUST_OPTIONAL_H