[gcc/devel/rust/master] util: Add Optional<T> wrapper class

[gcc/devel/rust/master] util: Add Optional<T> wrapper class

[Thomas Schwinge]

https://gcc.gnu.org/g:b088d47cdc1514d5f92801481cbb412d1e01aeeb

commit b088d47cdc1514d5f92801481cbb412d1e01aeeb
Author: Arthur Cohen <arthur.cohen@embecosm.com>
Date:   Thu May 5 15:18:08 2022 +0200

    util: Add Optional<T> wrapper class
    
    The class provides an easy to use alternative to raw pointers or member
    + bool pairs. It should be easy to extend and improve upon.

Diff:
---
 gcc/rust/Make-lang.in            |   3 +-
 gcc/rust/parse/rust-parse-impl.h |   2 +-
 gcc/rust/rust-lang.cc            |   2 +
 gcc/rust/util/rust-make-unique.h |   6 +-
 gcc/rust/util/rust-optional.h    | 241 +++++++++++++++++++++++++++++++++++++++
 5 files changed, 250 insertions(+), 4 deletions(-)

diff --git a/gcc/rust/Make-lang.in b/gcc/rust/Make-lang.in
index ae385e0bec9..89ab0c8e1bd 100644
--- a/gcc/rust/Make-lang.in
+++ b/gcc/rust/Make-lang.in
@@ -289,7 +289,8 @@ RUST_INCLUDES = -I $(srcdir)/rust \
 	-I $(srcdir)/rust/util \
 	-I $(srcdir)/rust/typecheck \
 	-I $(srcdir)/rust/privacy \
-	-I $(srcdir)/rust/lint
+	-I $(srcdir)/rust/lint \
+	-I $(srcdir)/rust/util
 
 # add files that require cross-folder includes - currently rust-lang.o, rust-lex.o
 CFLAGS-rust/rust-lang.o += $(RUST_INCLUDES)
diff --git a/gcc/rust/parse/rust-parse-impl.h b/gcc/rust/parse/rust-parse-impl.h
index 23ab32c832e..f74c66125b8 100644
--- a/gcc/rust/parse/rust-parse-impl.h
+++ b/gcc/rust/parse/rust-parse-impl.h
@@ -22,7 +22,7 @@ along with GCC; see the file COPYING3.  If not see
 
 #define INCLUDE_ALGORITHM
 #include "rust-diagnostics.h"
-#include "util/rust-make-unique.h"
+#include "rust-make-unique.h"
 
 namespace Rust {
 // Left binding powers of operations.
diff --git a/gcc/rust/rust-lang.cc b/gcc/rust/rust-lang.cc
index 73f9839ee3e..dd8c608789a 100644
--- a/gcc/rust/rust-lang.cc
+++ b/gcc/rust/rust-lang.cc
@@ -37,6 +37,7 @@
 #include "rust-cfg-parser.h"
 #include "rust-privacy-ctx.h"
 #include "rust-ast-resolve-item.h"
+#include "rust-optional.h"
 
 #include <mpfr.h>
 // note: header files must be in this order or else forward declarations don't
@@ -461,6 +462,7 @@ run_rust_tests ()
   rust_privacy_ctx_test ();
   rust_crate_name_validation_test ();
   rust_simple_path_resolve_test ();
+  rust_optional_test ();
 }
 } // namespace selftest
 
diff --git a/gcc/rust/util/rust-make-unique.h b/gcc/rust/util/rust-make-unique.h
index 77b203ee1c9..f33f9125f89 100644
--- a/gcc/rust/util/rust-make-unique.h
+++ b/gcc/rust/util/rust-make-unique.h
@@ -24,8 +24,10 @@
 namespace Rust {
 
 template <typename T, typename... Ts>
-std::unique_ptr<T> make_unique(Ts &&...params) {
-  return std::unique_ptr<T>(new T(std::forward<Ts>(params)...));
+std::unique_ptr<T>
+make_unique (Ts &&... params)
+{
+  return std::unique_ptr<T> (new T (std::forward<Ts> (params)...));
 }
 
 } // namespace Rust
diff --git a/gcc/rust/util/rust-optional.h b/gcc/rust/util/rust-optional.h
new file mode 100644
index 00000000000..c1b547a29d6
--- /dev/null
+++ b/gcc/rust/util/rust-optional.h
@@ -0,0 +1,241 @@
+// 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 (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);
+  }
+};
+
+} // namespace Rust
+
+#ifdef 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_test ()
+{
+  rust_optional_create ();
+  rust_optional_operators ();
+  rust_optional_take ();
+  rust_optional_map ();
+}
+
+#endif // !CHECKING_P
+
+#endif // !RUST_OPTIONAL_H