Make FunctionCallAnnotation::kind a SetOnce

libsolutil/SetOnce.h

@@ -0,0 +1,86 @@
+/*
+	This file is part of solidity.
+
+	solidity 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 of the License, or
+	(at your option) any later version.
+
+	solidity 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 solidity.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#pragma once
+
+#include <libsolutil/Assertions.h>
+#include <libsolutil/Exceptions.h>
+
+#include <memory>
+#include <optional>
+#include <utility>
+
+namespace solidity::util
+{
+
+DEV_SIMPLE_EXCEPTION(BadSetOnceReassignment);
+DEV_SIMPLE_EXCEPTION(BadSetOnceAccess);
+
+/// A class that stores a value that can only be set once
+/// \tparam T the type of the stored value
+template<typename T>
+class SetOnce
+{
+public:
+	/// Initializes the class to have no stored value.
+	SetOnce() = default;
+
+	// Not copiable
+	SetOnce(SetOnce const&) = delete;
+	SetOnce(SetOnce&&) = delete;
+
+	// Not movable
+	SetOnce& operator=(SetOnce const&) = delete;
+	SetOnce& operator=(SetOnce&&) = delete;
+
+	/// @brief Sets the stored value to \p _newValue
+	/// @throws BadSetOnceReassignment when the stored value has already been set
+	/// @return `*this`
+	constexpr SetOnce& operator=(T _newValue) &
+	{
+		assertThrow(
+			!m_value.has_value(),
+			BadSetOnceReassignment,
+			"Attempt to reassign to a SetOnce that already has a value."
+		);
+
+		m_value.emplace(std::move(_newValue));
+		return *this;
+	}
+
+	/// @return A reference to the stored value. The returned reference has the same lifetime as `*this`.
+	/// @throws BadSetOnceAccess when the stored value has not yet been set
+	T const& operator*() const
+	{
+		assertThrow(
+			m_value.has_value(),
+			BadSetOnceAccess,
+			"Attempt to access the value of a SetOnce that does not have a value."
+		);
+
+		return m_value.value();
+	}
+
+	/// @return A reference to the stored value. The referent of the returned pointer has the same lifetime as `*this`.
+	/// @throws BadSetOnceAccess when the stored value has not yet been set
+	T const* operator->() const { return std::addressof(**this); }
+
+private:
+	std::optional<T> m_value = std::nullopt;
+};
+
+}