Quick'n'dirty implementation of Rust's borrow checker for a C++Now Lightning Talk - not supposed to be used

borrow.cpp

#include <iostream>

#include "borrow_checker.hpp"

int main()
{
    auto i = 42;

    // borrow `i` under name `ref`
    borrow_var(ref, i)
    {
        // ++i; - error
        // *ref = 0; - error
        std::cout << *ref << ' ' << i << '\n';
    };
    
    // mutable borrow `i` under name `ref`
    mut_borrow_var(ref, i)
    {
        // auto var = i; - error
        *ref = 42;
    };

    ++i;
}

borrow_checker.hpp

#ifndef BORROW_CHECKER_HPP_INCLUDED
#define BORROW_CHECKER_HPP_INCLUDED

#include <initializer_list>
#include <new>
#include <type_traits>

namespace detail
{
    template <typename T>
    class ref
    {
    public:
        explicit ref(T& obj)
        : ptr_(&obj) {}

        T& operator*() const noexcept
        {
            return *ptr_;
        }

        T* operator->() const noexcept
        {
            return ptr_;
        }

    private:
        T* ptr_;
    };

    template <typename Object, typename Expression>
    struct borrow_lambda_invoker
    {
        Object& object; 
        Expression& expression;

        template <typename Lambda>
        auto call_lambda(int, const Lambda& l)
        -> decltype(l(object, ref<Expression>(expression)))
        {
            return l(object, ref<Expression>(expression));
        }

        template <typename Lambda>
        auto call_lambda(short, const Lambda& l)
        -> decltype(l({}, ref<Expression>(expression)))
        {
            return l({}, ref<Expression>(expression));
        }

        template <typename Lambda>
        void operator=(const Lambda& l)
        {
            call_lambda(0, l);
        }
    };

    struct error_variable_borrowed_as_mutable
    {
        error_variable_borrowed_as_mutable(const error_variable_borrowed_as_mutable&) = delete;
    };

    template <typename T>
    struct destructive_move_holder
    {
        std::aligned_storage_t<sizeof(T), alignof(T)> storage;
        bool should_destroy;

        template <typename ... Args>
        destructive_move_holder(Args&&... args)
        : should_destroy(true)
        {
            ::new(&storage) T(std::forward<Args>(args)...);
        }

        template <typename U>
        destructive_move_holder(std::initializer_list<U> list)
        : should_destroy(true)
        {
            ::new(&storage) T(std::move(list));
        }

        destructive_move_holder(const destructive_move_holder&) = delete;
        destructive_move_holder& operator=(const destructive_move_holder&) = delete;

        ~destructive_move_holder()
        {
            if (should_destroy)
                get().~T();
        }

        T& get()
        {
            void* mem = &storage;
            return *static_cast<T*>(mem);
        }

        T&& move()
        {
            should_destroy = false;
            return std::move(get());
        }
    };

    template <typename T>
    struct destructive_move_lambda_invoker
    {
        destructive_move_holder<T> holder;

        template <typename ... Args>
        destructive_move_lambda_invoker(Args&&... args)
        : holder(std::forward<Args>(args)...) {}

        template <typename U>
        destructive_move_lambda_invoker(std::initializer_list<U> list)
        : holder(std::move(list)) {}

        template <typename Lambda>
        void operator=(const Lambda& lambda)
        {
            lambda(holder, holder.get());
        }
    };
}

template <typename T>
using ref = detail::ref<const T>;

template <typename T>
using mut_ref = detail::ref<T>;

template <typename T>
ref<T> borrow(const T& obj)
{
    return ref<T>(obj);
}

template <typename T>
mut_ref<T> mut_borrow(T& obj)
{
    return mut_ref<T>(obj);
}

#define _borrow_lambda_invoker(Obj, ...) \
    detail::borrow_lambda_invoker<std::remove_reference_t<decltype(Obj)>, \
                          const std::remove_reference_t<decltype(__VA_ARGS__)>>{Obj, __VA_ARGS__}


#define _mut_borrow_lambda_invoker(Obj, ...) \
    detail::borrow_lambda_invoker<std::remove_reference_t<decltype(Obj)>, \
                          std::remove_reference_t<decltype(__VA_ARGS__)>>{Obj, __VA_ARGS__}

#define _borrow_lambda(Name, Obj) \
    [&]([[gnu::unused]] const auto& Obj, const auto& Name)

#define _mut_borrow_lambda(Name, Obj) \
    [&]([[gnu::unused]] const detail::error_variable_borrowed_as_mutable& Obj, const auto& Name)

#define _borrow(Name, Obj, ...) \
    _borrow_lambda_invoker(Obj, __VA_ARGS__) = _borrow_lambda(Name, Obj)

#define _mut_borrow(Name, Obj, ...) \
    _mut_borrow_lambda_invoker(Obj, __VA_ARGS__) = _mut_borrow_lambda(Name, Obj)

#define borrow_var(Name, Obj) _borrow(Name, Obj, Obj)
#define borrow_expr(Name, Obj, ...) _borrow(Name, Obj, __VA_ARGS__)
#define borrow_elem(Name, Obj, I) _borrow(Name, Obj, Obj[I])

#define mut_borrow_var(Name, Obj) _mut_borrow(Name, Obj, Obj)
#define mut_borrow_expr(Name, Obj, ...) _mut_borrow(Name, Obj, __VA_ARGS__)
#define mut_borrow_elem(Name, Obj, I) _mut_borrow(Name, Obj, Obj[I])

#define destructive_moveable(Type, Name, ...) \
    detail::destructive_move_lambda_invoker<Type>{__VA_ARGS__} \
        = [&]([[gnu::unused]] auto& _holder_##Name, auto& Name)

#define destructive_move(Name) \
    _holder_##Name.move()

#endif // BORROW_CHECKER_HPP_INCLUDED

borrow_func.cpp

#include <iostream>
#include <vector>

#include "borrow_checker.hpp"

void func_borrow(ref<std::vector<int>> vec)
{
    for (auto& el : *vec)
        std::cout << el << ' ';
    std::cout << '\n';
}

void func_mut_borrow(mut_ref<std::vector<int>> vec)
{
    vec->push_back(4);
}

void func_destructive_move(std::vector<int> vec)
{
    std::cout << "Now it's mine!\n";
}

int main()
{
    destructive_moveable(std::vector<int>, vec, {1, 2, 3})
    {
        vec.push_back(42);

        func_borrow(borrow(vec));
        func_mut_borrow(mut_borrow(vec));
        func_borrow(borrow(vec));

        func_destructive_move(destructive_move(vec));
    };
}

borrow_vec.cpp

#include <iostream>
#include <vector>

#include  "borrow_checker.hpp"

int main()
{
    std::vector<int> vec = {1, 2, 3};

    // borrow `vec[0]` as ref, locking `vec`
    borrow_expr(ref, vec, vec[0])
    {
        //*ref = 0; - error
        //vec.push_back(4); - error
        std::cout << vec.size() << ' ' << *ref << '\n';
    };

    vec.push_back(4);
}