Squashed 'third_party/immer/' content from commit ad3e3556d

git-subtree-dir: third_party/immer
git-subtree-split: ad3e3556d38bb75966dd24c61a774970a7c7957e

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diff --git a/immer/heap/thread_local_free_list_heap.hpp b/immer/heap/thread_local_free_list_heap.hpp
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+//
+// immer: immutable data structures for C++
+// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
+//
+// This software is distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
+//
+
+#pragma once
+
+#include <immer/heap/unsafe_free_list_heap.hpp>
+
+namespace immer {
+namespace detail {
+
+template <typename Heap>
+struct thread_local_free_list_storage
+{
+    struct head_t
+    {
+        free_list_node* data;
+        std::size_t count;
+
+        ~head_t() { Heap::clear(); }
+    };
+
+    static head_t& head()
+    {
+        thread_local static head_t head_{nullptr, 0};
+        return head_;
+    }
+};
+
+} // namespace detail
+
+/*!
+ * Adaptor that does not release the memory to the parent heap but
+ * instead it keeps the memory in a `thread_local` global free
+ * list. Must be preceded by a `with_data<free_list_node, ...>` heap
+ * adaptor.  When the current thread finishes, the memory is returned
+ * to the parent heap.
+ *
+ * @tparam Size  Maximum size of the objects to be allocated.
+ * @tparam Limit Maximum number of elements to keep in the free list.
+ * @tparam Base  Type of the parent heap.
+ */
+template <std::size_t Size, std::size_t Limit, typename Base>
+struct thread_local_free_list_heap
+    : detail::unsafe_free_list_heap_impl<detail::thread_local_free_list_storage,
+                                         Size,
+                                         Limit,
+                                         Base>
+{};
+
+} // namespace immer