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Mention required #![feature(const_fn)] attribute again for FixedSizeBlockAllocator

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The post explicitly allows readers to skip the `LinkedListAllocator` implementation, so we should not rely that the reader already enabled the unstable `const_fn` function there.

Reported in https://github.com/phil-opp/blog_os/issues/720#issuecomment-672625377.
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Philipp Oppermann
2020-08-19 09:29:46 +02:00
parent 9b61e061a0
commit 10d84faa92
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blog/content/second-edition/posts/11-allocator-designs/index.md
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@@ -969,6 +969,8 @@ The `new` function just initializes the `list_heads` array with empty nodes and
[`empty`]: https://docs.rs/linked_list_allocator/0.6.4/linked_list_allocator/struct.Heap.html#method.empty
If you haven't done so already for the `LinkedListAllocator` implementation, you also need to add **`#![feature(const_fn)]`** to the beginning of your `lib.rs`. The reason is that any use of mutable reference types in const functions is still unstable, including the `Option<&'static mut ListNode>` array element type of the `list_heads` field (even if we set it to `None`).
The unsafe `init` function only calls the [`init`] function of the `fallback_allocator` without doing any additional initialization of the `list_heads` array. Instead, we will initialize the lists lazily on `alloc` and `dealloc` calls.
[`init`]: https://docs.rs/linked_list_allocator/0.6.4/linked_list_allocator/struct.Heap.html#method.init