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Use EMPTY constant of removed const_in_array_repeat_expressions feature

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Philipp Oppermann
2021-02-02 11:50:35 +01:00
parent 6d648457b1
commit 563689dba5
1 changed files with 3 additions and 2 deletions
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5
blog/content/edition-2/posts/11-allocator-designs/index.md
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@@ -948,8 +948,9 @@ For constructing a `FixedSizeBlockAllocator`, we provide the same `new` and `ini
impl FixedSizeBlockAllocator {
/// Creates an empty FixedSizeBlockAllocator.
pub const fn new() -> Self {
const EMPTY: Option<&'static mut ListNode> = None;
FixedSizeBlockAllocator {
list_heads: [None; BLOCK_SIZES.len()],
list_heads: [EMPTY; BLOCK_SIZES.len()],
fallback_allocator: linked_list_allocator::Heap::empty(),
}
}
@@ -965,7 +966,7 @@ impl FixedSizeBlockAllocator {
}
```
The `new` function just initializes the `list_heads` array with empty nodes and creates an [`empty`] linked list allocator as `fallback_allocator`. Since array initializations using non-`Copy` types are still unstable, we need to add **`#![feature(const_in_array_repeat_expressions)]`** to the beginning of our `lib.rs`. The reason that `None` is not `Copy` in this case is that `ListNode` does not implement `Copy`. Thus, the `Option` wrapper and its `None` variant are not `Copy` either.
The `new` function just initializes the `list_heads` array with empty nodes and creates an [`empty`] linked list allocator as `fallback_allocator`. The `EMPTY` constant is needed because to tell the Rust compiler that we want to initialize the array with a constant value. Initializing the array directly as `[None; BLOCK_SIZES.len()]` does not work because then the compiler requires that `Option<&'static mut ListNode>` implements the `Copy` trait, which is does not. This is a current limitation of the Rust compiler, which might go away in the future.
[`empty`]: https://docs.rs/linked_list_allocator/0.8.0/linked_list_allocator/struct.Heap.html#method.empty