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cca3373706
Eliminate ZST allocations in `Box` and `Vec` This PR fixes 2 issues with `Box` and `RawVec` related to ZST allocations. Specifically, the `Allocator` trait requires that: - If you allocate a zero-sized layout then you must later deallocate it, otherwise the allocator may leak memory. - You cannot pass a ZST pointer to the allocator that you haven't previously allocated. These restrictions exist because an allocator implementation is allowed to allocate non-zero amounts of memory for a zero-sized allocation. For example, `malloc` in libc does this. Currently, ZSTs are handled differently in `Box` and `Vec`: - `Vec` never allocates when `T` is a ZST or if the vector capacity is 0. - `Box` just blindly passes everything on to the allocator, including ZSTs. This causes problems due to the free conversions between `Box<[T]>` and `Vec<T>`, specifically that ZST allocations could get leaked or a dangling pointer could be passed to `deallocate`. This PR fixes this by changing `Box` to not allocate for zero-sized values and slices. It also fixes a bug in `RawVec::shrink` where shrinking to a size of zero did not actually free the backing memory. |
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