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`oxc_allocator::Vec` is intended for storing AST types in the arena. `Vec` is intended to be non-drop, because all AST types are non-drop. If they were not, it would be a memory leak, because those types will not have `drop` called on them when the allocator is dropped. However, `oxc_allocator::Vec` is currently a wrapper around `allocator_api2::vec::Vec`, which *is* drop. That unintentionally makes `oxc_allocator::Vec` drop too. This PR fixes this by wrapping the inner `allocator_api2::vec::Vec` in `ManuallyDrop`. This makes `oxc_allocator::Vec` non-drop. The wider consequence of this change is that the compiler is now able to see that loads of other types which contain `oxc_allocator::Vec` are also non-drop. Once it can prove that, it can remove a load of code which handles dropping these types in the event of a panic. This probably also then allows it to make many further optimizations on that simplified code. Strictly speaking, this PR is a breaking change. If `oxc_allocator::Vec` is abused to store drop types, then in some circumstances this change could produce a memory leak where there was none before. However, we've always been clear that only non-drop types should be stored in the arena, so such usage was always a bug. #6622 fixes the only place in Oxc where we mistakenly stored non-drop types in `Vec`. The change to `oxc_prettier` is because compiler can now deduce that `Doc` is non-drop, which causes clippy to raise a warning about using `then` instead of `then_some`. As follow-up, we should: 1. Wrap other `allocator_api2` types (e.g. `IntoIter`) in `ManuallyDrop` too, so compiler can prove they are non-drop too (or reimplement `Vec` ourselves - #6488). 2. Introduce static checks to prevent non-drop types being used in `Box` and `Vec`, to make memory leaks impossible, and detect them at compile time. |
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| CHANGELOG.md | ||