TransparentWrapper supports UnsafeCell variance #1024
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This will allow us to implement `TransparentWrapper` for `T` where `T` and `T::Inner` do not have `UnsafeCell`s at the same byte ranges. This, in turn, will let us automatically implement traits for these types so long as those traits don't require reasoning about `UnsafeCell` ranges (in other words, all traits other than `NoCell`).
jswrenn
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Mar 6, 2024
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| // SAFETY: Per [1], `MaybeUninit<T>` has `UnsafeCell`s at the same byte | ||
| // ranges as `Inner = T`, and `UnsafeCell`s at the same byte offsets as `T`. |
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What's the distinction you're drawing here between ranges and offsets?
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I should perhaps say "covering" the same byte ranges. E.g., the type #[repr(C)] struct Foo(UnsafeCell<()>, usize) doesn't have any byte ranges which are covered by UnsafeCells, but there is a ZST UnsafeCell at byte offset 0. Until rust-lang/unsafe-code-guidelines#495 is resolved (and until we remove the restriction from NoCell that even ZST UnsafeCells are banned), this distinction is required for soundness.
jswrenn
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Mar 6, 2024
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This will allow us to implement
TransparentWrapperforTwhereTandT::Innerdo not haveUnsafeCells at the same byte ranges. This, in turn, will let us automatically implement traits for these types so long as those traits don't require reasoning aboutUnsafeCellranges (in other words, all traits other thanNoCell).