TransparentWrapper supports UnsafeCell variance #1024
Merged
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
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
reviewed
Mar 6, 2024
Comment on lines
+115
to
+116
| // 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`. |
Collaborator
There was a problem hiding this comment.
What's the distinction you're drawing here between ranges and offsets?
Member
Author
There was a problem hiding this comment.
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
approved these changes
Mar 6, 2024
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
2 participants
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
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).