[pointer] Relax UnsafeCell requirements #1211
Merged
Conversation
This file contains 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
Previously, we required there to always be "`UnsafeCell` agreement" between any `Ptr`s or references referencing a given region of memory. This was based on an overly-strict interpretation of the semantics of `UnsafeCell`. In this commit, we relax `Ptr` to only require "`UnsafeCell` agreement" when the aliasing model is `Shared`. All of the places that our internal invariants are "consumed" - ie, where we use them as safety preconditions for calling unsafe functions defined outside our crate - this relaxation is sufficient. This is based on what we (@jswrenn and I) believe to be a more accurate model of the semantics of `UnsafeCell`s. In particular, `UnsafeCell`s do not affect the semantics of loads or stores in Rust. All they do is affect the semantics of shared references. In particular, Rust assumes that the referent of a shared reference will not be stored to during the lifetime of any shared reference, but this assumption is not made for bytes which are covered by an `UnsafeCell`. This is entirely a runtime property. If two references refer to the same memory, but disagree on whether that memory is covered by an `UnsafeCell`, this results in UB if the `UnsafeCell` is used to store to the memory, violating the expectations of the non-`UnsafeCell` shared reference. This commit is consistent with the runtime nature of this property, but is inconsistent with Stacked Borrows (rust-lang/unsafe-code-guidelines#455). However, this is considered to be a bug in Stacked Borrows.
jswrenn
approved these changes
May 9, 2024
jswrenn
approved these changes
May 9, 2024
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
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.
TODO:
project