Skip PhantomData in Unsize checks #149968
Open
+42
−5
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 patch proposes relaxation on the type unsizing condition. `PhantomData` has been in the current type system exempted from being treated as carrying any data, including dropck, virtual call dispatch by `DispatchFromDyn` and unsizing container by `CoerceUnsize`. `PhantomData` is a special 1-ZST that really carries no "data" of the types it captures. I propose that we should also extend this interpretation to `Unsize`. Signed-off-by: Xiangfei Ding <[email protected]>
rustbot
added
S-waiting-on-review
Collaborator
theemathas
added
the
T-types
Contributor
|
FYI, with the way you currently worded this PR, if it were to be merged, #148727 would be closed. |
Contributor
|
This change is visible on stable. The following code fails to compile on nightly, but compiles with this PR use std::marker::PhantomData;
pub struct Thing<T: ?Sized>(PhantomData<T>, T);
pub fn foo(x: &Thing<i32>) -> &Thing<dyn Send> {
x
}I'm unsure whether this code starting to compiling might cause some library to become unsound. |
theemathas
added
the
needs-fcp
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
4 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Outdated suggestions cannot be applied.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 does not fix #148727, rather probably a good addition to how
Unsizebehaves. I am looking intoanti-fundamenetaltrait as suggested to solve a different problem.This patch proposes relaxation on the type unsizing condition.
PhantomDatahas been in the current type system exempted from being treated as carrying any data, including dropck, virtual call dispatch byDispatchFromDynand unsizing container byCoerceUnsize.PhantomDatais a special 1-ZST that really carries no "data" of the types it captures.I propose that we should also extend this interpretation to
Unsize.