Skip PhantomData in Unsize checks #149968
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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 <dingxiangfei2009@protonmail.ch>
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FYI, with the way you currently worded this PR, if it were to be merged, #148727 would be closed. |
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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. |
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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.