DispatchFromDyn: require additional checks #149068
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rustbot has assigned @WaffleLapkin. Use |
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@theemathas What do you think? Can we break |
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This code causes an ICE with this PR: #![feature(dispatch_from_dyn, unsize, arbitrary_self_types)]
use std::{
marker::{PhantomData, Unsize},
ops::{DispatchFromDyn, Receiver},
};
struct Ptr<T: ?Sized>(PhantomData<T>, Box<T>);
impl<'a, T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<&'a Ptr<U>> for &'a Ptr<T> {}
impl<T: ?Sized> Receiver for Ptr<T> {
type Target = T;
}
trait Trait {
fn method(self: &Ptr<Self>) {}
}
struct Thing;
impl Trait for Thing {}
fn main() {
let x: Ptr<dyn Trait> = Ptr(PhantomData, Box::new(Thing));
x.method();
}The compiler is probably attempting to do an dyn dispatch through two pointers, which is not possible Error output |
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The documentation for DispatchFromDyn also needs to be changed. |
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Author
I am not sure. We have an option to make it possible. The question is, would it be a bad idea? In any case, let me add a stop-gap to disallow this case. |
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This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed. Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers. |
The new checks are we check the pair of constituent types for same shapes structurally, and demand capability for dispatch for ADTs as usual; and we demand source generic parameter to be unsized of another for sanity. Signed-off-by: Xiangfei Ding <dingxiangfei2009@protonmail.ch>
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r? types |
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The new checks are we check the pair of constituent types for same shapes structurally, and demand capability for dispatch for ADTs as usual; and we demand source generic parameter to be unsized of another for sanity.
Fix #148727
cc @theemathas