DispatchFromDyn: require additional checks #149068
Open
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
rustbot
added
S-waiting-on-review
Collaborator
|
rustbot has assigned @WaffleLapkin. Use |
This comment has been minimized.
This comment has been minimized.
Contributor
Author
|
@theemathas What do you think? Can we break |
dingxiangfei2009
force-pushed
the
dispatch-from-dyn-due-diligence
branch
from
75de383 to
88de0f9
Compare
This comment has been minimized.
This comment has been minimized.
dingxiangfei2009
force-pushed
the
dispatch-from-dyn-due-diligence
branch
from
88de0f9 to
b7f84d2
Compare
Contributor
|
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 |
Contributor
|
The documentation for DispatchFromDyn also needs to be changed. |
Contributor
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. |
dingxiangfei2009
force-pushed
the
dispatch-from-dyn-due-diligence
branch
from
b7f84d2 to
8c95fd8
Compare
This comment has been minimized.
This comment has been minimized.
dingxiangfei2009
force-pushed
the
dispatch-from-dyn-due-diligence
branch
from
8c95fd8 to
fdf32a0
Compare
jackh726
added
T-types
Member
|
r? types |
Contributor
Author
|
Discussion under t-lang: #t-lang > Should we support dyn dispatch on `&Adt<_>`? I think we are quite convinced that |
Darksonn
reviewed
Nov 20, 2025
Comment on lines
+73
to
+74
| impl<'a, T: ?Sized, U: ?Sized> DispatchFromDyn<&'a Ptr<U>> for &'a Ptr<T> {} | ||
| //~^ ERROR the trait `DispatchFromDyn` does not allow dispatch through references |
Contributor
There was a problem hiding this comment.
Mentioning references here seems awfully specific. What about Box<Ptr<U>> or similar?
Contributor
Author
There was a problem hiding this comment.
I am in the middle of updating the documentation on this trait.
Box<Ptr<U>> falls in the second case of the new documentation. We would foremost require Ptr<T>: DispatchFromDyn<Ptr<U>> among the other requirements.
theemathas
reviewed
Nov 20, 2025
| .label = can't implement cross-crate trait for type in another crate | ||
| hir_analysis_dispatch_from_dyn_multiple_refs = the trait `DispatchFromDyn` does not allow dispatch through references | ||
| .note = the trait `DispatchFromDyn` may only be implemented when dispatch goes through at most one reference to a generic |
Contributor
There was a problem hiding this comment.
Why does this text not show up in the test error output?
Contributor
Author
There was a problem hiding this comment.
I forgot about a #[note] attribute on the error type. Now it should work.
Contributor
|
This code doesn't compile with this PR. Should it? #![feature(dispatch_from_dyn, arbitrary_self_types, unsize)]
use std::marker::{PhantomData, Unsize};
use std::ops::{DispatchFromDyn, Receiver};
pub struct Inner<T: ?Sized>(T);
#[repr(transparent)]
pub struct Outer<T: ?Sized>(PhantomData<T>, Inner<T>);
fn wrap<T: ?Sized>(x: &Inner<T>) -> &Outer<T> {
// SAFETY: We're using repr(transparent)
unsafe { &*(x as *const Inner<T> as *const Outer<T>) }
}
impl<'a, T: Unsize<U> + ?Sized, U: ?Sized> DispatchFromDyn<&'a Outer<U>> for &'a Outer<T> {}
impl<T: ?Sized> Receiver for Outer<T> {
type Target = T;
}
pub trait Trait {
fn method(self: &Outer<Self>);
}
struct Thing(i32);
impl Trait for Thing {
fn method(self: &Outer<Self>) {
println!("{}", self.1.0.0);
}
}
fn main() {
let inner: &Inner<dyn Trait> = &Inner(Thing(1));
let x: &Outer<dyn Trait> = wrap(inner);
x.method();
} |
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 <[email protected]>
dingxiangfei2009
force-pushed
the
dispatch-from-dyn-due-diligence
branch
from
fdf32a0 to
e0eee5f
Compare
Collaborator
|
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. |
Signed-off-by: Xiangfei Ding <[email protected]>
Contributor
Author
|
To answer the comment I have jotted down the reason that we do not want to compile the example in the trait documentation. |
Collaborator
|
The job Click to see the possible cause of the failure (guessed by this bot) |
Contributor
|
@dingxiangfei2009 Your comment in the code unfortunately does not answer my latest comment in this PR. My comment does not involve a double pointer indirection. |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
7 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.
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