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impl_trait_in_bindings is a breaking change #83021
Description
The following program compiles without the impl_trait_in_bindings feature, but not with that feature. I believe this is because the way impl_trait_in_bindings is set up. If you look at
rust/compiler/rustc_typeck/src/check/gather_locals.rs
Lines 61 to 65 in 9b2b02a
you can see that enabling the feature gate runs a completely different algorithm instead of emitting a feature error when the feature gate is not active. Before stabilizing
type_alias_impl_trait we need to make sure that we change all such occurrences of feature-gate based type processing.
#![allow(incomplete_features)]
#![feature(type_alias_impl_trait)]
#![feature(impl_trait_in_bindings)]
use std::marker::PhantomData;
trait MyIndex<T> {
type O;
fn my_index(self) -> Self::O;
}
trait MyFrom<T>: Sized {
type Error;
fn my_from(value: T) -> Result<Self, Self::Error>;
}
trait F {}
impl F for () {}
type DummyT<T> = impl F;
fn _dummy_t<T>() -> DummyT<T> {}
struct Phantom1<T>(PhantomData<T>);
struct Phantom2<T>(PhantomData<T>);
struct Scope<T>(Phantom2<DummyT<T>>);
impl<T> Scope<T> {
fn new() -> Self {
unimplemented!()
}
}
impl<T> MyFrom<Phantom2<T>> for Phantom1<T> {
type Error = ();
fn my_from(_: Phantom2<T>) -> Result<Self, Self::Error> {
unimplemented!()
}
}
impl<T: MyFrom<Phantom2<DummyT<U>>>, U> MyIndex<Phantom1<T>> for Scope<U> {
type O = T;
fn my_index(self) -> Self::O {
MyFrom::my_from(self.0).ok().unwrap()
}
}
fn main() {
let _pos: Phantom1<DummyT<()>> = Scope::new().my_index();
}Errors:
Compiling playground v0.0.1 (/playground)
error[E0282]: type annotations needed
--> src/main.rs:45:38
|
7 | type O;
| ------- `<Self as MyIndex<T>>::O` defined here
...
45 | let _pos: Phantom1<DummyT<()>> = Scope::new().my_index();
| ^^^^^^^^^^-------------
| |
| this method call resolves to `<Self as MyIndex<T>>::O`
| cannot infer type for type parameter `T`
error: aborting due to previous error
For more information about this error, try `rustc --explain E0282`.
error: could not compile `playground`
To learn more, run the command again with --verbose.
Activity
kpreid commentedon Jul 10, 2024
This could probably be closed, because the feature in question was removed by #86729.
Ralith commentedon Sep 25, 2025
The feature was reimplemented in #134185. Should this be reopened? The playground link still fails to compile, although the errors are different, and no longer seem to depend on the feature being enabled.
theemathas commentedon Sep 25, 2025
@Ralith Changing the code as follows causes the code to compile, with or without the impl_trait_in_bindings feature.
So, this issue can stay closed.