Open
Description
This is similar to the resolved issue #85636 , but details a newer iteration of that problem that has not yet been fixed.
Code example
trait MiniYokeable<'a> {
type Output;
}
struct MiniYoke<Y: for<'a> MiniYokeable<'a>> {
pub yokeable: Y,
}
impl<Y> Clone for MiniYoke<Y>
where
Y: for<'a> MiniYokeable<'a>,
for<'a> <Y as MiniYokeable<'a>>::Output: Clone
{
fn clone(&self) -> Self {
todo!()
}
}
trait MiniDataMarker {
type Yokeable: for<'a> MiniYokeable<'a>;
}
trait MiniDataProvider<M>
where
M: MiniDataMarker
{
fn mini_load_payload(&self) -> MiniYoke<M::Yokeable>;
}
struct MiniStructProvider<M>
where
M: MiniDataMarker,
{
pub yoke: MiniYoke<M::Yokeable>,
}
impl<M> MiniDataProvider<M> for MiniStructProvider<M>
where
M: MiniDataMarker,
for<'a> <M::Yokeable as MiniYokeable<'a>>::Output: Clone,
{
fn mini_load_payload(&self) -> MiniYoke<M::Yokeable> {
self.yoke.clone()
}
}
#[derive(Clone)]
struct SimpleStruct(pub u32);
impl<'a> MiniYokeable<'a> for SimpleStruct {
type Output = SimpleStruct;
}
impl MiniDataMarker for SimpleStruct {
type Yokeable = SimpleStruct;
}
fn main() {
let provider = MiniStructProvider {
yoke: MiniYoke {
yokeable: SimpleStruct(42)
}
};
let yoke: MiniYoke<SimpleStruct> = provider.mini_load_payload();
assert_eq!(yoke.yokeable.0, 42);
}(playpen)
This fails to compile on beta/nightly (stable does not have #85499 so we cannot expect this to compile) with:
error[E0599]: the method `mini_load_payload` exists for struct `MiniStructProvider<_>`, but its trait bounds were not satisfied
--> src/main.rs:65:49
|
30 | / struct MiniStructProvider<M>
31 | | where
32 | | M: MiniDataMarker,
33 | | {
34 | | pub yoke: MiniYoke<M::Yokeable>,
35 | | }
| | -
| | |
| |_method `mini_load_payload` not found for this
| doesn't satisfy `MiniStructProvider<_>: MiniDataProvider<_>`
...
65 | let yoke: MiniYoke<SimpleStruct> = provider.mini_load_payload();
| ^^^^^^^^^^^^^^^^^ method cannot be called on `MiniStructProvider<_>` due to unsatisfied trait bounds
|
= note: the following trait bounds were not satisfied:
`<_ as MiniYokeable<'a>>::Output: Clone`
which is required by `MiniStructProvider<_>: MiniDataProvider<_>`
The trait is implemented here: for<'a> <SimpleStruct as MiniYokeable>::Output is just SimpleStruct, which implements Clone.
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