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Allow constraining opaque types during subtyping in the trait system #125447

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Jun 19, 2024
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Allow constraining opaque types during subtyping in the trait system #125447

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4 changes: 2 additions & 2 deletions compiler/rustc_infer/src/infer/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -878,9 +878,9 @@ impl<'tcx> InferCtxt<'tcx> {

self.enter_forall(predicate, |ty::SubtypePredicate { a_is_expected, a, b }| {
if a_is_expected {
Ok(self.at(cause, param_env).sub(DefineOpaqueTypes::No, a, b))
Ok(self.at(cause, param_env).sub(DefineOpaqueTypes::Yes, a, b))
} else {
Ok(self.at(cause, param_env).sup(DefineOpaqueTypes::No, b, a))
Ok(self.at(cause, param_env).sup(DefineOpaqueTypes::Yes, b, a))
}
})
}
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67 changes: 16 additions & 51 deletions tests/ui/impl-trait/lazy_subtyping_of_opaques.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2,58 +2,23 @@
//! No hidden types are being constrained in the subtyping predicate, but type and
//! lifetime variables get subtyped in the generic parameter list of the opaque.
use std::iter;

mod either {
pub enum Either<L, R> {
Left(L),
Right(R),
}

impl<L: Iterator, R: Iterator<Item = L::Item>> Iterator for Either<L, R> {
type Item = L::Item;
fn next(&mut self) -> Option<Self::Item> {
todo!()
}
}
pub use self::Either::{Left, Right};
}

pub enum BabeConsensusLogRef<'a> {
NextEpochData(BabeNextEpochRef<'a>),
NextConfigData,
}

impl<'a> BabeConsensusLogRef<'a> {
pub fn scale_encoding(
&self,
) -> impl Iterator<Item = impl AsRef<[u8]> + Clone + 'a> + Clone + 'a {
//~^ ERROR is not satisfied
//~| ERROR is not satisfied
//~| ERROR is not satisfied
match self {
BabeConsensusLogRef::NextEpochData(digest) => either::Left(either::Left(
digest.scale_encoding().map(either::Left).map(either::Left),
)),
BabeConsensusLogRef::NextConfigData => either::Right(
// The Opaque type from ``scale_encoding` gets used opaquely here, while the `R`
// generic parameter of `Either` contains type variables that get subtyped and the
// opaque type contains lifetime variables that get subtyped.
iter::once(either::Right(either::Left([1])))
.chain(std::iter::once([1]).map(either::Right).map(either::Right)),
),
}
}
}

pub struct BabeNextEpochRef<'a>(&'a ());

impl<'a> BabeNextEpochRef<'a> {
pub fn scale_encoding(
&self,
) -> impl Iterator<Item = impl AsRef<[u8]> + Clone + 'a> + Clone + 'a {
std::iter::once([1])
//@ check-pass

fn foo() -> impl Default + Copy {
if false {
let x = Default::default();
// add `Subtype(?x, ?y)` obligation
let y = x;

// Make a tuple `(?x, ?y)` and equate it with `(impl Default, u32)`.
// For us to try and prove a `Subtype(impl Default, u32)` obligation,
// we have to instantiate both `?x` and `?y` without any
// `select_where_possible` calls inbetween.
let mut tup = &mut (x, y);
let assign_tup = &mut (foo(), 1u32);
tup = assign_tup;
}
1u32
}

fn main() {}
21 changes: 0 additions & 21 deletions tests/ui/impl-trait/lazy_subtyping_of_opaques.stderr
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3 changes: 2 additions & 1 deletion tests/ui/type-alias-impl-trait/lazy_subtyping_of_opaques.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -7,9 +7,10 @@
type Tait = impl FnOnce() -> ();

fn reify_as_tait() -> Thunk<Tait> {
//~^ ERROR: expected a `FnOnce()` closure, found `()`
Thunk::new(|cont| cont)
//~^ ERROR: mismatched types
//~| ERROR: mismatched types
//~| ERROR: expected a `FnOnce()` closure, found `()`
}

struct Thunk<F>(F);
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31 changes: 18 additions & 13 deletions tests/ui/type-alias-impl-trait/lazy_subtyping_of_opaques.stderr
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@@ -1,26 +1,31 @@
error[E0308]: mismatched types
--> $DIR/lazy_subtyping_of_opaques.rs:10:23
error[E0277]: expected a `FnOnce()` closure, found `()`
--> $DIR/lazy_subtyping_of_opaques.rs:11:23
|
LL | type Tait = impl FnOnce() -> ();
| ------------------- the found opaque type
...
LL | Thunk::new(|cont| cont)
| ^^^^ expected `()`, found opaque type
| ^^^^ expected an `FnOnce()` closure, found `()`
|
= note: expected unit type `()`
found opaque type `Tait`
= help: the trait `FnOnce()` is not implemented for `()`
= note: wrap the `()` in a closure with no arguments: `|| { /* code */ }`

error[E0308]: mismatched types
--> $DIR/lazy_subtyping_of_opaques.rs:10:5
error[E0277]: expected a `FnOnce()` closure, found `()`
--> $DIR/lazy_subtyping_of_opaques.rs:9:23
|
LL | fn reify_as_tait() -> Thunk<Tait> {
| ----------- expected `Thunk<_>` because of return type
| ^^^^^^^^^^^ expected an `FnOnce()` closure, found `()`
|
= help: the trait `FnOnce()` is not implemented for `()`
= note: wrap the `()` in a closure with no arguments: `|| { /* code */ }`

error[E0308]: mismatched types
--> $DIR/lazy_subtyping_of_opaques.rs:11:5
|
LL | Thunk::new(|cont| cont)
| ^^^^^^^^^^^^^^^^^^^^^^^ expected `Thunk<_>`, found `()`
|
= note: expected struct `Thunk<_>`
found unit type `()`

error: aborting due to 2 previous errors
error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0308`.
Some errors have detailed explanations: E0277, E0308.
For more information about an error, try `rustc --explain E0277`.
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