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Rollup merge of #118914 - compiler-errors:eager-alias-relate, r=lcnr
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Unconditionally register alias-relate in projection goal

Follow-up to #118725, which subtly broke closure signature inference on combinators like `Result::map` which I noticed in syn.

Essentially, instead of using `eq` which will eagerly infer `?1 := <?2 as Trait>::Assoc`, we can directly emit an alias-relate goal, which will stay ambiguous for as long as `?2` is ambiguous.

This also more closely models the conceptual framing that projects-to acts like an alias-relate when solving, and like a normalizes-to when in a param env.

r? lcnr
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matthiaskrgr authored Dec 14, 2023
2 parents 9ec6205 + 929d632 commit accaea2
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Showing 6 changed files with 71 additions and 13 deletions.
34 changes: 23 additions & 11 deletions compiler/rustc_trait_selection/src/solve/project_goals.rs
Original file line number Diff line number Diff line change
Expand Up @@ -8,16 +8,28 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
&mut self,
goal: Goal<'tcx, ProjectionPredicate<'tcx>>,
) -> QueryResult<'tcx> {
match goal.predicate.term.unpack() {
ty::TermKind::Ty(term) => {
let alias = goal.predicate.projection_ty.to_ty(self.tcx());
self.eq(goal.param_env, alias, term)?;
self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
}
// FIXME(associated_const_equality): actually do something here.
ty::TermKind::Const(_) => {
self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
}
}
let tcx = self.tcx();
let projection_term = match goal.predicate.term.unpack() {
ty::TermKind::Ty(_) => goal.predicate.projection_ty.to_ty(tcx).into(),
ty::TermKind::Const(_) => ty::Const::new_unevaluated(
tcx,
ty::UnevaluatedConst::new(
goal.predicate.projection_ty.def_id,
goal.predicate.projection_ty.args,
),
tcx.type_of(goal.predicate.projection_ty.def_id)
.instantiate(tcx, goal.predicate.projection_ty.args),
)
.into(),
};
self.add_goal(goal.with(
tcx,
ty::PredicateKind::AliasRelate(
projection_term,
goal.predicate.term,
ty::AliasRelationDirection::Equate,
),
));
self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
}
}
21 changes: 21 additions & 0 deletions tests/ui/traits/next-solver/closure-signature-inference-2.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,21 @@
// compile-flags: -Znext-solver
// check-pass

fn map<T: Default, U, F: FnOnce(T) -> U>(f: F) {
f(T::default());
}

fn main() {
map::<i32, _ /* ?U */, _ /* ?F */>(|x| x.to_string());
// PREVIOUSLY when confirming the `map` call, we register:
//
// (1.) ?F: FnOnce<(i32,)>
// (2.) <?F as FnOnce<(i32,)>>::Output projects-to ?U
//
// While (1.) is ambiguous, (2.) immediately gets processed
// and we infer `?U := <?F as FnOnce<(i32,)>>::Output`.
//
// Thus, the only pending obligation that remains is (1.).
// Since it is a trait obligation, we don't use it to deduce
// the closure signature, and we fail!
}
15 changes: 15 additions & 0 deletions tests/ui/traits/next-solver/closure-signature-inference.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,15 @@
// compile-flags: -Znext-solver
// check-pass

struct A;
impl A {
fn hi(self) {}
}

fn hello() -> Result<(A,), ()> {
Err(())
}

fn main() {
let x = hello().map(|(x,)| x.hi());
}
Original file line number Diff line number Diff line change
@@ -1,8 +1,18 @@
error[E0284]: type annotations needed: cannot satisfy `<<Rigid as IdHigherRankedBound>::Assoc as WithAssoc<<Wrapper<Leaf> as Id>::Assoc>>::Assoc normalizes-to <<Leaf as WithAssoc<_>>::Assoc as Id>::Assoc`
error[E0284]: type annotations needed
--> $DIR/generalize-proj-new-universe-index-2.rs:74:5
|
LL | bound::<<Rigid as IdHigherRankedBound>::Assoc, <Wrapper<Leaf> as Id>::Assoc, _>()
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot satisfy `<<Rigid as IdHigherRankedBound>::Assoc as WithAssoc<<Wrapper<Leaf> as Id>::Assoc>>::Assoc normalizes-to <<Leaf as WithAssoc<_>>::Assoc as Id>::Assoc`
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type of the type parameter `V` declared on the function `bound`
|
= note: cannot satisfy `<<Rigid as IdHigherRankedBound>::Assoc as WithAssoc<<Wrapper<Leaf> as Id>::Assoc>>::Assoc == _`
note: required by a bound in `bound`
--> $DIR/generalize-proj-new-universe-index-2.rs:69:21
|
LL | fn bound<T: ?Sized, U: ?Sized, V: ?Sized>()
| ----- required by a bound in this function
LL | where
LL | T: WithAssoc<U, Assoc = V>,
| ^^^^^^^^^ required by this bound in `bound`

error: aborting due to 1 previous error

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