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Un-shadow object bound candidate in NormalizesTo goal if self_ty is trait object #148292

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Nov 6, 2025
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Un-shadow object bound candidate in NormalizesTo goal if self_ty is trait object #148292

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2fbb751
Un-shadow object bound candidate in `NormalizesTo` goal
adwinwhite Oct 30, 2025
e2ac9d9
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adwinwhite Oct 31, 2025
d2cfc47
add test for alias self_ty
adwinwhite Nov 5, 2025
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6 changes: 6 additions & 0 deletions compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1143,6 +1143,12 @@ where
// See `tests/ui/winnowing/norm-where-bound-gt-alias-bound.rs`.
if candidates.iter().any(|c| matches!(c.source, CandidateSource::ParamEnv(_))) {
candidates.retain(|c| matches!(c.source, CandidateSource::ParamEnv(_)));
} else if matches!(goal.predicate.self_ty().kind(), ty::Dynamic(..)) {
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@lcnr lcnr Oct 31, 2025

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I think we should move this into assemble_and_evaluate_candidates as rn this doesn't handle cases where the self type is an alias, e.g. the following should still fail with ur PR

trait Trait {
    type Assoc;
}
trait Id {
    type This;
}
impl<T: ?Sized> Id for T {
    type This = T;
}

fn foo<T>(x: <dyn Trait<Assoc = T> as Trait>::Assoc) -> T
where
    <dyn Trait<Assoc = T> as Id>::This: Trait,
{
    x
}

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@adwinwhite adwinwhite Oct 31, 2025 •
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This doesn't fail (after fixing ?Sized bound). And the Assoc projection's self_ty is not alias?
The trait goal in param env has alias self_ty but it's irrelevant for the NormalizesTo goal.

Besides, the following doesn't fail either. Perhaps the self_ty is normalized somehow?

trait Trait {
    type Assoc;
}
trait Id {
    type This: ?Sized;
}
impl<T: ?Sized> Id for T {
    type This = T;
}

fn foo<T>(x: <<dyn Trait<Assoc = T> as Id>::This as Trait>::Assoc) -> T
where
    <dyn Trait<Assoc = T> as Id>::This: Trait,
{
    x
}

Nonetheless, I moved the code into assemble_and_evaluate_candidates and removed comments specific to NormalizesTo goal since trait goals also use this method.

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@lcnr lcnr Nov 3, 2025 •
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you're right. I messed up the example.

does the following fail with the old impl? I think the issue is that we otherwise first normalize the inner, and then the outer alias 🤔

trait Trait {
    type Assoc;
}
trait Id<'a> {
    type This: ?Sized;
}
impl<T: ?Sized> Id<'_> for T {
    type This = T;
}

fn foo<T>(x: for<'a> fn(
    <<dyn Trait<Assoc = T> as Id<'a>>::This as Trait>::Assoc
)) -> fn(T)
where
    dyn Trait<Assoc = T>: Trait,
{
    x
}

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@adwinwhite adwinwhite Nov 5, 2025

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This doesn't fail with my first commit.

It turns out when we add NormalizesTo goal for projection Assoc, we also add a nested AliasRelate goal for the inner projection This.
And we evaluate the inner projection before the outer one.
Thus when we evaluate the NormalizesTo goal of Assoc, the self_ty is already normalized.

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@lcnr lcnr Nov 5, 2025

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ah yeah. we never actually need to normalize the self type for normalization goals 😅 can you still add that one as a test?

// Object candidate may be shadowed by where-bound for the trait goal, see
// `tests/ui/traits/next-solver/normalization-shadowing/use_object_if_empty_env.rs`.
// Trait objects always have their associated types specified so `candidates`
// won't be empty.
self.assemble_object_bound_candidates(goal, &mut candidates);
} else if candidates.is_empty() {
// If the trait goal has been proven by using the environment, we want to treat
// aliases as rigid if there are no applicable projection bounds in the environment.
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17 changes: 17 additions & 0 deletions tests/ui/traits/next-solver/normalization-shadowing/use_object_if_empty_env.rs
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@@ -0,0 +1,17 @@
//@ compile-flags: -Znext-solver
//@ check-pass

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@lcnr lcnr Oct 30, 2025

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pls add // Regression test for trait-system-refactor-initiative#whatever

trait Trait {
type Assoc;
}

// We have param env candidate for the trait goal but not the projection.
// Under such circumstance, consider object candidate if the self_ty is trait object.
fn foo<T>(x: <dyn Trait<Assoc = T> as Trait>::Assoc) -> T
where
dyn Trait<Assoc = T>: Trait,
{
x
}

fn main() {}
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