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trait_sel: resolve vars in host effects #137604

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Feb 26, 2025
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trait_sel: resolve vars in host effects #137604

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2 changes: 2 additions & 0 deletions compiler/rustc_trait_selection/src/traits/effects.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -31,6 +31,8 @@ pub fn evaluate_host_effect_obligation<'tcx>(
);
}

let ref obligation = selcx.infcx.resolve_vars_if_possible(obligation.clone());

// Force ambiguity for infer self ty.
if obligation.predicate.self_ty().is_ty_var() {
return Err(EvaluationFailure::Ambiguous);
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36 changes: 36 additions & 0 deletions tests/ui/traits/const-traits/unconstrained-var-specialization.rs
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On nightly this emits

error[E0283]: type annotations needed: cannot satisfy `Iter: Tr`
  --> src/lib.rs:31:33
   |
31 | impl<A: const Foo, Iter> Tr for Iter
   |                                 ^^^^
   |
note: multiple `impl`s satisfying `Iter: Tr` found
  --> src/lib.rs:24:1
   |
24 | / impl<A: const Foo, Iter> Tr for Iter
25 | |     where
26 | |         Iter: Iterator<Item = A>,
   | |_________________________________^
...
31 | / impl<A: const Foo, Iter> Tr for Iter
32 | |     where
33 | |         Iter: MoreSpecificThanIterator<Item = A>,
   | |_________________________________________________^

Original file line number Diff line number Diff line change
@@ -0,0 +1,36 @@
//@ check-pass
//@ compile-flags: --crate-type=lib
#![no_std]
#![allow(internal_features)]
#![feature(rustc_attrs, min_specialization, const_trait_impl)]

// In the default impl below, `A` is constrained by the projection predicate, and if the host effect
// predicate for `const Foo` doesn't resolve vars, then specialization will fail.

#[const_trait]
trait Foo {}

pub trait Iterator {
type Item;
}

#[rustc_unsafe_specialization_marker]
pub trait MoreSpecificThanIterator: Iterator {}

pub trait Tr {
fn foo();
}

impl<A: const Foo, Iter> Tr for Iter
where
Iter: Iterator<Item = A>,
{
default fn foo() {}
}

impl<A: const Foo, Iter> Tr for Iter
where
Iter: MoreSpecificThanIterator<Item = A>,
{
fn foo() {}
}
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