marker_trait_attr: discard eval modulo regions in favor of eval to ok #84922
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May 5, 2021
| @@ -1494,7 +1494,18 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> { | |||
| // will then correctly report an inference error, since the | |||
| // existence of multiple marker trait impls tells us nothing | |||
| // about which one should actually apply. | |||
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Can this cause inference variables to be arbitrarily constrained, as described in the comments above?
e.g. if we had
#[marker]
pub trait F {}
impl F for u32 {}
impl<'a> F for &'a str where 'a: 'static {}Would this change cause the <_#0t as F> obligation to unify _#0t with u32?
(contrived example because I don't know enough about trait selection/winnowing to provide an uncontrived one)
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no, because the !needs_infer condition still applies.
We drop strictly fewer candidates with this change.
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derp. boolean logic, it's no joke
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both the current state and this PR are unsound. Going to close this until we come up with an actually sound approach (cc #84955) |
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fixes #84917
cc https://rust-lang.zulipchat.com/#narrow/stream/144729-wg-traits/topic/marker_trait_attr.20stabilization/near/237371224