Function rustc_trait_selection::traits::coherence::impl_intersection_has_impossible_obligation
source · fn impl_intersection_has_impossible_obligation<'a, 'cx, 'tcx>(
selcx: &mut SelectionContext<'cx, 'tcx>,
obligations: &'a [PredicateObligation<'tcx>],
) -> IntersectionHasImpossibleObligations<'tcx>
Expand description
Check if both impls can be satisfied by a common type by considering whether any of either impl’s obligations is not known to hold.
For example, given these two impls:
impl From<MyLocalType> for Box<dyn Error>
(in my crate)
impl<E> From<E> for Box<dyn Error> where E: Error
(in libstd)
After replacing both impl headers with inference vars (which happens before
this function is called), we get:
Box<dyn Error>: From<MyLocalType>
Box<dyn Error>: From<?E>
This gives us ?E = MyLocalType
. We then certainly know that MyLocalType: Error
never holds in intercrate mode since a local impl does not exist, and a
downstream impl cannot be added – therefore can consider the intersection
of the two impls above to be empty.
Importantly, this works even if there isn’t a impl !Error for MyLocalType
.