rustc_trait_selection::traits::coherence

Function 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.