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bc439ad092
Implements some of the changes from proposal #5168. * The data structure for complete facet types has been repurposed for identified facet types. Identified facet types are now a concept in the toolchain, but without named constraint support they are not substantially different from incomplete facet types. * Identified facet types keep the list of required specific interfaces in sorted order, for efficiency improvements in impl lookup. Found another way to identify the interface to impl (or number of impls if not 1). * Forward `impl` declarations of identified but incomplete facet types are allowed unless the facet type has rewrites. An incomplete facet type with rewrites is already either an error or has more than one interface and so can't be implemented, so this case can't be exercised very well yet. * Forward `impl` declarations of interface without rewrites use a placeholder inst block for the witness. * Changed some machinery to use RequireIdentifiedFacetType to access the interfaces of the facet type so we only need to add support for expanding named constraints into interfaces in one place. --------- Co-authored-by: Josh L <josh11b@users.noreply.github.com> Co-authored-by: Dana Jansens <danakj@orodu.net>