Function where_predicate_must_hold 

pub fn where_predicate_must_hold<'db>(
    db: &'db dyn HirDatabase,
    resolver: &Resolver<'db>,
    store: &'db ExpressionStore,
    def: ExpressionStoreOwnerId,
    generic_def: GenericDefId,
    env: ParamEnvAndCrate<'db>,
    predicate: &WherePredicate,
) -> WherePredicateEvaluation

This should not be used in hir-ty, only in hir. This is exposed to allow the IDE to evaluate arbitrary predicates.