This API is part of the Hack Standard Library, not HHVM itself.
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This function is available as Vec\gen_filter() in Facebook's www repository.

Returns a new vec containing only the values for which the given async predicate returns true

namespace HH\Lib\Vec;

function filter_async<Tv>(
    Container<Tv> $container,
  (function(Tv):Awaitable<bool>) $value_predicate,
): Awaitable<vec<Tv>>;

For non-async predicates, see Vec\filter().