This API is part of the Hack Standard Library, not HHVM itself.

Returns a new vec containing each element of the given Traversable exactly once

namespace HH\Lib\Vec;

function unique<Tv as arraykey>(
    Traversable<Tv> $traversable,
): vec<Tv>;

The Traversable must contain arraykey values, and strict equality will be used.

For non-arraykey elements, see Vec\unique_by().