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

Returns the first element of the result of the given Awaitable, or null if the Traversable is empty

namespace HH\Lib\C;

function first_async<T>(
    Awaitable<Traversable<T>> $awaitable,
): Awaitable<?T>;

For non-Awaitable Traversables, see C\first.