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

Returns a new vec where each value is the result of calling the given async function on the original value

namespace HH\Lib\Vec;

function map_async<Tv1, Tv2>(
    Traversable<Tv1> $traversable,
  (function(Tv1):Awaitable<Tv2>) $async_func,
): Awaitable<vec<Tv2>>;

For non-async functions, see Vec\map().