This is a function for developers to use when extending vctrs. It casts
a list to a more specific vectoring type, keeping the length constant.
It does this by discarding (with a warning), any elements after the 1.
It is called from vec_cast.XYZ.list() methods to preserve symmetry with
vec_cast.list.XYZ().
vec_list_cast(x, to)
| x | A list |
|---|---|
| to | Type to coerce to |
See vignette("s3-vector") for details.