@quinedot thank you! I'm in dependency hell on this particular project, and had made a tactical decision to carry on with the kludge of using back-reved versions of several packages so that I wouldn't get into a version conflict with ndarray (long story -- see the link).
I've been strictly looking at the documentation for 0.13.0, because that's what works with the package combination I have. You made me look at the current documentation, and voila! as_array_mut is safe in the latest version.
There's a suggestion in that thread on how to resolve things which I had not taken -- I'll be trying that, and see if it makes the sun break through the clouds for me.