In the style guide underSlicing with Ranges it's said that

[0..-1] inary[0..-1] is redundant and simply synonymous withary.

However, one makes a shallow copy while the other doesn't:

ary=[1,2,3]with_slice=ary[0..-1]without_slice=aryary[0]=:changed# [1, 2, 3]putswith_slice.inspect# [:changed, 2, 3]putswithout_slice.inspect

By analogy to the Python idiomary[:], I would guess that the "bad" form is often written specifically to make a copy rather than an alias.

I suggest that the advice be changed to say something likeary.clone,ary.dup or[*ary] is preferred (whatever is considered better style), or that simplyary is preferred if a copy isn't needed. Ifclone is a good alternative then it's worth adding that it isn't exactly synonymous with a full slice, since evenary.clone(freeze: false) clones the singleton class, whereas slicing doesn't.