I want to point out a recent discussion bit (though the thread itself is about something else) about avoiding CoW: [ANN] CeedNumerics released (SN's ShapedArray API discussion) - #31 by Lantua.
Essentially there is a problem when the yielding value is not a simple element value, but a more complex variable containing self like most Subsequence.
In those cases modify couldn't help avoiding CoW due to the similar problem the safe variance of first example has.
It could be orthogonal to the modify (as a separate accessor), somewhat related (borrow context block + modify accessor), or even as part of the future direction (move-only type?). So I think it's worth pointing out.