rename_dims to an existing dimension name

[dcherian]

ds = xr.Dataset({"a": ("x", np.arange(10)), "b": ("y", np.arange(10))})
ds.rename_dims({"x": "y"})

ValueError: Cannot rename x to y because y already exists. Try using swap_dims instead.

This should be possible since both x and y have the same lengths.
Thoughts?

For indexed dimensions, should we check that the indexes are equal?

[keewis]

When I added the error, my reasoning was that renaming dimensions (choosing a different name for a dimension) should not be mixed with swapping dimensions (using a different dimension for a variable).

see also #3645 and #3438

[dcherian]

Ah i didn't think swap_dims would work but it does! Thanks.,