Allow ... in non-final assignment location

[adkabo]

It would be convenient to have more iterable unpacking.

Julia 1.7:

julia> x, (ys...) = (1,2,3,4);
julia> x, ys
(1, (2, 3, 4))
julia> x, (ys...), z = (1,2,3,4)
ERROR: syntax: invalid "..." on non-final assignment location "ys" around REPL[2]:1

Python (feature specification):

In [1]: x, *y, z = [1,2,3,4]

In [2]: x, y, z
Out[2]: (1, [2, 3], 4)

[martinholters]

Similar to but not exactly duplicate of #42036.

[adkabo]

Maybe #42036 can be for function signatures and this issue can have the smaller scope of only unpacking assignment.

# first position
(a..., b) = _
# interior position
(a, b..., c) = _

[simeonschaub]

This is addressed by #42902, but that still needs to be finished up.