[RFC/WIP] Handle :compact context in 3-arg show for arrays

[tkf]

Edit: now this PR fixes only permutedims([Int[], Int[]]) case #34733 (comment)

---

Before:

julia> [Int[]]
1-element Array{Array{Int64,1},1}:
 0-element Array{Int64,1}

After:

julia> [Int[]]
1-element Array{Array{Int64,1},1}:
 []

I'm sending this as a WIP I was meant to send this as a WIP PR before fixing the tests as I don't know if the core devs think this is the way to go.

fix (a related bug of) #34659

[tkf]

I'm not sure if people are happy with removing length(axes(X, 2)) > 1.

(BTW I think this is a shortcoming of the way :compact is used. It's not possible to declare compactness of the width and height separately. This is why I've been suggesting to use :displaysize instead of :compact #33260 (comment))

[JeffBezanson]

This is a separate issue and will probably be controversial, so I wouldn't change this for now.

[JeffBezanson]

Is this really necessary? I don't think it's common for 2-arg show methods to call 3-arg ones.

[tkf]

@JeffBezanson Note that both changes are required if you want to fix the bug #34659 this way. The problem is that 3-arg show is used for the inner array when calling 3-arg show for the outer array show(io, "text/plain", [Int[]]) due to

julia/base/arrayshow.jl

Lines 105 to 111 in b0d1c1a

	# First try 3-arg show
	sx = sprint(show, "text/plain", x, context=io, sizehint=0)
	# If the output contains line breaks, try 2-arg show instead.
	if occursin('\n', sx)
	sx = sprint(show, x, context=io, sizehint=0)
	end

That is to say, since 3-arg show of the inner array show(io, "text/plain", Int[]) does not print multiple lines, we don't use the if occursin('\n', sx) branch. This is why we need if get(io, :compact, false) === true #34733 (comment). However, for the inner array to be able to check that it is invoked via 3-arg show, we need to set :compat always even with vectors #34733 (comment).

Of course, there may be another solution for fixing #34659. Please close this PR if there is a better way.

[JeffBezanson]

You're right; it would only fix this case:

julia> permutedims([Int[], Int[]])
1×2 Array{Array{Int64,1},2}:
 0-element Array{Int64,1}  0-element Array{Int64,1}

but I think that's worth doing.
I don't see any way to fix the 1-column case except adding a showelement function and overloading it for arrays.

[tkf]

OK, I put length(axes(X, 2)) > 1 back. Let's see how many tests fail...