Union{} slots are not 0 bits, they are currently managed pointers

[andyferris]

This issue is possibly a subset of some other issues involving Unions of isbits types (things like #29289) but I suspect it is easier to satisfy with a special case for Union{} rather than a full generic solution for all possible Union types.

It is becoming more frequent to use the Union{} type as slots of (immutable) structs and Arrays, especially as we move away from containers constructed with eltype coming from Core.Compiler.return_type and while also correctly dealing with empty containers (or starting an algorithm with an empty container and filling elements).

Unfortunately, this represents a bit of a performance problem in certain cases. On my 64-bit intel system, I get 8 bytes allocated per Union{} slot, presumably filled with null pointers that can never be set. Cases where this can be particularly important include creation of SVectors and DataValues - in the latter case the NA type is, unfortunately, not singleton like missing. In both those examples we are often aiming for allocation-free algorithms for speed, friendliness for GPUs, etc. Another case is when experimenting with removing all usage of return_type from collect or map or whatever, starting with an eltype of Union{} with zero filled elements and adding elements and widening the eltype from there - which means in the simple case of perfect inference of a simple bits type you get an extra allocated array.

Some minimal examples.

julia> sizeof(Vector{Union{}}(undef, 1_000_000))
8000000

julia> struct A
           a::Union{}
       end

julia> A.isbitstype
false

julia> A.size
8

I would hope for 0, true and 0 respectively.