Seeing allocations only when assigning to a Vector of Union{Nothing, Struct} when Struct is immutable, but contains mutable data

General Usage Performance
10

Since there is no direct solution, you may try to use a structure like:

struct A
  isnothing::Bool
  x::Vector{Int}
end

Base.isnothing(a::A) = a.isnothing

work with containers of that type instead of containers of Union types, and adapt the code accordingly.

Well, if that turns out to be critical and you can deal with that data structure instead.