Apologies if there's already an issue open for this already. I took a quick look but.

Many times when I define a mutable struct, but I know there's only one or two fields in it that will ever change, I feel slightly guilty. I suspect there must be optimizations that could become available if Julia knew which of the fields might change, and which might not. For example, in the following example, perhaps I know that b might change, but a will not.

mutable struct A
    a::Int
    b::Int
end

So I could imagine writing it like this instead:

struct A
    a::Int
    mutable b::Int
end

Of course, it's possible to make b a Ref, but perhaps the solution above seems a bit more general?