Undefined behavior when modifying NullableArray internals

[davidagold]

julia> X = NullableArray([1:10...]);

julia> splice!(X.isnull, 1)
false

julia> X
10-element NullableArrays.NullableArray{Int64,1}:
    Nullable(1)
    Nullable(2)
    Nullable(3)
    Nullable(4)
    Nullable(5)
    Nullable(6)
    Nullable(7)
    Nullable(8)
    Nullable(9)
 #undef        

julia> length(X)
10

julia> length(X.isnull)
9

Okay, that's undefined behavior. But I can imagine the following (or something like it) biting somebody trying to do something reasonable:

julia> A = [1:10...];

julia> X = NullableArray(A);

julia> splice!(A, 1)
1

julia> X
9-element NullableArrays.NullableArray{Int64,1}:
 Nullable(2) 
 Nullable(3) 
 Nullable(4) 
 Nullable(5) 
 Nullable(6) 
 Nullable(7) 
 Nullable(8) 
 Nullable(9) 
 Nullable(10)

julia> X.isnull
10-element Array{Bool,1}:
 false
 false
 false
 false
 false
 false
 false
 false
 false
 false

Thoughts? Really it shouldn't be unexpected since Julia passes by reference, but I'm wondering if there's anything we ought to do to make it more obvious that you might have broken something. Then again, the best (and maybe only feasible) thing to do may just be to include a caveat in the documentation.