This looks like a job for the @SArray (or @SVector) macro. Unfortunately, it doesn’t quite work:

julia> @SArray [I1[i] for i in 1:3]
3-element SVector{3, Int64} with indices SOneTo(3):
1
2
3
julia> @SArray [I1[i] for i in 1:3, j in 1:3]
3×3 SMatrix{3, 3, Int64, 9} with indices SOneTo(3)×SOneTo(3):
1 1 1
2 2 2
3 3 3
julia> @SArray [I1[i] for i in 1:3 for j in 1:3]
ERROR: LoadError: Expected generator in comprehension, e.g. [f(i,j) for i = 1:3, j = 1:3]

It seems like this particular syntax just isn’t supported. An alternative is to create a vector and convert to SVector:

julia> SVector{9}( [I1[i] for i in 1:3 for j in 1:3] )
9-element SVector{9, Int64} with indices SOneTo(9):
1
1
1
2
2
2
3
3
3

but this allocates an intermediate vector. A more efficient, but less elegant, workaround is:

julia> vec(@SArray [I1[i] for j in 1:3, i in 1:3]) # notice switched indexing order
9-element SVector{9, Int64} with indices SOneTo(9):
1
1
1
2
2
2
3
3
3

This should be close to maximally efficient, I think.