Struct containing a StructArray is not type stable

[robsmith11]

After converting my project to use StructArrays, I was surprised to see that there are now millions of allocations. The culprit seems to be a type instability with StructArrays, which causes some functions like searchsorted to allocate:

julia> struct ST
       x::StructArray{typeof((a=0,))}
       end

julia> s = ST(StructArray{typeof((a=0,))}(undef, 0))
ST(NamedTuple{(:a,),Tuple{Int64}}[])

julia> @code_warntype s.x
Variables
  #self#::Core.Compiler.Const(getproperty, false)
  x::ST
  f::Symbol

Body::StructArray{NamedTuple{(:a,),Tuple{Int64}},N,C,I} where I where C<:Union{Tuple, NamedTuple} where N
1 ─ %1 = Base.getfield(x, f)::StructArray{NamedTuple{(:a,),Tuple{Int64}},N,C,I} where I where C<:Union{Tuple, NamedTuple} where N
└──      return %1

julia> @btime searchsorted($s.x.a, 0)
  111.993 ns (1 allocation: 32 bytes)
1:0

julia> @btime searchsorted($s.x.a::Vector{Int}, 0)
  63.711 ns (0 allocations: 0 bytes)
1:0

[robsmith11]

I was able to fix the type stability by specifying the full very verbose type in the struct definition. Is there any more compact alternative?

julia> struct ST2
       x::StructArray{NamedTuple{(:a,),Tuple{Int64}},1,NamedTuple{(:a,),Tuple{Array{Int64,1}}},Int64}
       end

julia> s = ST2(StructArray{NamedTuple{(:a,),Tuple{Int64}}}(undef, 0))
ST2(NamedTuple{(:a,),Tuple{Int64}}[])

julia> @code_warntype s.x
Variables
  #self#::Core.Compiler.Const(getproperty, false)
  x::ST2
  f::Symbol

Body::StructArray{NamedTuple{(:a,),Tuple{Int64}},1,NamedTuple{(:a,),Tuple{Array{Int64,1}}},Int64}
1 ─ %1 = Base.getfield(x, f)::StructArray{NamedTuple{(:a,),Tuple{Int64}},1,NamedTuple{(:a,),Tuple{Array{Int64,1}}},Int64}
└──      return %1

julia> @btime searchsorted($s.x.a, 0)
  9.175 ns (0 allocations: 0 bytes)
1:0

[KristofferC]

I was able to fix the type stability by specifying the full very verbose type in the struct definition. Is there any more compact alternative?

You need your field to be concrete so this isn't very surprising. You can write it as

struct ST2{ST <: StructArray}
    x::ST
end

[robsmith11]

Yeah I guess that makes sense. My mistake was incorrectly thinking that StructArray{Foo} was a concrete type.

[SteffenPL]

Maybe not relevant, but I came across this and found

typeof(StructArray(YourType[]))

as an easy way to get the concrete type.