julia> itr = ("foo" for _ in 1:100)
Base.Generator{UnitRange{Int64}, var"#1#2"}(var"#1#2"(), 1:100)

julia> @code_warntype join(itr)
MethodInstance for join(::Base.Generator{UnitRange{Int64}, var"#1#2"})
  from join(iterator) @ Base strings/io.jl:374
Arguments
  #self#::Core.Const(join)
  iterator::Base.Generator{UnitRange{Int64}, var"#1#2"}
Body::Union{Base.AnnotatedString{String}, String}