UniStr constructor is not type stable

[bkamins]

This probably will be a problem when dynamically constructing UniStr:

julia> @code_warntype Strs.UniStr("1")
Variables:
  str::String

Body:
  begin
      # meta: location D:\DEV\Julia\Strs.jl\src\encode.jl convert 82
      Core.SSAValue(1) = $(Expr(:invoke, MethodInstance for _str(::String), :(Main.Strs._str), :(str)))::Any
      # meta: pop location
      return Core.SSAValue(1)
  end::Any

[ScottPJones]

Since UniStr is a Union, it can't ever be type stable, however I may be able to annotate the return so that it knows that it is the UniStr union type, or at least Str, and not just Any.

[bkamins]

This is what I mean. UniStr type should be identified as a return type - only then - inside functions we can hope to have "small union" benefit (i.e. we construct a UniStr and then use it in the same function - now we need barrier functions to have it properly type-inferred in hot loops).
Probably some benchmarking is needed here as simply annotating the return value of constructor might also harm performance.