Sampling directly from varied-sized prior is buggy

[xukai92]

MEW:

using Random: AbstractRNG
using Distributions: DiscreteMultivariateDistribution
import Distributions: rand, logpdf

struct DSL <: DiscreteMultivariateDistribution
end

function logpdf(dsl::DSL, dsl_numeric::AbstractVector{Int})
    return sum([log(0.5) * 0.5^i for i in 1:length(dsl_numeric)])
end

function rand(rng::AbstractRNG, dsl::DSL)
    fst = rand(rng, [0, 1])
    dsl_numeric = [fst]
    while rand() < 0.5
        push!(dsl_numeric, rand(rng, [0, 1]))
    end
    return dsl_numeric
end

using Turing

@model function mwe()
    dsl ~ DSL()
end

chain = sample(mwe(), PG(10), 500)

gives

Stacktrace in the failed task:
tried to assign 2 elements to 7 destinations
throw_setindex_mismatch(::Array{Int64,1}, ::Tuple{Int64}) at indices.jl:191
setindex_shape_check at indices.jl:242 [inlined]
setindex! at array.jl:850 [inlined]
setval!(::DynamicPPL.VarInfo{NamedTuple{(:dsl,),Tuple{DynamicPPL.Metadata{Dict{DynamicPPL.VarName{:dsl},Int64},Array{DSL,1},Array{DynamicPPL.VarName{:dsl},1},Array{Int64,1},Array{Set{DynamicPPL.Selector},1}}}},Float64}, ::Array{Int64,1}, ::DynamicPPL.VarName{:dsl}) at varinfo.jl:284
setindex!(::DynamicPPL.VarInfo{NamedTuple{(:dsl,),Tuple{DynamicPPL.Metadata{Dict{DynamicPPL.VarName{:dsl},Int64},Array{DSL,1},Array{DynamicPPL.VarName{:dsl},1},Array{Int64,1},Array{Set{DynamicPPL.Selector},1}}}},Float64}, ::Array{Int64,1}, ::DynamicPPL.VarName{:dsl}) at varinfo.jl:939
assume(::DynamicPPL.Sampler{PG{()},Turing.Inference.PGState{DynamicPPL.VarInfo{NamedTuple{(:dsl,),Tuple{DynamicPPL.Metadata{Dict{DynamicPPL.VarName{:dsl},Int64},Array{DSL,1},Array{DynamicPPL.VarName{:dsl},1},Array{Int64,1},Array{Set{DynamicPPL.Selector},1}}}},Float64},Float64}}, ::DSL, ::DynamicPPL.VarName{:dsl}, ::DynamicPPL.VarInfo{NamedTuple{(:dsl,),Tuple{DynamicPPL.Metadata{Dict{DynamicPPL.VarName{:dsl},Int64},Array{DSL,1},Array{DynamicPPL.VarName{:dsl},1},Array{Int64,1},Array{Set{DynamicPPL.Selector},1}}}},Float64}) at AdvancedSMC.jl:272
_tilde at context_implementations.jl:39 [inlined]
tilde at context_implementations.jl:18 [inlined]
macro expansion at compiler.jl:348 [inlined]
macro expansion at In[4]:9 [inlined]
##inner_function#340#6 at compiler.jl:493 [inlined]
#_#5 at model.jl:24 [inlined]
Model at model.jl:24 [inlined]
Model at model.jl:23 [inlined]
#3 at container.jl:37 [inlined]
(::Libtask.var"#1#2"{Turing.Core.var"#3#4"{DynamicPPL.Model{var"##inner_function#340#6",NamedTuple{(),Tuple{}},DynamicPPL.ModelGen{(),var"###mwe#348",NamedTuple{(),Tuple{}}},Val{()}},DynamicPPL.Sampler{PG{()},Turing.Inference.PGState{DynamicPPL.VarInfo{NamedTuple{(:dsl,),Tuple{DynamicPPL.Metadata{Dict{DynamicPPL.VarName{:dsl},Int64},Array{DSL,1},Array{DynamicPPL.VarName{:dsl},1},Array{Int64,1},Array{Set{DynamicPPL.Selector},1}}}},Float64},Float64}},DynamicPPL.VarInfo{NamedTuple{(:dsl,),Tuple{DynamicPPL.Metadata{Dict{DynamicPPL.VarName{:dsl},Int64},Array{DSL,1},Array{DynamicPPL.VarName{:dsl},1},Array{Int64,1},Array{Set{DynamicPPL.Selector},1}}}},Float64}}})() at taskcopy.jl:38