Introduce TaggedOneOf

[Drvi]

WIP

The idea is to generate a custom struct for each oneof field which would contain a tag and logic to access the active union member in a type-stable way with PB.active_value(x) do val; ...; end

Example:

syntax = "proto3";
message A { oneof field_name { string b = 3; uint64 c = 4; bytes d = 5; int32 e = 6; int32 g = 7; } }

struct var"A.FieldName" <: PB.TaggedOneOf
    tag::UInt8
    bit::Union{Nothing,UInt64,Int32}
    ptr::Union{Nothing,String,Vector{UInt8}}
end
var"A.FieldName"(::Val{:b}, x) = var"A.FieldName"(UInt8(1), nothing, convert(String, x))
var"A.FieldName"(::Val{:c}, x) = var"A.FieldName"(UInt8(2), convert(UInt64, x), nothing)
var"A.FieldName"(::Val{:d}, x) = var"A.FieldName"(UInt8(3), nothing, convert(Vector{UInt8}, x))
var"A.FieldName"(::Val{:e}, x) = var"A.FieldName"(UInt8(4), convert(Int32, x), nothing)
var"A.FieldName"(::Val{:g}, x) = var"A.FieldName"(UInt8(5), convert(Int32, x), nothing)
PB.unsafe_getproperty(x::var"A.FieldName", ::Val{:b}) = Base.getfield(x, :ptr)::String
PB.unsafe_getproperty(x::var"A.FieldName", ::Val{:c}) = Base.getfield(x, :bit)::UInt64
PB.unsafe_getproperty(x::var"A.FieldName", ::Val{:d}) = Base.getfield(x, :ptr)::Vector{UInt8}
PB.unsafe_getproperty(x::var"A.FieldName", ::Val{:e}) = Base.getfield(x, :bit)::Int32
PB.unsafe_getproperty(x::var"A.FieldName", ::Val{:g}) = Base.getfield(x, :bit)::Int32
PB.field_to_tag(::Type{var"A.FieldName"}) = (;b = UInt8(1), c = UInt8(2), d = UInt8(3), e = UInt8(4), g = UInt8(5))
function PB.active_name(f::F, x::var"A.FieldName") where {F}
    tag = getfield(x, :tag)
    if     tag == UInt8(1); return f(Val(:b))
    elseif tag == UInt8(2); return f(Val(:c))
    elseif tag == UInt8(3); return f(Val(:d))
    elseif tag == UInt8(4); return f(Val(:e))
    elseif tag == UInt8(5); return f(Val(:g))
    else   PB.throw_bad_tag(var"A.FieldName", tag)
    end
end
function PB.active_value(f::F, x::var"A.FieldName") where {F}
    tag = getfield(x, :tag)
    if     tag == UInt8(1); return f(Base.getfield(x, :ptr)::String)
    elseif tag == UInt8(2); return f(Base.getfield(x, :bit)::UInt64)
    elseif tag == UInt8(3); return f(Base.getfield(x, :ptr)::Vector{UInt8})
    elseif tag == UInt8(4); return f(Base.getfield(x, :bit)::Int32)
    elseif tag == UInt8(5); return f(Base.getfield(x, :bit)::Int32)
    else   PB.throw_bad_tag(var"A.FieldName", tag)
    end
end

struct A
    field_name::var"A.FieldName"
end
# ...

A big remaining todo is handling dependency cycles.