wip that works, committing in case I f it up

config/config.exs

@@ -1 +1,4 @@
 use Mix.Config
+
+config :protobuf, extensions: :enabled
+

lib/elixir.pb.ex

@@ -0,0 +1,20 @@
+defmodule Brex.Elixir.FieldOptions do
+  @moduledoc false
+  use Protobuf, syntax: :proto2
+
+  @type t :: %__MODULE__{
+          extype: String.t()
+        }
+  defstruct [:extype]
+
+  field :extype, 1, optional: true, type: :string
+end
+
+defmodule Brex.Elixir.PbExtension do
+  @moduledoc false
+  use Protobuf, syntax: :proto2
+
+  extend Google.Protobuf.FieldOptions, :field, 65_007,
+    optional: true,
+    type: Brex.Elixir.FieldOptions
+end

lib/protobuf/extension/props.ex

@@ -5,7 +5,7 @@ defmodule Protobuf.Extension.Props do
     @moduledoc false
     @type t :: %__MODULE__{
             extendee: module,
-            field_props: FieldProps.T
+            field_props: FieldProps.t
           }
     defstruct extendee: nil,
               field_props: nil

lib/protobuf/field_props.ex

@@ -17,7 +17,8 @@ defmodule Protobuf.FieldProps do
           packed?: boolean,
           map?: boolean,
           deprecated?: boolean,
-          encoded_fnum: iodata
+          encoded_fnum: iodata,
+          options: List.t() | nil
         }
   defstruct fnum: nil,
             name: nil,
@@ -34,5 +35,6 @@ defmodule Protobuf.FieldProps do
             packed?: nil,
             map?: false,
             deprecated?: false,
-            encoded_fnum: nil
+            encoded_fnum: nil,
+            options: nil
 end

lib/protobuf/message_props.ex

@@ -6,7 +6,7 @@ defmodule Protobuf.MessageProps do
   @type t :: %__MODULE__{
           ordered_tags: [integer],
           tags_map: %{integer => integer},
-          field_props: %{integer => FieldProps.T},
+          field_props: %{integer => FieldProps.t},
           field_tags: %{atom => integer},
           repeated_fields: [atom],
           embedded_fields: [atom],

lib/protobuf/protoc/generator/message.ex

@@ -12,6 +12,7 @@ defmodule Protobuf.Protoc.Generator.Message do
 
   def generate(ctx, desc) do
     msg_struct = parse_desc(ctx, desc)
+    IO.inspect(:stderr, msg_struct, label: :message_struct)
     ctx = %{ctx | namespace: msg_struct[:new_namespace]}
     {nested_enums, nested_msgs} = Enum.unzip(gen_nested_msgs(ctx, desc))
 
@@ -177,8 +178,10 @@ defmodule Protobuf.Protoc.Generator.Message do
   end
 
   def get_field(ctx, f, nested_maps, oneofs) do
+
     opts = field_options(f)
     map = nested_maps[f.type_name]
+
     opts = if map, do: Map.put(opts, :map, true), else: opts
 
     opts =
@@ -288,8 +291,13 @@ defmodule Protobuf.Protoc.Generator.Message do
   end
 
   defp merge_field_options(opts, f) do
+    extype_options = Google.Protobuf.FieldOptions.get_extension(f.options, Brex.Elixir.PbExtension, :field)
+
+    IO.inspect(:stderr, {opts, f.options, extype_options}, label: :together)
+
     opts
     |> Map.put(:packed, f.options.packed)
     |> Map.put(:deprecated, f.options.deprecated)
+    |> Map.put(:options, "#{inspect(extype_options)}")
   end
 end

src/elixir.proto

@@ -0,0 +1,24 @@
+syntax = "proto2";
+
+// we should call just elixirpb and merge with Tony's
+// https://github.com/tony612/protobuf-elixir/blob/e1d4a23baa1f51c2da58db24f58a26dc4d6511b2/src/elixirpb.proto
+package brex.elixir;
+
+import "google/protobuf/descriptor.proto";
+
+// Define an extension to specify the elixir type generated for the given field.
+
+// Field level options
+//
+// For example,
+// google.protobuf.StringValue my_string = 1 [(brex.elixirpb.field).extype="String.t"];
+
+message FieldOptions {
+  // Specify an elixir type to generate for this field. This will override usual type.
+  optional string extype = 1;
+}
+
+extend google.protobuf.FieldOptions {
+  // number to change
+  optional FieldOptions field = 65007;
+}

src/elixirpb.proto

@@ -28,3 +28,4 @@ message FileOptions {
 extend google.protobuf.FileOptions {
   optional FileOptions file = 1047;
 }
+

test/protobuf/dsl_test.exs

@@ -223,4 +223,9 @@ defmodule Protobuf.DSLTest do
     assert msg_props.field_props[4].oneof == 1
     refute msg_props.field_props[5].oneof
   end
+
+  test "Extension use case" do
+    msg_props = TestMsg.Ext.UseCase.__message_props__()
+    assert %Protobuf.FieldProps{options: [extype: "String.t"]} = msg_props.field_props[1]
+  end
 end

test/protobuf/protoc/generator/message_test.exs

@@ -173,7 +173,35 @@ defmodule Protobuf.Protoc.Generator.MessageTest do
     assert msg =~ "field :a, 1, optional: true, type: :int32, deprecated: true\n"
   end
 
-  test "generete/2 supports message type field" do
+  test "generate/2 supports extensions on field options" do
+    ctx = %Context{package: ""}
+
+    opts = Google.Protobuf.FieldOptions.new()
+    custom_opts = Brex.Elixir.FieldOptions.new(extype: "String.t")
+
+    opts =
+      Google.Protobuf.FieldOptions.put_extension(opts, Brex.Elixir.PbExtension, :field, custom_opts)
+
+    desc =
+      Google.Protobuf.DescriptorProto.new(
+        name: "Foo",
+        field: [
+          Google.Protobuf.FieldDescriptorProto.new(
+            name: "my_string",
+            number: 1,
+            type: :TYPE_MESSAGE,
+            # type_name: ".google.protobuf.StringValue",
+            label: :LABEL_OPTIONAL,
+            options: opts
+          )
+        ]
+      )
+
+    {[], [msg]} = Generator.generate(ctx, desc)
+    assert msg =~ "field :my_string, 1, optional: true, type: :message, options: %Brex.Elixir.FieldOptions{extype: \"String.t\"}\n"
+  end
+
+  test "generate/2 supports message type field" do
     ctx = %Context{
       package: "",
       dep_type_mapping: %{

test/support/test_msg.ex

@@ -212,4 +212,16 @@ defmodule TestMsg do
     extend Ext.Foo2, :bar, 1047, optional: true, type: :string
     extend Ext.Foo1, :"Parent.foo", 1048, optional: true, type: Ext.EnumFoo, enum: true
   end
+
+  defmodule Ext.UseCase do
+    @moduledoc false
+    use Protobuf, syntax: :proto3
+
+    @type t :: %__MODULE__{
+            my_string: Google.Protobuf.StringValue.t() | nil
+          }
+    defstruct [:my_string]
+
+    field :my_string, 1, type: Google.Protobuf.StringValue, options: [extype: "String.t"]
+  end
 end