Great :)

So at a high level, this will work as follow: Dune will install a file <libdir>/toplevel/dune. This file will invoke dune in the current directory in order to:

build all the relevant libraries
get a list of include directories and cma files to load

We can make dune print a file that the toplevel will evaluate. For instance, we could run dune toplevel-init-file, capture its output and use Toploop.use_file to evaluate it.

To add a toplevel-init-file command, you can add a file bin/toplevel_init_file.ml with the following contents:

open Stdune
open Dune
open Import
open Fiber.O

let doc = "..."

let man =
  [ `S "DESCRIPTION"
  ; `P "..."
  ; `Blocks Common.help_secs
  ]

let info = Term.info "toplevel-init-file" ~doc ~man

let term =
  let+ common = Common.term  in
  Common.set_common common ~targets:[];
  Scheduler.go ~common (fun () ->
    let* setup = Import.Main.setup common in
    let sctx =
      Context_name.Map.find setup.super_contexts "default"
      |> Option.value_exn
    in
    let dir =
      Path.Build.relative
        (Super_context.build_dir sctx)
        (Common.prefix_target common dir)
    in
    ... (* insert code similar to the one in the [setup] function in
           [src/dune/utop] to compute [requires] *)
    let include_paths = Lib.L.include_paths requires in
    let files = Lib.L.link_deps requires ~mode:Link_mode.Byte in
    let* () = do_build (Build.paths files) in
    let files_to_load =
      List.filter files ~f:(fun p ->
        match Path.Build.extension p with
          | ".cma" | ".cmo" -> true
          | _ -> false)
    in
    Toplevel.print_toplevel_init_file ~include_paths ~files_to_load;
    Fiber.return ())

let command = (term, info)

Where:

Lib.L.link_deps is a new function to write that returns the list of files needed when linking. You can take inspiration from Lib.L.compile_and_link_flags for this, it's pretty much the same except that we want to return just files rather than command line arguments
Toplevel.print_toplevel_init_file is a new function to write that print the file to evaluate in the toplevel, composed of #directory and #load directives

Then you just have to add Toplevel_init.file.command to the all list in bin/main.ml.

Add support for dune rtop alongside dune utop #2694

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Add support for `dune rtop` alongside `dune utop` #2694