After a public, then private discussion with @rgrinberg , I'm submitting this issue about dune utop. dune utop is very handy for our students but it's slow (> 5s on my 2019 laptop for a project only printing "hello world") when it needs to be rebuilt. The slow part is the linking phase building an ad hoc utop.exe.

While dune utop is perhaps (?) not that useful to an everyday developer, this is the command our students use the most and it seems important, in this respect, to make it fast.

@rgrinberg suggested using TM=$(mktemp) && dune ocaml top > "$TM" && utop -init "$TM" to have a fast experience. If this is indeed equivalent to dune utop in all cases, it might be interesting to redesign dune utop to perform these steps rather than rebuilding utop.exe.