Specify -index 0 in type-enclosing queries.

[voodoos]

This change greatly improves performances in some cases where Merlin would spend a lot of time formatting big types that were discarded by ocaml-lsp anyway.

This fixes type-enclosing taking 10s in Irmin:

❯ ocamlmerlin single type-enclosing -position 146:39 -filename src/irmin-pack/mem/irmin_pack_mem.ml <src/irmin-pack/mem/irmin_pack_mem.ml | jq '.timing'
{
  "clock": 11399,
  "cpu": 11321,
  "query": 11153,
  "pp": 0,
  "reader": 1,
  "ppx": 23,
  "typer": 144,
  "error": 0
}

❯ ocamlmerlin single type-enclosing -position 146:39 -filename src/irmin-pack/mem/irmin_pack_mem.ml -index 0 <src/irmin-pack/mem/irmin_pack_mem.ml | jq '.timing'
{
  "clock": 238,
  "cpu": 183,
  "query": 17,
  "pp": 0,
  "reader": 1,
  "ppx": 22,
  "typer": 142,
  "error": 0
}

This does not fix every issue on Irmin's codebase: hovering on some modules causes ocaml-lsp to hang at 100% cpu.
This happens for example with this query, which is instantaneous in Merlin:

❯ ocamlmerlin single type-enclosing -position 43:40 -index 0 -filename src/irmin-pack/mem/irmin_pack_mem.ml <src/irmin-pack/mem/irmin_pack_mem.ml | jq '.timing'
{
  "clock": 263,
  "cpu": 207,
  "query": 40,
  "pp": 0,
  "reader": 1,
  "ppx": 23,
  "typer": 144,
  "error": 0
}

Cancellation is not working and the process must be manually killed.

I will open an issue for this, it is independent from the problem solved by that PR that should increase performance without drawbacks in many cases.

[rgrinberg]

formatting big types that were discarded by ocaml-lsp anyway.

Formatting as in pretty printing? I'm curious how this can be so slow.

By the way, is it possible to add a test that demonstrates there's a change in behavior?

[voodoos]

Formatting as in pretty printing? I'm curious how this can be so slow.

I agree that this is surprising, Merlin's relies on Printtyp to pretty print.

By the way, is it possible to add a test that demonstrates there's a change in behavior?

There is no change in behavior since ocaml-lsp only uses the first result in the list:

https://github.com/ocaml/ocaml-lsp/blob/master/ocaml-lsp-server/src/document.ml#L306-L311