Fully utilize autocompletion ordering for hrsh7th/nvim-cmp

[rochala]

Hello everyone!
As you know, nvim-metals uses Language Server Protocol to provide IDE capabilities.
The one I'm going to create discussion about is completion with specifications available here.

nvim-metals plugin is not responsible for displaying the tooltips, only fetching them from the server. This is where things get interesting. LSP provides a way to return completion order by setting the sortText string value. If it is undefined, all ordering and sorting is handled by the client, in this case by hrsh7th/nvim-cmp plugin.

The default sorting configuration matches the completions in the following way:

    sorting = {
      priority_weight = 2,
      comparators = {
        compare.offset,
        compare.exact,
        compare.score,
        compare.recently_used,
        compare.kind,
        compare.sort_text,
        compare.length,
        compare.order,
      },
    },

As you see, the sort_text is present in this configuration and by default it should work as intended... but it isn't.

As an example, we will check the completion is build.sbt file:

And here the package is a preselected value. If we check the ordering returned from metals we can see that it returns package with sortText = "00000" so it should be our first result.

[Trace - 04:59:28 PM] Sending response 'textDocument/completion - (4)'. Processing request took 400ms
Result: {
  "isIncomplete": true,
  "items": [
    {
      "label": "`package`: TaskKey[File]",
      "kind": 12,
      "detail": ": TaskKey[File]",
      "preselect": true,
      "sortText": "00000",
      "filterText": "package",
      "insertTextFormat": 2,
      "textEdit": {
        "range": {
          "start": {
            "line": 10,
            "character": 0
          },
          "end": {
            "line": 10,
            "character": 1
          }
        },
        "newText": "`package`"
      },
      "data": {
        "symbol": "sbt/Keys.package.",
        "target": "file:/Users/jrochala/Projects/playground/project/?id\u003dplayground-build",
        "kind": 0
      }
    },

So let's get a quick summary before going further:

• completions are not ordered properly,
• this leads to a preselected item to being in the middle of the completion list, which is super annoying

Let's fix this, so we can have the correct ordering from the server:

For this, we can't easily order only by sortText as metals optimizes the calls to the server.
In short, to not recompute file on every letter typed, the response from completion also contains the isIncomplete field. It only does recompute when it is true, and usually 3 letters are enough to get all available completions and further letters typed no longer are sorted by server...
This is what happens after the 4th letter:

Screen.Recording.2023-02-27.at.17.13.07.mov

As you can see, our selection randomly jumped to the middle of the list. This is caused because our ordering has higher precedence of compare.offset than our compare.sort_text and it doesn't refresh preselect!

The fix is to add the following lines to your nvim-cmp configuration (it will affect other LSP configurations, but I leave it to you to figure out how to enable it only for metals):

cmp.setup({
  preselect = cmp.PreselectMode.None, -- disable preselection
  sorting = {
    priority_weight = 2,
    comparators = {
      compare.offset, -- we still want offset to be higher to order after 3rd letter
      compare.score, -- same as above
      compare.sort_text, -- add higher precedence for sort_text, it must be above `kind`
      compare.recently_used,
      compare.kind,
      compare.length,
      compare.order,
    },
  },
  -- if you want to add preselection you have to set completeopt to new values
  completion = {
    -- completeopt = 'menu,menuone,noselect', <---- this is default value,
    completeopt = 'menu,menuone', -- remove noselect
  },
})

In my opinion, this is not a bug in hrsh7th/nvim-cmp but rather a problem with the unique metals client implementation. This should improve your completions.

[ckipp01]

Thanks a lot for this @rochala!