feat(completion)!: make default LSP item processing to maybe fuzzy match

Details:
- Do fuzzy matching (both filter and sort) of LSP completion items
  (i.e. of 'textDocument/completion' response) if 'completeopt' option
  contains "fuzzy" flag (requires Neovim>=0.11); use previous `'prefix'`
  method otherwise.

  This makes experience more seamless with `completeopt+=fuzzy` and
  servers which might return `isIncomplete=true`. Using same fuzzy
  matching algorithm (`:h matchfuzzy()` and `:h fuzzy-matching`) allows
  typing in common case of "first several `isIncomplete=true` responses
  followed by built-in candidates matching after a `isIncomplete=false`
  response" feel the same.

  One difference is that initial LSP item processing respects both
  `filterText` and `label` fields while built-in completion menu
  filtering will respect only `label` (as there is no concept of
  `filterText` in built-in completion items).

Author: echasnovski

CHANGELOG.md

@@ -44,7 +44,7 @@
     - Show `labelDetails` as a part of the popup menu via `menu` completion item field.
 - BREAKING: prefer to not use functions as default config values. In particular, for `lsp_completion.process_items` (use `nil` with explicit fallback) and `fallback_action` (use `'<C-n>'`). This should not have any user facing effects and marked as breaking only because a structure of a default config has changed to be more aligned with other modules.
 - BREAKING FEATURE: update behavior and capabilities of `default_process_items()`:
-    - Add `filtersort` option to control how items are filtered and/or sorted.
+    - Add `filtersort` option to control how items are filtered and/or sorted. Its default value has new (breaking) value: do fuzzy matching if 'completeopt' option contains "fuzzy" entry; same as before otherwise.
     - Use `filterText` and `label` item fields during matching (instead of `textEdit.newText`, `insertText`, and `label` as before). This is more aligned with LSP specification.
 
 ## mini.doc

doc/mini-completion.txt

@@ -361,7 +361,7 @@ Parameters ~
       - `'fuzzy'` - filter and sort with |matchfuzzy| using `filterText`.
       - `'none'` - no filter and no sort.
     If callable, should take `items` and `base` arguments and return items array.
-    Default: `'prefix'`.
+    Default: `'fuzzy'` if 'completeopt' contains "fuzzy", `'prefix'` otherwise.
 
 Return ~
 `(table)` Array of processed items from LSP response.

lua/mini/completion.lua

@@ -482,14 +482,14 @@ end
 ---       - `'fuzzy'` - filter and sort with |matchfuzzy| using `filterText`.
 ---       - `'none'` - no filter and no sort.
 ---     If callable, should take `items` and `base` arguments and return items array.
----     Default: `'prefix'`.
+---     Default: `'fuzzy'` if 'completeopt' contains "fuzzy", `'prefix'` otherwise.
 ---
 ---@return table Array of processed items from LSP response.
 MiniCompletion.default_process_items = function(items, base, opts)
   opts = opts or {}
 
   -- NOTE: custom filter+sort is important with frequent `isIncomplete`
-  local fs = opts.filtersort or 'prefix'
+  local fs = opts.filtersort or (vim.o.completeopt:find('fuzzy') and 'fuzzy' or 'prefix')
   if type(fs) == 'string' then fs = H.filtersort_methods[fs] end
   if not vim.is_callable(fs) then H.error('`filtersort` should be callable or one of "prefix", "fuzzy", "none"') end
   local res = fs(items, base)

tests/test_completion.lua

@@ -291,6 +291,11 @@ local ref_fuzzy_items = {
 T['default_process_items()']['works'] = function()
   -- Should use 'prefix' filtersort if no 'fuzzy' in 'completeopt'
   eq(child.lua_get('MiniCompletion.default_process_items(_G.items, "M")'), ref_prefix_items)
+
+  -- Should use 'fuzzy' filtersort if there is 'fuzzy' in 'completeopt'
+  if child.fn.has('nvim-0.11') == 0 then MiniTest.skip("Only Neovim>=0.11 has 'fuzzy' flag in 'completeopt'") end
+  child.o.completeopt = 'menuone,noselect,fuzzy'
+  eq(child.lua_get('MiniCompletion.default_process_items(_G.items, "l")'), ref_fuzzy_items)
 end
 
 T['default_process_items()']["highlights LSP kind if 'mini.icons' is enabled"] = function()