fix(lsp): use filterText as word if textEdit/label doesn't match

Problem:

With language servers like lemminx, completing xml tags like `<mo` first
shows the right candidates (`modules`) but after typing `d` the
candidates disappear.

This is because the server returns:

    [...]
    filterText = "<module",
    label = "module",
    textEdit = {
      newText = "<module>$1</module>$0",

Which resulted in `module` being used as `word`, and `module` doesn't
match the prefix `<mo`. Typing `d` causes the `complete()` filtering
mechanism to kick in and remove the entry.

Solution:

Use `<module` from the `filterText` as `word` if the textEdit/label
heuristic doesn't match.

runtime/lua/vim/lsp/completion.lua

@@ -127,8 +127,10 @@ end
 --- See https://microsoft.github.io/language-server-protocol/specifications/specification-current/#textDocument_completion
 ---
 --- @param item lsp.CompletionItem
+--- @param prefix string
+--- @param match fun(text: string, prefix: string):boolean
 --- @return string
-local function get_completion_word(item)
+local function get_completion_word(item, prefix, match)
   if item.insertTextFormat == protocol.InsertTextFormat.Snippet then
     if item.textEdit then
       -- Use label instead of text if text has different starting characters.
@@ -146,7 +148,12 @@ local function get_completion_word(item)
       --
       -- Typing `i` would remove the candidate because newText starts with `t`.
       local text = parse_snippet(item.insertText or item.textEdit.newText)
-      return #text < #item.label and vim.fn.matchstr(text, '\\k*') or item.label
+      local word = #text < #item.label and vim.fn.matchstr(text, '\\k*') or item.label
+      if item.filterText and not match(word, prefix) then
+        return item.filterText
+      else
+        return word
+      end
     elseif item.insertText and item.insertText ~= '' then
       return parse_snippet(item.insertText)
     else
@@ -276,7 +283,7 @@ function M._lsp_to_complete_items(result, prefix, client_id)
   local user_convert = vim.tbl_get(buf_handles, bufnr, 'convert')
   for _, item in ipairs(items) do
     if matches(item) then
-      local word = get_completion_word(item)
+      local word = get_completion_word(item, prefix, match_item_by_value)
       local hl_group = ''
       if
         item.deprecated

test/functional/plugin/lsp/completion_spec.lua

@@ -216,6 +216,38 @@ describe('vim.lsp.completion: item conversion', function()
       })
     end)
 
+    it('uses filterText as word if label/newText would not match', function()
+      local items = {
+        {
+          filterText = '<module',
+          insertTextFormat = 2,
+          kind = 10,
+          label = 'module',
+          sortText = 'module',
+          textEdit = {
+            newText = '<module>$1</module>$0',
+            range = {
+              start = {
+                character = 0,
+                line = 0,
+              },
+              ['end'] = {
+                character = 0,
+                line = 0,
+              },
+            },
+          },
+        },
+      }
+      local expected = {
+        {
+          abbr = 'module',
+          word = '<module',
+        },
+      }
+      assert_completion_matches('<mo', items, expected)
+    end)
+
     it('fuzzy matches on label when filterText is missing', function()
       assert_completion_matches('fo', {
         { label = 'foo' },