fix(lsp): pre-filter matches on label if filterText is missing (#29491)

Although the built-in pum completion mechanism will filter anyway on the
next input it is odd if the initial popup shows entries which don't
match the current prefix.

Using fuzzy match on the label/prefix is compatible with
`completeopt+=fuzzy` and also doesn't seem to break postfix snippet
cases

Closes https://github.com/neovim/neovim/issues/29287

runtime/lua/vim/lsp/completion.lua

@@ -235,14 +235,20 @@ function M._lsp_to_complete_items(result, prefix, client_id)
     return {}
   end
 
-  local matches = prefix == '' and function()
-    return true
-  end or function(item)
-    if item.filterText then
-      return next(vim.fn.matchfuzzy({ item.filterText }, prefix))
+  ---@type fun(item: lsp.CompletionItem):boolean
+  local matches
+  if prefix == '' then
+    matches = function(_)
+      return true
+    end
+  else
+    ---@param item lsp.CompletionItem
+    matches = function(item)
+      local text = item.filterText or item.label
+      return next(vim.fn.matchfuzzy({ text }, prefix)) ~= nil
     end
-    return true
   end
+
   local candidates = {}
   for _, item in ipairs(items) do
     if matches(item) then

test/functional/plugin/lsp/completion_spec.lua

@@ -141,6 +141,27 @@ describe('vim.lsp.completion: item conversion', function()
     eq(expected, result)
   end)
 
+  it('filters on label if filterText is missing', function()
+    local completion_list = {
+      { label = 'foo' },
+      { label = 'bar' },
+    }
+    local result = complete('fo|', completion_list)
+    local expected = {
+      {
+        abbr = 'foo',
+        word = 'foo',
+      },
+    }
+    result = vim.tbl_map(function(x)
+      return {
+        abbr = x.abbr,
+        word = x.word,
+      }
+    end, result.items)
+    eq(expected, result)
+  end)
+
   it('trims trailing newline or tab from textEdit', function()
     local range0 = {
       start = { line = 0, character = 0 },