fix(lsp): when prefix is non word add all result into matches (#30044)

Problem: prefix can be a symbol like period, the fuzzy matching can't
handle it correctly.

Solution: when prefix is empty or a symbol add all lsp completion
result into matches.

runtime/lua/vim/lsp/completion.lua

@@ -238,7 +238,7 @@ function M._lsp_to_complete_items(result, prefix, client_id)
 
   ---@type fun(item: lsp.CompletionItem):boolean
   local matches
-  if prefix == '' then
+  if not prefix:find('%w') then
     matches = function(_)
       return true
     end

test/functional/plugin/lsp/completion_spec.lua

@@ -18,35 +18,36 @@ local create_server_definition = t_lsp.create_server_definition
 ---@param candidates lsp.CompletionList|lsp.CompletionItem[]
 ---@param lnum? integer 0-based, defaults to 0
 ---@return {items: table[], server_start_boundary: integer?}
-local function complete(line, candidates, lnum)
+local function complete(line, candidates, lnum, server_boundary)
   lnum = lnum or 0
   -- nvim_win_get_cursor returns 0 based column, line:find returns 1 based
   local cursor_col = line:find('|') - 1
   line = line:gsub('|', '')
   return exec_lua(
     [[
-    local line, cursor_col, lnum, result = ...
+    local line, cursor_col, lnum, result, server_boundary = ...
     local line_to_cursor = line:sub(1, cursor_col)
     local client_start_boundary = vim.fn.match(line_to_cursor, '\\k*$')
-    local items, server_start_boundary = require("vim.lsp.completion")._convert_results(
+    local items, new_server_boundary = require("vim.lsp.completion")._convert_results(
       line,
       lnum,
       cursor_col,
       1,
       client_start_boundary,
-      nil,
+      server_boundary,
       result,
       "utf-16"
     )
     return {
       items = items,
-      server_start_boundary = server_start_boundary
+      server_start_boundary = new_server_boundary
     }
   ]],
     line,
     cursor_col,
     lnum,
-    candidates
+    candidates,
+    server_boundary
   )
 end
 
@@ -162,6 +163,26 @@ describe('vim.lsp.completion: item conversion', function()
     eq(expected, result)
   end)
 
+  it('works on non word prefix', function()
+    local completion_list = {
+      { label = ' foo', insertText = '->foo' },
+    }
+    local result = complete('wp.|', completion_list, 0, 2)
+    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 },