lua: add error autocompletion

Let's add an `__autocomplete` method to enable error autocompletion and
make the error object easier to use. It will suggest error object fields
(including payload fields) and methods.

Closes tarantool#9107

NO_DOC=<not a documentable feature>

changelogs/unreleased/gh-9107-error-autocomplete.md

@@ -0,0 +1,3 @@
+## feature/lua
+
+* Added autocompletion (including payload fields) to `box.error` (gh-9107).

src/lua/error.lua

@@ -173,6 +173,18 @@ local error_methods = {
     ["set_prev"] = error_set_prev;
 }
 
+local function error_autocomplete(err)
+    local err_unpacked = err:unpack()
+    for key, method in pairs(error_methods) do
+        if key:match('__.+') == nil then
+            err_unpacked[key] = method
+        end
+    end
+    return err_unpacked
+end
+
+error_methods["__autocomplete"] = error_autocomplete
+
 local function error_index(err, key)
     local method = error_methods[key]
     if method ~= nil then

test/box-luatest/error_subsystem_improvements_test.lua

@@ -1,4 +1,6 @@
 local t = require('luatest')
+local console = require('console')
+
 local g = t.group()
 
 -- Test the `prev' argument to the table constructor of `box.error.new'
@@ -138,3 +140,39 @@ g.test_payload_inheritance = function()
     t.assert_equals(e3.bar, 33) -- Inherited from e2.
     t.assert_equals(e3.baz, 55) -- Own payload field.
 end
+
+--
+-- Get tab completion for @s
+-- @param s string
+-- @return tab completion
+local function tabcomplete(s)
+    return console.completion_handler(s, 0, #s)
+end
+
+-- Test `box.error` autocompletion (gh-9107).
+g.test_autocomplete = function()
+    -- tabcomplete always uses global table
+    rawset(_G, 'err', box.error.new{'message', foo = 777})
+
+    local r = tabcomplete('err.')
+    t.assert_items_equals(r, {'err.',
+                              'err.base_type',
+                              'err.code',
+                              'err.foo',
+                              'err.match(',
+                              'err.message',
+                              'err.raise(',
+                              'err.set_prev(',
+                              'err.trace',
+                              'err.type',
+                              'err.unpack(',
+                              })
+
+    r = tabcomplete('err:')
+    t.assert_items_equals(r, {'err:',
+                              'err:match(',
+                              'err:raise(',
+                              'err:set_prev(',
+                              'err:unpack(',
+                              })
+end