Bug Report: to_string is not defined when using bad formated hosts inventory

[Yannig]

Ansible Version:

last devel

Ansible Configuration:

none

Environment:

none

Summary:

When using user@host inventory, Ansible try to call to_string and crash

Steps To Reproduce:

ansible -m debug -i test@127.0.0.1, all

Expected Results:

test@127.0.0.1 | SUCCESS => {
    "msg": "Hello world!"
}

Actual Results:

Unexpected Exception: global name 'to_string' is not defined
to see the full traceback, use -vvv