How to enter automatic indentation after ":"

Neovim
1

(The following is Google Translate, please forgive me for the misconduct)
How do I automatically indent after “:”, like the following,

if true: |

CR

if true:
    |
2

The smalest way to do that (replace 4 with wanted indenation level):
:set indentexpr=(getline(v:lnum-1)=~':$')*4