How to turn off the dashlines indicating indent areas in insert mode?

[hopezh]

As highlighted by the arrow in the image below, three dash lines --- show up in the indent area entering the insert mode. (Not related to the space_char_blanklin option which sets the char for indent spaces in normal mode.)

May I ask if this is related to indent-blankline? And, if it is, which option I shall use to disable it?

Thanks.

[lukas-reineke]

This is not indent-blankline, looks like :help lcs-trail maybe?