Function migration_begin_line 

pub fn migration_begin_line(id: &str, description: &str) -> String

Migration ids (m.id(), e.g. "0020_agent_color_backfill") are a fixed, engineer-chosen identifier format with no spaces — never free text — so splitting id:<id> description:<text> on the first space is safe and exact, unlike description itself which is flattened via one_line.