Constant OLD_SOURCE 

pub const OLD_SOURCE: usize = _; // 18_446_744_073_709_551_614usize

The source number a column of a trigger’s OLD row carries.

Like EXCLUDED_SOURCE, it is not a FROM term: OLD and NEW are the row the write is about, which the compiler already holds in registers by the time a trigger fires. Numbering them where no real source can reach means a compiler that forgot to substitute one is caught by the verifier rather than quietly reading whatever cursor happened to be open.