Constant EXCLUDED_SOURCE 

pub const EXCLUDED_SOURCE: usize = usize::MAX; // 18_446_744_073_709_551_615usize

The source number a column of an upsert’s excluded row carries.

It is not a FROM term: excluded is the row the INSERT was about to write, which lives in registers rather than under a cursor. Giving it a number no real source can have means the compiler must substitute it - and a compiler that forgot to would try to open a cursor two billion and be refused by the verifier, rather than reading the wrong row.