Constant SYNTHESIZED_FILE_ID 

pub const SYNTHESIZED_FILE_ID: u16 = u16::MAX; // 65_535u16

Sentinel file_id indicating a directive was synthesized by a plugin rather than parsed from a source file.

Regular source files get sequential IDs starting at 0 (see rustledger_loader::SourceMap::add_file), so this sentinel is safely out of the normal range. Code that formats error locations or looks up files in a SourceMap should treat this as “no source location” and, where appropriate, hint to the user that a plugin generated the directive.

See issue #896.