Function prelude_user_offset 

pub fn prelude_user_offset(
    segments: &[PreludeSegment],
    pos: usize,
) -> Option<usize>

The offset in the author’s own text that byte pos of the prelude-expanded text came from, through the runs crate::loader::apply_prelude_mapped reports. A position inside spliced stdlib text came from no byte the author wrote and maps to None; a run’s END maps too, so an exclusive span end and the end of the file translate.