Function shift_position 

pub fn shift_position(
    p: Position,
    edit: &Edit,
    line_len: impl Fn(usize) -> usize,
    rows: usize,
) -> Option<Position>

Rewrite p so it still points at the same text after edit lands, or None when the edit’s geometry is not modelled and the position must be dropped rather than guessed.

line_len returns the pre-edit char length of a row, and rows the pre-edit row count. Only the row-restructuring edits consult them.

Units

Works in char columns, which is what Edit and View::cursor both speak. Deliberately not expressed over crate::types::Selection, whose Pos::col counts graphemes: doing this arithmetic in grapheme columns would silently mis-shift every position sitting after a multi-byte character. Converting between the two units needs the buffer, so it belongs at the call boundary, not here.