Function build_preview 

pub fn build_preview<S: AsRef<str>>(
    pattern: &FindPattern,
    rows: impl Iterator<Item = S>,
    replacement: &str,
    current: Option<(usize, usize)>,
) -> Preview

Build the replace preview: every match replaced, plus where each replacement landed so the renderer can colour it.

current is the buffer-coordinate (row, char_col) start of the current match, so the span covering it can be flagged. It is passed in buffer coordinates because that is what the editor knows; the remap to preview coordinates happens here, where the length delta is being accumulated anyway.

The line count never changes — the pattern cannot span a newline and the replace field is single-line — which is why callers may keep their scroll offset and row indices across a preview.