Function stream_lines 

pub fn stream_lines<F>(path: &Path, line_fn: F) -> Result<usize>
where
    F: FnMut(usize, &str) -> Result<()>,

Processes a file by reading line-by-line with constant memory.

Calls line_fn for each line. The line buffer is reused across iterations — memory does not grow with file length.

Errors

Returns an error if the file cannot be read.