//! Width guards for transcript rendering with fixed prefix columns.//!//! Several rendering paths reserve a fixed number of columns for bullets,//! gutters, or labels before laying out content.  When the terminal is very//! narrow, those reserved columns can consume the entire width, leaving zero//! or negative space for content.//!//! These helpers centralise the subtraction and enforce a strict-positive//! contract: they return `Some(n)` where `n > 0`, or `None` when no usable//! content width remains.  Callers treat `None` as "render prefix-only//! fallback" rather than attempting wrapped rendering at zero width, which//! would produce empty or unstable output./// Returns usable content width after reserving fixed columns.////// Guarantees a strict positive width (`Some(n)` where `n > 0`) or `None` when/// the reserved columns consume the full width.////// Treat `None` as "render prefix-only fallback". Coercing it to `0` and still/// attempting wrapped rendering often produces empty or unstable output at very/// narrow terminal widths.pub(crate) fn usable_content_width(total_width: usize, reserved_cols: usize) -> Option<usize> {    total_width        .checked_sub(reserved_cols)        .filter(|remaining| *remaining > 0)}/// `u16` convenience wrapper around [`usable_content_width`].////// This keeps width math at callsites that receive terminal dimensions as/// `u16` while preserving the same `None` contract for exhausted width.pub(crate) fn usable_content_width_u16(total_width: u16, reserved_cols: u16) -> Option<usize> {    usable_content_width(usize::from(total_width), usize::from(reserved_cols))}#[cfg(test)]mod tests {    use super::*;    use pretty_assertions::assert_eq;    #[test]    fn usable_content_width_returns_none_when_reserved_exhausts_width() {        assert_eq!(            usable_content_width(/*total_width*/ 0, /*reserved_cols*/ 0),            None        );        assert_eq!(            usable_content_width(/*total_width*/ 2, /*reserved_cols*/ 2),            None        );        assert_eq!(            usable_content_width(/*total_width*/ 3, /*reserved_cols*/ 4),            None        );        assert_eq!(            usable_content_width(/*total_width*/ 5, /*reserved_cols*/ 4),            Some(1)        );    }    #[test]    fn usable_content_width_u16_matches_usize_variant() {        assert_eq!(            usable_content_width_u16(/*total_width*/ 2, /*reserved_cols*/ 2),            None        );        assert_eq!(            usable_content_width_u16(/*total_width*/ 5, /*reserved_cols*/ 4),            Some(1)        );    }}