Enum UnderlineStyle 

#[repr(u8)]
pub enum UnderlineStyle {
    None = 0,
    Single = 1,
    Double = 2,
    Curly = 3,
    Dotted = 4,
    Dashed = 5,
}

How a cell’s underline is drawn — SGR 4 : Ps.

This is the storage, and None is a member of it. There is no second boolean saying whether the cell is underlined: CellFlags::UNDERLINE survives as a derived view bit so existing consumers keep working, and the style is its only writer. So the two cannot disagree — which is not a stylistic preference but the defect three of the four references demonstrably pay for. alacritty’s display layer inserts a plain UNDERLINE over a cell that already carries a curl and its renderer draws both rects; xterm.js has two readers that resolve the same conflict in opposite directions, pinned by a test; xterm leaves both bits set after CSI 4m; CSI 21m and lets each consumer pick a resolution. ghostty is the one where it is not representable — its underline is an enum(u3) with none among the members and there is no underline: bool — and that is the shape here.

Stored in the content word (bits 26..=28), not in a row side map: the packed cell is not full, so the side-map invariant does not apply, and packing is what makes the style ride along for free everywhere a cell moves — reflow, scroll, and the blank a wrapping wide glyph leaves behind. One path needed an explicit carry and is worth naming rather than counting in: promote_cluster_to_wide / relocate_cluster_wide synthesise the pair’s spacer from the pen rather than moving it, so they take the style from the lead the same way they already take its extended attrs (ADR-0025 D4). A refuting pass found that one; the other three are free.

Variants

None = 0

Not underlined. The absence is a member rather than a separate flag.

Single = 1

SGR 4 or 4:1 — one straight line.

Double = 2

Two straight lines — 4:2, and also the legacy SGR 21, which is the only value with a second spelling. Both land on this field, so SGR 24 clears either of them.

The legacy form is not unanimous in the prior art and the spec is what settles it: vte reads 21 as cancel bold, so an application meaning “stop bold” gets a double underline here and keeps its bold. SGR 22 is the arm that cancels bold.

Curly = 3

4:3 — a curl.

Dotted = 4

4:4 — a dotted line. Drawn with a whole number of dots per cell, so the pattern does not restart at a cell boundary.

Dashed = 5

4:5 — a dashed line. One period per cell, with the dash split across the boundary so adjacent cells’ dashes join.

Trait Implementations

impl Clone for UnderlineStyle

fn clone(&self) -> Self

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for UnderlineStyle

impl Debug for UnderlineStyle

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for UnderlineStyle

fn default() -> Self

Returns the “default value” for a type.

impl Eq for UnderlineStyle

impl Hash for UnderlineStyle

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl PartialEq for UnderlineStyle

fn eq(&self, other: &Self) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl StructuralPartialEq for UnderlineStyle