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 (#829).

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 (#830). 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. The mark #829 carries end to end.

Dotted = 4

4:4 — a dotted line (#830). 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 (#830). 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) -> UnderlineStyle

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() -> UnderlineStyle

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: &UnderlineStyle) -> bool

Equality operator ==.

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

Inequality operator !=.

impl StructuralPartialEq for UnderlineStyle