Document

doc.blocks
    .iter()
    .filter_map(|block| match &block.kind {
        BlockKind::Heading { level, text } => Some((*level, text.text.clone())),
        _ => None,
    })
    .collect()

let marks = markdown::Marks::new().with("highlight", "==").with("underline", "++");

let doc = markdown::parse_with("a ==lit== word", &marks);
assert_eq!(markdown::serialize_with(&doc, &marks), "a ==lit== word");

// markdown — pure, so the registry is a parameter rather than a global.

pub fn parse_with(source: &str, marks: &Marks) -> Doc;
pub fn serialize_with(doc: &Doc, marks: &Marks) -> String;

/// The pair a caret crosses on. Both put a sentinel where the caret is, so
/// neither can drift from the serializer or parser it rides on.
pub fn parse_at(source: &str, offset: usize, marks: &Marks) -> (Doc, Cursor);
pub fn serialize_at(doc: &Doc, at: Cursor, marks: &Marks) -> (String, usize);

// The gpui-side half the editing surface reads. `set_mark_paint` is paint only
// — colour, background, weight, italic, underline, strike. A name nothing
// paints round trips and reads as the text it wraps.
pub fn set_marks(cx: &mut App, marks: Marks);
pub fn set_mark_paint(cx: &mut App, paint: Painter);

/// Classifies markdown *source* without a grammar, which is what gives a
/// source view colour in a browser build.
pub fn source_spans(source: &str) -> Vec<(Range<usize>, HighlightKind)>;

/// A long fence line wraps by default, since the caret reads a fence in the
/// editor and a sideways scroller can hold it off the right edge.
pub fn set_layout(cx: &mut App, layout: Layout);

// ...