Move layout traits into library

1 files changed, 789 insertions, 0 deletions

diff --git a/src/library/layout/mod.rs b/src/library/layout/mod.rs
index 02276f22..000cb212 100644
--- a/src/library/layout/mod.rs
+++ b/src/library/layout/mod.rs
@@ -23,3 +23,792 @@ pub use place::*;
 pub use spacing::*;
 pub use stack::*;
 pub use transform::*;
+
+use std::mem;
+
+use comemo::Tracked;
+use typed_arena::Arena;
+
+use crate::diag::SourceResult;
+use crate::frame::Frame;
+use crate::geom::*;
+use crate::library::structure::{DocNode, ListItem, ListNode, DESC, ENUM, LIST};
+use crate::library::text::{
+    LinebreakNode, ParChild, ParNode, ParbreakNode, SmartQuoteNode, SpaceNode, TextNode,
+};
+use crate::model::{
+    capability, Barrier, Content, Interruption, Node, SequenceNode, Show, StyleChain,
+    StyleEntry, StyleMap, StyleVec, StyleVecBuilder, StyledNode, Target,
+};
+use crate::World;
+
+/// The root-level layout.
+#[capability]
+pub trait Layout: 'static + Sync + Send {
+    /// Layout into one frame per page.
+    fn layout(&self, world: Tracked<dyn World>) -> SourceResult<Vec<Frame>>;
+}
+
+impl Layout for Content {
+    #[comemo::memoize]
+    fn layout(&self, world: Tracked<dyn World>) -> SourceResult<Vec<Frame>> {
+        let styles = StyleChain::with_root(&world.config().styles);
+        let scratch = Scratch::default();
+
+        let mut builder = Builder::new(world, &scratch, true);
+        builder.accept(self, styles)?;
+
+        let (doc, shared) = builder.into_doc(styles)?;
+        doc.layout(world, shared)
+    }
+}
+
+/// Block-level layout.
+#[capability]
+pub trait LayoutBlock: 'static + Sync + Send {
+    /// Layout into one frame per region.
+    fn layout_block(
+        &self,
+        world: Tracked<dyn World>,
+        regions: &Regions,
+        styles: StyleChain,
+    ) -> SourceResult<Vec<Frame>>;
+}
+
+impl LayoutBlock for Content {
+    #[comemo::memoize]
+    fn layout_block(
+        &self,
+        world: Tracked<dyn World>,
+        regions: &Regions,
+        styles: StyleChain,
+    ) -> SourceResult<Vec<Frame>> {
+        if let Some(node) = self.to::<dyn LayoutBlock>() {
+            let barrier = StyleEntry::Barrier(Barrier::new(self.id()));
+            let styles = barrier.chain(&styles);
+            return node.layout_block(world, regions, styles);
+        }
+
+        let scratch = Scratch::default();
+        let mut builder = Builder::new(world, &scratch, false);
+        builder.accept(self, styles)?;
+        let (flow, shared) = builder.into_flow(styles)?;
+        flow.layout_block(world, regions, shared)
+    }
+}
+
+/// Inline-level layout.
+#[capability]
+pub trait LayoutInline: 'static + Sync + Send {
+    /// Layout into a single frame.
+    fn layout_inline(
+        &self,
+        world: Tracked<dyn World>,
+        regions: &Regions,
+        styles: StyleChain,
+    ) -> SourceResult<Vec<Frame>>;
+}
+
+impl LayoutInline for Content {
+    #[comemo::memoize]
+    fn layout_inline(
+        &self,
+        world: Tracked<dyn World>,
+        regions: &Regions,
+        styles: StyleChain,
+    ) -> SourceResult<Vec<Frame>> {
+        if let Some(node) = self.to::<dyn LayoutInline>() {
+            let barrier = StyleEntry::Barrier(Barrier::new(self.id()));
+            let styles = barrier.chain(&styles);
+            return node.layout_inline(world, regions, styles);
+        }
+
+        if let Some(node) = self.to::<dyn LayoutBlock>() {
+            let barrier = StyleEntry::Barrier(Barrier::new(self.id()));
+            let styles = barrier.chain(&styles);
+            return node.layout_block(world, regions, styles);
+        }
+
+        let scratch = Scratch::default();
+        let mut builder = Builder::new(world, &scratch, false);
+        builder.accept(self, styles)?;
+        let (flow, shared) = builder.into_flow(styles)?;
+        flow.layout_block(world, regions, shared)
+    }
+}
+
+/// A sequence of regions to layout into.
+#[derive(Debug, Clone, Hash)]
+pub struct Regions {
+    /// The (remaining) size of the first region.
+    pub first: Size,
+    /// The base size for relative sizing.
+    pub base: Size,
+    /// The height of followup regions. The width is the same for all regions.
+    pub backlog: Vec<Abs>,
+    /// The height of the final region that is repeated once the backlog is
+    /// drained. The width is the same for all regions.
+    pub last: Option<Abs>,
+    /// Whether nodes should expand to fill the regions instead of shrinking to
+    /// fit the content.
+    pub expand: Axes<bool>,
+}
+
+impl Regions {
+    /// Create a new region sequence with exactly one region.
+    pub fn one(size: Size, base: Size, expand: Axes<bool>) -> Self {
+        Self {
+            first: size,
+            base,
+            backlog: vec![],
+            last: None,
+            expand,
+        }
+    }
+
+    /// Create a new sequence of same-size regions that repeats indefinitely.
+    pub fn repeat(size: Size, base: Size, expand: Axes<bool>) -> Self {
+        Self {
+            first: size,
+            base,
+            backlog: vec![],
+            last: Some(size.y),
+            expand,
+        }
+    }
+
+    /// Create new regions where all sizes are mapped with `f`.
+    ///
+    /// Note that since all regions must have the same width, the width returned
+    /// by `f` is ignored for the backlog and the final region.
+    pub fn map<F>(&self, mut f: F) -> Self
+    where
+        F: FnMut(Size) -> Size,
+    {
+        let x = self.first.x;
+        Self {
+            first: f(self.first),
+            base: f(self.base),
+            backlog: self.backlog.iter().map(|&y| f(Size::new(x, y)).y).collect(),
+            last: self.last.map(|y| f(Size::new(x, y)).y),
+            expand: self.expand,
+        }
+    }
+
+    /// Whether the first region is full and a region break is called for.
+    pub fn is_full(&self) -> bool {
+        Abs::zero().fits(self.first.y) && !self.in_last()
+    }
+
+    /// Whether the first region is the last usable region.
+    ///
+    /// If this is true, calling `next()` will have no effect.
+    pub fn in_last(&self) -> bool {
+        self.backlog.len() == 0 && self.last.map_or(true, |height| self.first.y == height)
+    }
+
+    /// Advance to the next region if there is any.
+    pub fn next(&mut self) {
+        if let Some(height) = (!self.backlog.is_empty())
+            .then(|| self.backlog.remove(0))
+            .or(self.last)
+        {
+            self.first.y = height;
+            self.base.y = height;
+        }
+    }
+
+    /// An iterator that returns the sizes of the first and all following
+    /// regions, equivalently to what would be produced by calling
+    /// [`next()`](Self::next) repeatedly until all regions are exhausted.
+    /// This iterater may be infinite.
+    pub fn iter(&self) -> impl Iterator<Item = Size> + '_ {
+        let first = std::iter::once(self.first);
+        let backlog = self.backlog.iter();
+        let last = self.last.iter().cycle();
+        first.chain(backlog.chain(last).map(|&h| Size::new(self.first.x, h)))
+    }
+}
+
+/// Builds a document or a flow node from content.
+struct Builder<'a> {
+    /// The core context.
+    world: Tracked<'a, dyn World>,
+    /// Scratch arenas for building.
+    scratch: &'a Scratch<'a>,
+    /// The current document building state.
+    doc: Option<DocBuilder<'a>>,
+    /// The current flow building state.
+    flow: FlowBuilder<'a>,
+    /// The current paragraph building state.
+    par: ParBuilder<'a>,
+    /// The current list building state.
+    list: ListBuilder<'a>,
+}
+
+/// Temporary storage arenas for building.
+#[derive(Default)]
+struct Scratch<'a> {
+    /// An arena where intermediate style chains are stored.
+    styles: Arena<StyleChain<'a>>,
+    /// An arena where intermediate content resulting from show rules is stored.
+    templates: Arena<Content>,
+}
+
+impl<'a> Builder<'a> {
+    pub fn new(
+        world: Tracked<'a, dyn World>,
+        scratch: &'a Scratch<'a>,
+        top: bool,
+    ) -> Self {
+        Self {
+            world,
+            scratch,
+            doc: top.then(|| DocBuilder::default()),
+            flow: FlowBuilder::default(),
+            par: ParBuilder::default(),
+            list: ListBuilder::default(),
+        }
+    }
+
+    pub fn into_doc(
+        mut self,
+        styles: StyleChain<'a>,
+    ) -> SourceResult<(DocNode, StyleChain<'a>)> {
+        self.interrupt(Interruption::Page, styles, true)?;
+        let (pages, shared) = self.doc.unwrap().pages.finish();
+        Ok((DocNode(pages), shared))
+    }
+
+    pub fn into_flow(
+        mut self,
+        styles: StyleChain<'a>,
+    ) -> SourceResult<(FlowNode, StyleChain<'a>)> {
+        self.interrupt(Interruption::Par, styles, false)?;
+        let (children, shared) = self.flow.0.finish();
+        Ok((FlowNode(children), shared))
+    }
+
+    pub fn accept(
+        &mut self,
+        content: &'a Content,
+        styles: StyleChain<'a>,
+    ) -> SourceResult<()> {
+        if let Some(text) = content.downcast::<TextNode>() {
+            if let Some(realized) = styles.apply(self.world, Target::Text(&text.0))? {
+                let stored = self.scratch.templates.alloc(realized);
+                return self.accept(stored, styles);
+            }
+        } else if let Some(styled) = content.downcast::<StyledNode>() {
+            return self.styled(styled, styles);
+        } else if let Some(seq) = content.downcast::<SequenceNode>() {
+            return self.sequence(seq, styles);
+        } else if content.has::<dyn Show>() {
+            if self.show(&content, styles)? {
+                return Ok(());
+            }
+        }
+
+        if self.list.accept(content, styles) {
+            return Ok(());
+        }
+
+        self.interrupt(Interruption::List, styles, false)?;
+
+        if content.is::<ListItem>() {
+            self.list.accept(content, styles);
+            return Ok(());
+        }
+
+        if self.par.accept(content, styles) {
+            return Ok(());
+        }
+
+        self.interrupt(Interruption::Par, styles, false)?;
+
+        if self.flow.accept(content, styles) {
+            return Ok(());
+        }
+
+        let keep = content
+            .downcast::<PagebreakNode>()
+            .map_or(false, |pagebreak| !pagebreak.weak);
+        self.interrupt(Interruption::Page, styles, keep)?;
+
+        if let Some(doc) = &mut self.doc {
+            doc.accept(content, styles);
+        }
+
+        // We might want to issue a warning or error for content that wasn't
+        // handled (e.g. a pagebreak in a flow building process). However, we
+        // don't have the spans here at the moment.
+        Ok(())
+    }
+
+    fn show(
+        &mut self,
+        content: &'a Content,
+        styles: StyleChain<'a>,
+    ) -> SourceResult<bool> {
+        if let Some(mut realized) = styles.apply(self.world, Target::Node(content))? {
+            let mut map = StyleMap::new();
+            let barrier = Barrier::new(content.id());
+            map.push(StyleEntry::Barrier(barrier));
+            map.push(StyleEntry::Barrier(barrier));
+            realized = realized.styled_with_map(map);
+            let stored = self.scratch.templates.alloc(realized);
+            self.accept(stored, styles)?;
+            Ok(true)
+        } else {
+            Ok(false)
+        }
+    }
+
+    fn styled(
+        &mut self,
+        styled: &'a StyledNode,
+        styles: StyleChain<'a>,
+    ) -> SourceResult<()> {
+        let stored = self.scratch.styles.alloc(styles);
+        let styles = styled.map.chain(stored);
+        let intr = styled.map.interruption();
+
+        if let Some(intr) = intr {
+            self.interrupt(intr, styles, false)?;
+        }
+
+        self.accept(&styled.sub, styles)?;
+
+        if let Some(intr) = intr {
+            self.interrupt(intr, styles, true)?;
+        }
+
+        Ok(())
+    }
+
+    fn interrupt(
+        &mut self,
+        intr: Interruption,
+        styles: StyleChain<'a>,
+        keep: bool,
+    ) -> SourceResult<()> {
+        if intr >= Interruption::List && !self.list.is_empty() {
+            mem::take(&mut self.list).finish(self)?;
+        }
+
+        if intr >= Interruption::Par {
+            if !self.par.is_empty() {
+                mem::take(&mut self.par).finish(self);
+            }
+        }
+
+        if intr >= Interruption::Page {
+            if let Some(doc) = &mut self.doc {
+                if !self.flow.is_empty() || (doc.keep_next && keep) {
+                    mem::take(&mut self.flow).finish(doc, styles);
+                }
+                doc.keep_next = !keep;
+            }
+        }
+
+        Ok(())
+    }
+
+    fn sequence(
+        &mut self,
+        seq: &'a SequenceNode,
+        styles: StyleChain<'a>,
+    ) -> SourceResult<()> {
+        for content in &seq.0 {
+            self.accept(content, styles)?;
+        }
+        Ok(())
+    }
+}
+
+/// Accepts pagebreaks and pages.
+struct DocBuilder<'a> {
+    /// The page runs built so far.
+    pages: StyleVecBuilder<'a, PageNode>,
+    /// Whether to keep a following page even if it is empty.
+    keep_next: bool,
+}
+
+impl<'a> DocBuilder<'a> {
+    fn accept(&mut self, content: &'a Content, styles: StyleChain<'a>) {
+        if let Some(pagebreak) = content.downcast::<PagebreakNode>() {
+            self.keep_next = !pagebreak.weak;
+        }
+
+        if let Some(page) = content.downcast::<PageNode>() {
+            self.pages.push(page.clone(), styles);
+            self.keep_next = false;
+        }
+    }
+}
+
+impl Default for DocBuilder<'_> {
+    fn default() -> Self {
+        Self {
+            pages: StyleVecBuilder::new(),
+            keep_next: true,
+        }
+    }
+}
+
+/// Accepts flow content.
+#[derive(Default)]
+struct FlowBuilder<'a>(CollapsingBuilder<'a, FlowChild>);
+
+impl<'a> FlowBuilder<'a> {
+    fn accept(&mut self, content: &'a Content, styles: StyleChain<'a>) -> bool {
+        // Weak flow elements:
+        // Weakness | Element
+        //    0     | weak colbreak
+        //    1     | weak fractional spacing
+        //    2     | weak spacing
+        //    3     | generated weak spacing
+        //    4     | generated weak fractional spacing
+        //    5     | par spacing
+
+        if let Some(_) = content.downcast::<ParbreakNode>() {
+            /* Nothing to do */
+        } else if let Some(colbreak) = content.downcast::<ColbreakNode>() {
+            if colbreak.weak {
+                self.0.weak(FlowChild::Colbreak, styles, 0);
+            } else {
+                self.0.destructive(FlowChild::Colbreak, styles);
+            }
+        } else if let Some(vertical) = content.downcast::<VNode>() {
+            let child = FlowChild::Spacing(vertical.amount);
+            let frac = vertical.amount.is_fractional();
+            if vertical.weak {
+                let weakness = 1 + u8::from(frac) + 2 * u8::from(vertical.generated);
+                self.0.weak(child, styles, weakness);
+            } else if frac {
+                self.0.destructive(child, styles);
+            } else {
+                self.0.ignorant(child, styles);
+            }
+        } else if content.has::<dyn LayoutBlock>() {
+            let child = FlowChild::Block(content.clone());
+            if content.is::<PlaceNode>() {
+                self.0.ignorant(child, styles);
+            } else {
+                self.0.supportive(child, styles);
+            }
+        } else {
+            return false;
+        }
+
+        true
+    }
+
+    fn par(&mut self, par: ParNode, styles: StyleChain<'a>, indent: bool) {
+        let amount = if indent && !styles.get(ParNode::SPACING_AND_INDENT) {
+            styles.get(ParNode::LEADING).into()
+        } else {
+            styles.get(ParNode::SPACING).into()
+        };
+
+        self.0.weak(FlowChild::Spacing(amount), styles, 5);
+        self.0.supportive(FlowChild::Block(par.pack()), styles);
+        self.0.weak(FlowChild::Spacing(amount), styles, 5);
+    }
+
+    fn finish(self, doc: &mut DocBuilder<'a>, styles: StyleChain<'a>) {
+        let (flow, shared) = self.0.finish();
+        let styles = if flow.is_empty() { styles } else { shared };
+        let node = PageNode(FlowNode(flow).pack());
+        doc.pages.push(node, styles);
+    }
+
+    fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+}
+
+/// Accepts paragraph content.
+#[derive(Default)]
+struct ParBuilder<'a>(CollapsingBuilder<'a, ParChild>);
+
+impl<'a> ParBuilder<'a> {
+    fn accept(&mut self, content: &'a Content, styles: StyleChain<'a>) -> bool {
+        // Weak par elements:
+        // Weakness | Element
+        //    0     | weak fractional spacing
+        //    1     | weak spacing
+        //    2     | space
+
+        if content.is::<SpaceNode>() {
+            self.0.weak(ParChild::Text(' '.into()), styles, 2);
+        } else if let Some(linebreak) = content.downcast::<LinebreakNode>() {
+            let c = if linebreak.justify { '\u{2028}' } else { '\n' };
+            self.0.destructive(ParChild::Text(c.into()), styles);
+        } else if let Some(horizontal) = content.downcast::<HNode>() {
+            let child = ParChild::Spacing(horizontal.amount);
+            let frac = horizontal.amount.is_fractional();
+            if horizontal.weak {
+                let weakness = u8::from(!frac);
+                self.0.weak(child, styles, weakness);
+            } else if frac {
+                self.0.destructive(child, styles);
+            } else {
+                self.0.ignorant(child, styles);
+            }
+        } else if let Some(quote) = content.downcast::<SmartQuoteNode>() {
+            self.0.supportive(ParChild::Quote { double: quote.double }, styles);
+        } else if let Some(text) = content.downcast::<TextNode>() {
+            self.0.supportive(ParChild::Text(text.0.clone()), styles);
+        } else if content.has::<dyn LayoutInline>() {
+            self.0.supportive(ParChild::Inline(content.clone()), styles);
+        } else {
+            return false;
+        }
+
+        true
+    }
+
+    fn finish(self, parent: &mut Builder<'a>) {
+        let (mut children, shared) = self.0.finish();
+        if children.is_empty() {
+            return;
+        }
+
+        // Paragraph indent should only apply if the paragraph starts with
+        // text and follows directly after another paragraph.
+        let indent = shared.get(ParNode::INDENT);
+        if !indent.is_zero()
+            && children
+                .items()
+                .find_map(|child| match child {
+                    ParChild::Spacing(_) => None,
+                    ParChild::Text(_) | ParChild::Quote { .. } => Some(true),
+                    ParChild::Inline(_) => Some(false),
+                })
+                .unwrap_or_default()
+            && parent
+                .flow
+                .0
+                .items()
+                .rev()
+                .find_map(|child| match child {
+                    FlowChild::Spacing(_) => None,
+                    FlowChild::Block(content) => Some(content.is::<ParNode>()),
+                    FlowChild::Colbreak => Some(false),
+                })
+                .unwrap_or_default()
+        {
+            children.push_front(ParChild::Spacing(indent.into()));
+        }
+
+        parent.flow.par(ParNode(children), shared, !indent.is_zero());
+    }
+
+    fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+}
+
+/// Accepts list / enum items, spaces, paragraph breaks.
+struct ListBuilder<'a> {
+    /// The list items collected so far.
+    items: StyleVecBuilder<'a, ListItem>,
+    /// Whether the list contains no paragraph breaks.
+    tight: bool,
+    /// Whether the list can be attached.
+    attachable: bool,
+    /// Trailing content for which it is unclear whether it is part of the list.
+    staged: Vec<(&'a Content, StyleChain<'a>)>,
+}
+
+impl<'a> ListBuilder<'a> {
+    fn accept(&mut self, content: &'a Content, styles: StyleChain<'a>) -> bool {
+        if self.items.is_empty() {
+            if content.is::<ParbreakNode>() {
+                self.attachable = false;
+            } else if !content.is::<SpaceNode>() && !content.is::<ListItem>() {
+                self.attachable = true;
+            }
+        }
+
+        if let Some(item) = content.downcast::<ListItem>() {
+            if self
+                .items
+                .items()
+                .next()
+                .map_or(true, |first| item.kind() == first.kind())
+            {
+                self.items.push(item.clone(), styles);
+                self.tight &= self.staged.drain(..).all(|(t, _)| !t.is::<ParbreakNode>());
+            } else {
+                return false;
+            }
+        } else if !self.items.is_empty()
+            && (content.is::<SpaceNode>() || content.is::<ParbreakNode>())
+        {
+            self.staged.push((content, styles));
+        } else {
+            return false;
+        }
+
+        true
+    }
+
+    fn finish(self, parent: &mut Builder<'a>) -> SourceResult<()> {
+        let (items, shared) = self.items.finish();
+        let kind = match items.items().next() {
+            Some(item) => item.kind(),
+            None => return Ok(()),
+        };
+
+        let tight = self.tight;
+        let attached = tight && self.attachable;
+        let content = match kind {
+            LIST => ListNode::<LIST> { tight, attached, items }.pack(),
+            ENUM => ListNode::<ENUM> { tight, attached, items }.pack(),
+            DESC | _ => ListNode::<DESC> { tight, attached, items }.pack(),
+        };
+
+        let stored = parent.scratch.templates.alloc(content);
+        parent.accept(stored, shared)?;
+
+        for (content, styles) in self.staged {
+            parent.accept(content, styles)?;
+        }
+
+        parent.list.attachable = true;
+
+        Ok(())
+    }
+
+    fn is_empty(&self) -> bool {
+        self.items.is_empty()
+    }
+}
+
+impl Default for ListBuilder<'_> {
+    fn default() -> Self {
+        Self {
+            items: StyleVecBuilder::default(),
+            tight: true,
+            attachable: true,
+            staged: vec![],
+        }
+    }
+}
+
+/// A wrapper around a [`StyleVecBuilder`] that allows to collapse items.
+struct CollapsingBuilder<'a, T> {
+    /// The internal builder.
+    builder: StyleVecBuilder<'a, T>,
+    /// Staged weak and ignorant items that we can't yet commit to the builder.
+    /// The option is `Some(_)` for weak items and `None` for ignorant items.
+    staged: Vec<(T, StyleChain<'a>, Option<u8>)>,
+    /// What the last non-ignorant item was.
+    last: Last,
+}
+
+/// What the last non-ignorant item was.
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+enum Last {
+    Weak,
+    Destructive,
+    Supportive,
+}
+
+impl<'a, T> CollapsingBuilder<'a, T> {
+    /// Create a new style-vec builder.
+    pub fn new() -> Self {
+        Self {
+            builder: StyleVecBuilder::new(),
+            staged: vec![],
+            last: Last::Destructive,
+        }
+    }
+
+    /// Whether the builder is empty.
+    pub fn is_empty(&self) -> bool {
+        self.builder.is_empty() && self.staged.is_empty()
+    }
+
+    /// Can only exist when there is at least one supportive item to its left
+    /// and to its right, with no destructive items in between. There may be
+    /// ignorant items in between in both directions.
+    ///
+    /// Between weak items, there may be at least one per layer and among the
+    /// candidates the strongest one (smallest `weakness`) wins. When tied,
+    /// the one that compares larger through `PartialOrd` wins.
+    pub fn weak(&mut self, item: T, styles: StyleChain<'a>, weakness: u8)
+    where
+        T: PartialOrd,
+    {
+        if self.last == Last::Destructive {
+            return;
+        }
+
+        if self.last == Last::Weak {
+            if let Some(i) =
+                self.staged.iter().position(|(prev_item, _, prev_weakness)| {
+                    prev_weakness.map_or(false, |prev_weakness| {
+                        weakness < prev_weakness
+                            || (weakness == prev_weakness && item > *prev_item)
+                    })
+                })
+            {
+                self.staged.remove(i);
+            } else {
+                return;
+            }
+        }
+
+        self.staged.push((item, styles, Some(weakness)));
+        self.last = Last::Weak;
+    }
+
+    /// Forces nearby weak items to collapse.
+    pub fn destructive(&mut self, item: T, styles: StyleChain<'a>) {
+        self.flush(false);
+        self.builder.push(item, styles);
+        self.last = Last::Destructive;
+    }
+
+    /// Allows nearby weak items to exist.
+    pub fn supportive(&mut self, item: T, styles: StyleChain<'a>) {
+        self.flush(true);
+        self.builder.push(item, styles);
+        self.last = Last::Supportive;
+    }
+
+    /// Has no influence on other items.
+    pub fn ignorant(&mut self, item: T, styles: StyleChain<'a>) {
+        self.staged.push((item, styles, None));
+    }
+
+    /// Iterate over the contained items.
+    pub fn items(&self) -> impl DoubleEndedIterator<Item = &T> {
+        self.builder.items().chain(self.staged.iter().map(|(item, ..)| item))
+    }
+
+    /// Return the finish style vec and the common prefix chain.
+    pub fn finish(mut self) -> (StyleVec<T>, StyleChain<'a>) {
+        self.flush(false);
+        self.builder.finish()
+    }
+
+    /// Push the staged items, filtering out weak items if `supportive` is
+    /// false.
+    fn flush(&mut self, supportive: bool) {
+        for (item, styles, meta) in self.staged.drain(..) {
+            if supportive || meta.is_none() {
+                self.builder.push(item, styles);
+            }
+        }
+    }
+}
+
+impl<'a, T> Default for CollapsingBuilder<'a, T> {
+    fn default() -> Self {
+        Self::new()
+    }
+}