amend! parser: determine tight vs loose lists

block: determine tight vs loose lists

src/block.rs

@@ -13,7 +13,6 @@ use ListType::*;
 
 pub type Tree = tree::Tree<Node, Atom>;
 pub type TreeBuilder = tree::Builder<Node, Atom>;
-pub type Element = tree::Element<Node, Atom>;
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum Node {
@@ -81,8 +80,11 @@ pub enum Container {
     /// Span is class specifier, possibly empty.
     Div,
 
+    /// Span is `:`.
+    DescriptionList,
+
     /// Span is the list marker of the first list item in the list.
-    List(ListType),
+    List { ty: ListType, tight: bool },
 
     /// Span is the list marker.
     ListItem(ListType),
@@ -96,7 +98,6 @@ pub enum ListType {
     Unordered(u8),
     Ordered(crate::OrderedListNumbering, crate::OrderedListStyle),
     Task,
-    Description,
 }
 
 #[derive(Debug)]
@@ -107,6 +108,8 @@ struct OpenList {
     /// Depth in the tree where the direct list items of the list are. Needed to determine when to
     /// close the list.
     depth: u16,
+    /// Index to node in tree, required to update tightness.
+    node: tree::NodeIndex,
 }
 
 /// Parser for block-level tree structure of entire document.
@@ -114,7 +117,10 @@ struct TreeParser<'s> {
     src: &'s str,
     tree: TreeBuilder,
 
-    lists_open: Vec<OpenList>,
+    /// The previous block element was a blank line.
+    prev_blankline: bool,
+    /// Stack of currently open lists.
+    open_lists: Vec<OpenList>,
 }
 
 impl<'s> TreeParser<'s> {
@@ -123,7 +129,8 @@ impl<'s> TreeParser<'s> {
         Self {
             src,
             tree: TreeBuilder::new(),
-            lists_open: Vec::new(),
+            prev_blankline: false,
+            open_lists: Vec::new(),
         }
     }
 
@@ -138,7 +145,7 @@ impl<'s> TreeParser<'s> {
             }
             line_pos += line_count;
         }
-        for _ in self.lists_open.drain(..) {
+        for _ in self.open_lists.drain(..) {
             self.tree.exit(); // list
         }
         self.tree.finish()
@@ -176,6 +183,45 @@ impl<'s> TreeParser<'s> {
                     lines
                 };
 
+                // close list if a non list item or a list item of new type appeared
+                if let Some(OpenList { ty, depth, .. }) = self.open_lists.last() {
+                    assert!(usize::from(*depth) <= self.tree.depth());
+                    if self.tree.depth() == (*depth).into()
+                        && !matches!(
+                            kind,
+                            Block::Container(Container::ListItem(ty_new)) if *ty == ty_new,
+                        )
+                    {
+                        self.tree.exit(); // list
+                        self.open_lists.pop();
+                    }
+                }
+
+                // set list to loose if blankline discovered
+                if matches!(kind, Block::Atom(Atom::Blankline)) {
+                    self.prev_blankline = true;
+                } else {
+                    if self.prev_blankline {
+                        for OpenList { node, depth, .. } in &self.open_lists {
+                            if usize::from(*depth) < self.tree.depth()
+                                && matches!(kind, Block::Container(Container::ListItem { .. }))
+                            {
+                                continue;
+                            }
+                            if let tree::Element::Container(Node::Container(Container::List {
+                                tight,
+                                ..
+                            })) = self.tree.elem_mut(*node)
+                            {
+                                *tight = false;
+                            } else {
+                                panic!();
+                            }
+                        }
+                    }
+                    self.prev_blankline = false;
+                }
+
                 match kind {
                     Block::Atom(a) => self.tree.atom(a, span),
                     Block::Leaf(l) => {
@@ -210,7 +256,7 @@ impl<'s> TreeParser<'s> {
                     Block::Container(c) => {
                         let (skip_chars, skip_lines_suffix) = match c {
                             Blockquote => (2, 0),
-                            List(..) => panic!(),
+                            List{..} | DescriptionList => panic!(),
                             ListItem(..) | Footnote => (indent, 0),
                             Div => (0, 1),
                         };
@@ -234,16 +280,20 @@ impl<'s> TreeParser<'s> {
 
                         if let Container::ListItem(ty) = c {
                             if self
-                                .lists_open
+                                .open_lists
                                 .last()
                                 .map_or(true, |OpenList { depth, .. }| {
                                     usize::from(*depth) < self.tree.depth()
                                 })
                             {
-                                self.tree.enter(Node::Container(Container::List(ty)), span);
+                                let tight = true;
-                                self.lists_open.push(OpenList {
+                                let node = self
+                                    .tree
+                                    .enter(Node::Container(Container::List { ty, tight }), span);
+                                self.open_lists.push(OpenList {
                                     ty,
                                     depth: self.tree.depth().try_into().unwrap(),
+                                    node,
                                 });
                             }
                         }
@@ -254,11 +304,11 @@ impl<'s> TreeParser<'s> {
                             l += self.parse_block(&mut lines[l..line_count_inner]);
                         }
 
-                        if let Some(OpenList { depth, .. }) = self.lists_open.last() {
+                        if let Some(OpenList { depth, .. }) = self.open_lists.last() {
                             assert!(usize::from(*depth) <= self.tree.depth());
                             if self.tree.depth() == (*depth).into() {
                                 self.tree.exit(); // list
-                                self.lists_open.pop();
+                                self.open_lists.pop();
                             }
                         }
 
@@ -368,10 +418,9 @@ impl BlockParser {
                     )
                 }
             }),
-            ':' if chars.clone().next().map_or(true, char::is_whitespace) => Some((
+            ':' if chars.clone().next().map_or(true, char::is_whitespace) => {
-                Block::Container(ListItem(Description)),
+                Some((Block::Container(DescriptionList), Span::by_len(start, 1)))
-                Span::by_len(start, 1),
+            }
-            )),
             f @ ('`' | ':' | '~') => {
                 let fence_length = (&mut chars).take_while(|c| *c == f).count() + 1;
                 fence = Some((f, fence_length));
@@ -445,7 +494,7 @@ impl BlockParser {
                 !((&mut c).take(fence_length).all(|c| c == fence)
                     && c.next().map_or(true, char::is_whitespace))
             }
-            Block::Container(List(..)) => panic!(),
+            Block::Container(List { .. } | DescriptionList) => panic!(),
         }
     }
 
@@ -818,42 +867,153 @@ mod test {
     #[test]
     fn parse_list_single_item() {
         test_parse!(
-            concat!(
+            "- abc",
-                "- abc\n",
+            (
-                "\n",
+                Enter(Container(List {
-                "\n", //
+                    ty: Unordered(b'-'),
+                    tight: true
+                })),
+                "-"
             ),
-            (Enter(Container(List(Unordered(b'-')))), "-"),
             (Enter(Container(ListItem(Unordered(b'-')))), "-"),
             (Enter(Leaf(Paragraph)), ""),
             (Inline, "abc"),
             (Exit(Leaf(Paragraph)), ""),
-            (Atom(Blankline), "\n"),
-            (Atom(Blankline), "\n"),
             (Exit(Container(ListItem(Unordered(b'-')))), "-"),
-            (Exit(Container(List(Unordered(b'-')))), "-"),
+            (
+                Exit(Container(List {
+                    ty: Unordered(b'-'),
+                    tight: true
+                })),
+                "-"
+            ),
         );
     }
 
     #[test]
-    fn parse_list_multi_item() {
+    fn parse_list_tight() {
         test_parse!(
-            "- abc\n\n\n- def\n\n",
+            concat!(
-            (Enter(Container(List(Unordered(b'-')))), "-"),
+                "- a\n", //
+                "- b\n", //
+            ),
+            (
+                Enter(Container(List {
+                    ty: Unordered(b'-'),
+                    tight: true,
+                })),
+                "-"
+            ),
             (Enter(Container(ListItem(Unordered(b'-')))), "-"),
             (Enter(Leaf(Paragraph)), ""),
-            (Inline, "abc"),
+            (Inline, "a"),
+            (Exit(Leaf(Paragraph)), ""),
+            (Exit(Container(ListItem(Unordered(b'-')))), "-"),
+            (Enter(Container(ListItem(Unordered(b'-')))), "-"),
+            (Enter(Leaf(Paragraph)), ""),
+            (Inline, "b"),
+            (Exit(Leaf(Paragraph)), ""),
+            (Exit(Container(ListItem(Unordered(b'-')))), "-"),
+            (
+                Exit(Container(List {
+                    ty: Unordered(b'-'),
+                    tight: true,
+                })),
+                "-"
+            ),
+        );
+    }
+
+    #[test]
+    fn parse_list_loose() {
+        test_parse!(
+            concat!(
+                "- a\n", //
+                "- b\n", //
+                "\n",    //
+                "- c\n", //
+            ),
+            (
+                Enter(Container(List {
+                    ty: Unordered(b'-'),
+                    tight: false,
+                })),
+                "-"
+            ),
+            (Enter(Container(ListItem(Unordered(b'-')))), "-"),
+            (Enter(Leaf(Paragraph)), ""),
+            (Inline, "a"),
+            (Exit(Leaf(Paragraph)), ""),
+            (Exit(Container(ListItem(Unordered(b'-')))), "-"),
+            (Enter(Container(ListItem(Unordered(b'-')))), "-"),
+            (Enter(Leaf(Paragraph)), ""),
+            (Inline, "b"),
             (Exit(Leaf(Paragraph)), ""),
-            (Atom(Blankline), "\n"),
             (Atom(Blankline), "\n"),
             (Exit(Container(ListItem(Unordered(b'-')))), "-"),
             (Enter(Container(ListItem(Unordered(b'-')))), "-"),
             (Enter(Leaf(Paragraph)), ""),
-            (Inline, "def"),
+            (Inline, "c"),
+            (Exit(Leaf(Paragraph)), ""),
+            (Exit(Container(ListItem(Unordered(b'-')))), "-"),
+            (
+                Exit(Container(List {
+                    ty: Unordered(b'-'),
+                    tight: false,
+                })),
+                "-"
+            ),
+        );
+    }
+
+    #[test]
+    fn parse_list_tight_nest() {
+        test_parse!(
+            concat!(
+                "- a\n",    //
+                "\n",       //
+                "  + aa\n", //
+                "  + ab\n", //
+                "\n",       //
+                "- b\n",    //
+            ),
+            (
+                Enter(Container(List {
+                    ty: Unordered(b'-'),
+                    tight: true,
+                })),
+                "-"
+            ),
+            (Enter(Container(ListItem(Unordered(b'-')))), "-"),
+            (Enter(Leaf(Paragraph)), ""),
+            (Inline, "a"),
+            (Exit(Leaf(Paragraph)), ""),
+            (Atom(Blankline), "\n"),
+            (
+                Enter(Container(List {
+                    ty: Unordered(b'+'),
+                    tight: true,
+                })),
+                "+",
+            ),
+            (Enter(Container(ListItem(Unordered(b'+')))), "+"),
+            (Enter(Leaf(Paragraph)), ""),
+            (Inline, "aa"),
+            (Exit(Leaf(Paragraph)), ""),
+            (Exit(Container(ListItem(Unordered(b'+')))), "+"),
+            (Enter(Container(ListItem(Unordered(b'+')))), "+"),
+            (Enter(Leaf(Paragraph)), ""),
+            (Inline, "ab"),
             (Exit(Leaf(Paragraph)), ""),
             (Atom(Blankline), "\n"),
             (Exit(Container(ListItem(Unordered(b'-')))), "-"),
-            (Exit(Container(List(Unordered(b'-')))), "-"),
+            (
+                Exit(Container(List {
+                    ty: Unordered(b'-'),
+                    tight: true,
+                })),
+                "-"
+            ),
         );
     }
 
@@ -862,34 +1022,135 @@ mod test {
         test_parse!(
             concat!(
                 "- a\n",     //
-                "\n",        //
+                "    \n",    //
-                "   - aa\n", //
+                "  + b\n",   //
-                "\n",        //
+                "      \n",  //
-                "\n",        //
+                "    * c\n", //
-                "- b\n",     //
+            ),
+            (
+                Enter(Container(List {
+                    ty: Unordered(b'-'),
+                    tight: true,
+                })),
+                "-"
             ),
-            (Enter(Container(List(Unordered(b'-')))), "-"),
-            (Enter(Container(ListItem(Unordered(b'-')))), "-"),
-            (Enter(Leaf(Paragraph)), ""),
-            (Inline, "a"),
-            (Exit(Leaf(Paragraph)), ""),
-            (Atom(Blankline), "\n"),
-            (Enter(Container(List(Unordered(b'-')))), "-"),
-            (Enter(Container(ListItem(Unordered(b'-')))), "-"),
-            (Enter(Leaf(Paragraph)), ""),
-            (Inline, "aa"),
-            (Exit(Leaf(Paragraph)), ""),
-            (Atom(Blankline), "\n"),
-            (Atom(Blankline), "\n"),
-            (Exit(Container(ListItem(Unordered(b'-')))), "-"),
-            (Exit(Container(List(Unordered(b'-')))), "-"),
-            (Exit(Container(ListItem(Unordered(b'-')))), "-"),
             (Enter(Container(ListItem(Unordered(b'-')))), "-"),
             (Enter(Leaf(Paragraph)), ""),
             (Inline, "b"),
             (Exit(Leaf(Paragraph)), ""),
             (Exit(Container(ListItem(Unordered(b'-')))), "-"),
-            (Exit(Container(List(Unordered(b'-')))), "-"),
+            (
+                Exit(Container(List {
+                    ty: Unordered(b'-'),
+                    tight: true,
+                })),
+                "-"
+            ),
+        );
+    }
+
+    #[test]
+    fn parse_list_post() {
+        test_parse!(
+            concat!(
+                "- a\n",   //
+                "\n",      //
+                "  * b\n", //
+                "cd\n",    //
+            ),
+            (
+                Enter(Container(List {
+                    ty: Unordered(45),
+                    tight: true
+                })),
+                "-"
+            ),
+            (Enter(Container(ListItem(Unordered(45)))), "-"),
+            (Enter(Leaf(Paragraph)), ""),
+            (Inline, "a"),
+            (Exit(Leaf(Paragraph)), ""),
+            (Atom(Blankline), "\n"),
+            (
+                Enter(Container(List {
+                    ty: Unordered(42),
+                    tight: true
+                })),
+                "*"
+            ),
+            (Enter(Container(ListItem(Unordered(42)))), "*"),
+            (Enter(Leaf(Paragraph)), ""),
+            (Inline, "b\n"),
+            (Inline, "cd"),
+            (Exit(Leaf(Paragraph)), ""),
+            (Exit(Container(ListItem(Unordered(42)))), "*"),
+            (
+                Exit(Container(List {
+                    ty: Unordered(42),
+                    tight: true
+                })),
+                "*"
+            ),
+            (Exit(Container(ListItem(Unordered(45)))), "-"),
+            (
+                Exit(Container(List {
+                    ty: Unordered(45),
+                    tight: true
+                })),
+                "-"
+            ),
+        );
+    }
+    #[test]
+    fn parse_list_mixed() {
+        test_parse!(
+            concat!(
+                "- a\n", //
+                "+ b\n", //
+                "+ c\n", //
+            ),
+            (
+                Enter(Container(List {
+                    ty: Unordered(b'-'),
+                    tight: true
+                })),
+                "-"
+            ),
+            (Enter(Container(ListItem(Unordered(b'-')))), "-"),
+            (Enter(Leaf(Paragraph)), ""),
+            (Inline, "a"),
+            (Exit(Leaf(Paragraph)), ""),
+            (Exit(Container(ListItem(Unordered(b'-')))), "-"),
+            (
+                Exit(Container(List {
+                    ty: Unordered(b'-'),
+                    tight: true
+                })),
+                "-"
+            ),
+            (
+                Enter(Container(List {
+                    ty: Unordered(b'+'),
+                    tight: true
+                })),
+                "+"
+            ),
+            (Enter(Container(ListItem(Unordered(b'+')))), "+"),
+            (Enter(Leaf(Paragraph)), ""),
+            (Inline, "b"),
+            (Exit(Leaf(Paragraph)), ""),
+            (Exit(Container(ListItem(Unordered(b'+')))), "+"),
+            (Enter(Container(ListItem(Unordered(b'+')))), "+"),
+            (Enter(Leaf(Paragraph)), ""),
+            (Inline, "c"),
+            (Exit(Leaf(Paragraph)), ""),
+            (Exit(Container(ListItem(Unordered(b'+')))), "+"),
+            (
+                Exit(Container(List {
+                    ty: Unordered(b'+'),
+                    tight: true
+                })),
+                "+"
+            ),
         );
     }
 
@@ -1081,7 +1342,7 @@ mod test {
 
     #[test]
     fn block_list_description() {
-        test_block!(": abc\n", Block::Container(ListItem(Description)), ":", 1);
+        test_block!(": abc\n", Block::Container(DescriptionList), ":", 1);
     }
 
     #[test]

src/html.rs

@@ -103,18 +103,14 @@ impl<'s, I: Iterator<Item = Event<'s>>, W: std::fmt::Write> Writer<'s, I, W> {
                     }
                     match &c {
                         Container::Blockquote => self.out.write_str("<blockquote")?,
-                        Container::List {
+                        Container::List { kind, tight } => {
-                            kind: ListKind::Unordered | ListKind::Task,
+                            self.list_tightness.push(*tight);
-                            ..
+                            match kind {
-                        } => {
+                                ListKind::Unordered | ListKind::Task => {
-                            self.out.write_str("<ul")?;
+                                    self.out.write_str("<ul")?
                                 }
-                        Container::List {
-                            kind:
                                 ListKind::Ordered {
                                     numbering, start, ..
-                                },
-                            ..
                                 } => {
                                     self.out.write_str("<ol")?;
                                     if *start > 1 {
@@ -130,6 +126,8 @@ impl<'s, I: Iterator<Item = Event<'s>>, W: std::fmt::Write> Writer<'s, I, W> {
                                         write!(self.out, r#" type="{}""#, ty)?;
                                     }
                                 }
+                            }
+                        }
                         Container::ListItem | Container::TaskListItem { .. } => {
                             self.out.write_str("<li")?;
                         }

src/lib.rs

@@ -293,9 +293,6 @@ pub struct Parser<'s> {
     /// Inline parser, recreated for each new inline.
     inline_parser: Option<inline::Parser<span::InlineCharsIter<'s>>>,
 
-    /// Stack of tightnesses for current open lists.
-    list_tightness: Vec<bool>,
-
     /// Footnote references in the order they were encountered, without duplicates.
     footnote_references: Vec<&'s str>,
     /// Cache of footnotes to emit at the end.
@@ -336,7 +333,6 @@ impl<'s> Parser<'s> {
             src,
             link_definitions,
             tree: branch,
-            list_tightness: Vec::new(),
             footnote_references: Vec::new(),
             footnotes: std::collections::HashMap::new(),
             footnote_index: 0,
@@ -506,10 +502,8 @@ impl<'s> Parser<'s> {
                                 self.footnotes.insert(content, self.tree.take_branch());
                                 continue;
                             }
-                            block::Container::List(block::ListType::Description) => {
+                            block::Container::DescriptionList => Container::DescriptionList,
-                                Container::DescriptionList
+                            block::Container::List { ty, tight } => {
-                            }
-                            block::Container::List(ty) => {
                                 let kind = match ty {
                                     block::ListType::Unordered(..) => ListKind::Unordered,
                                     block::ListType::Ordered(numbering, style) => {
@@ -523,30 +517,6 @@ impl<'s> Parser<'s> {
                                         }
                                     }
                                     block::ListType::Task => ListKind::Task,
-                                    block::ListType::Description => unreachable!(),
-                                };
-                                let tight = if enter {
-                                    let tight = !self.tree.linear().any(|elem| {
-                                        matches!(elem, block::Element::Atom(block::Atom::Blankline))
-                                    }) && !self.tree.linear_containers().any(
-                                        |(c, tree)| {
-                                            matches!(
-                                                c,
-                                                block::Node::Container(block::Container::ListItem(
-                                                    ..
-                                                ))
-                                            ) && tree.linear().any(|elem| {
-                                                matches!(
-                                                    elem,
-                                                    block::Element::Atom(block::Atom::Blankline)
-                                                )
-                                            })
-                                        },
-                                    );
-                                    self.list_tightness.push(tight);
-                                    tight
-                                } else {
-                                    self.list_tightness.pop().unwrap()
                                 };
                                 Container::List { kind, tight }
                             }

src/tree.rs

@@ -8,10 +8,9 @@ pub enum EventKind<C, A> {
     Atom(A),
 }
 
-#[derive(Debug, Clone)]
+pub enum Element<'a, C, A> {
-pub enum Element<C, A> {
+    Container(&'a mut C),
-    Container(C),
+    Atom(&'a mut A),
-    Atom(A),
     Inline,
 }
 
@@ -29,14 +28,6 @@ pub struct Tree<C: 'static, A: 'static> {
 }
 
 impl<C: Clone, A: Clone> Tree<C, A> {
-    fn with_head(&self, head: Option<NodeIndex>) -> Self {
-        Self {
-            nodes: self.nodes.clone(),
-            branch: Vec::new(),
-            head,
-        }
-    }
-
     pub fn empty() -> Self {
         Self {
             nodes: vec![].into_boxed_slice().into(),
@@ -57,42 +48,6 @@ impl<C: Clone, A: Clone> Tree<C, A> {
         count
     }
 
-    /// Retrieve upcoming direct events without entering branches.
-    pub fn linear(&self) -> impl Iterator<Item = Element<C, A>> + '_ {
-        let mut head = self.head;
-        std::iter::from_fn(move || {
-            head.take().map(|h| {
-                let n = &self.nodes[h.index()];
-                head = n.next;
-                match &n.kind {
-                    NodeKind::Root => unreachable!(),
-                    NodeKind::Container(c, ..) => Element::Container(c.clone()),
-                    NodeKind::Atom(a) => Element::Atom(a.clone()),
-                    NodeKind::Inline => Element::Inline,
-                }
-            })
-        })
-    }
-
-    /// Retrieve the upcoming branches.
-    pub fn linear_containers(&self) -> impl Iterator<Item = (C, Self)> + '_ {
-        let mut head = self.head;
-        std::iter::from_fn(move || {
-            while let Some(h) = head.take() {
-                let n = &self.nodes[h.index()];
-                head = n.next;
-                match &n.kind {
-                    NodeKind::Root => unreachable!(),
-                    NodeKind::Container(c, child) => {
-                        return Some((c.clone(), self.with_head(*child)));
-                    }
-                    NodeKind::Atom(_) | NodeKind::Inline => continue,
-                }
-            }
-            None
-        })
-    }
-
     /// Split off the remaining part of the current branch. The returned [`Tree`] will continue on
     /// the branch, this [`Tree`] will skip over the current branch.
     pub fn take_branch(&mut self) -> Self {
@@ -162,7 +117,7 @@ impl<C: Clone, A: Clone> Iterator for Tree<C, A> {
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
-struct NodeIndex(std::num::NonZeroUsize);
+pub struct NodeIndex(std::num::NonZeroUsize);
 
 impl NodeIndex {
     fn new(i: usize) -> Self {
@@ -232,13 +187,13 @@ impl<C: Clone, A: Clone> Builder<C, A> {
         });
     }
 
-    pub(super) fn enter(&mut self, c: C, span: Span) {
+    pub(super) fn enter(&mut self, c: C, span: Span) -> NodeIndex {
         self.depth += 1;
         self.add_node(Node {
             span,
             kind: NodeKind::Container(c, None),
             next: None,
-        });
+        })
     }
 
     pub(super) fn exit(&mut self) {
@@ -251,6 +206,19 @@ impl<C: Clone, A: Clone> Builder<C, A> {
         }
     }
 
+    pub(super) fn depth(&self) -> usize {
+        self.depth
+    }
+
+    pub(super) fn elem_mut(&mut self, ni: NodeIndex) -> Element<C, A> {
+        match &mut self.nodes[ni.index()].kind {
+            NodeKind::Root => unreachable!(),
+            NodeKind::Container(c, ..) => Element::Container(c),
+            NodeKind::Atom(a) => Element::Atom(a),
+            NodeKind::Inline => Element::Inline,
+        }
+    }
+
     pub(super) fn finish(self) -> Tree<C, A> {
         assert_eq!(self.depth, 0);
         let head = self.nodes[NodeIndex::root().index()].next;
@@ -261,11 +229,7 @@ impl<C: Clone, A: Clone> Builder<C, A> {
         }
     }
 
-    pub(super) fn depth(&self) -> usize {
+    fn add_node(&mut self, node: Node<C, A>) -> NodeIndex {
-        self.depth
-    }
-
-    fn add_node(&mut self, node: Node<C, A>) {
         let ni = NodeIndex::new(self.nodes.len());
         self.nodes.push(node);
         if let Some(head_ni) = &mut self.head {
@@ -291,6 +255,7 @@ impl<C: Clone, A: Clone> Builder<C, A> {
             panic!()
         }
         self.head = Some(ni);
+        ni
     }
 }