tree: add tree branch reference

allow multiple independent iterators for the same underlying tree

safety: not very good, but should work because original tree is kept in
Parser and branches are only used during its lifetime

src/block.rs

@@ -418,7 +418,7 @@ mod test {
     macro_rules! test_parse {
             ($src:expr $(,$($event:expr),* $(,)?)?) => {
                 let t = super::TreeParser::new($src).parse();
-                let actual = t.map(|ev| (ev.kind, ev.span.of($src))).collect::<Vec<_>>();
+                let actual = t.root().map(|ev| (ev.kind, ev.span.of($src))).collect::<Vec<_>>();
                 let expected = &[$($($event),*,)?];
                 assert_eq!(actual, expected, "\n\n{}\n\n", $src);
             };

src/lib.rs

@@ -265,10 +265,12 @@ impl<'s> Container<'s> {
 
 pub struct Parser<'s> {
     src: &'s str,
-    tree: block::Tree,
+    tree: tree::Branch<block::Node, block::Atom>,
     inlines: span::InlineSpans<'s>,
     inline_parser: Option<inline::Parser<span::InlineCharsIter<'s>>>,
     link_definitions: std::collections::HashMap<&'s str, String>,
+
+    _tree_data: block::Tree,
 }
 
 impl<'s> Parser<'s> {
@@ -277,27 +279,30 @@ impl<'s> Parser<'s> {
         let tree = block::parse(src);
 
         let link_definitions = {
-            let mut tree = tree.clone(); // TODO avoid clone
+            let mut branch = tree.root();
             let mut defs = std::collections::HashMap::new();
-            while let Some(e) = tree.next() {
+            while let Some(e) = branch.next() {
                 if let tree::EventKind::Enter(block::Node::Leaf(block::Leaf::LinkDefinition)) =
                     e.kind
                 {
                     let tag = e.span.of(src);
                     // TODO borrow url string if single inline
-                    let url = tree.inlines().map(|sp| sp.of(src).trim()).collect();
+                    let url = branch.take_inlines().map(|sp| sp.of(src).trim()).collect();
                     defs.insert(tag, url);
                 }
             }
             defs
         };
 
+        let branch = tree.root();
+
         Self {
             src,
-            tree,
+            tree: branch,
             inlines: span::InlineSpans::new(src),
             inline_parser: None,
             link_definitions,
+            _tree_data: tree,
         }
     }
 }
@@ -423,12 +428,12 @@ impl<'s> Parser<'s> {
                     if matches!(c, block::Node::Leaf(block::Leaf::LinkDefinition)) =>
                 {
                     // ignore link definitions
-                    self.tree.inlines().last();
+                    self.tree.take_inlines().last();
                     continue;
                 }
                 tree::EventKind::Enter(c) => match c {
                     block::Node::Leaf(l) => {
-                        self.inlines.set_spans(self.tree.inlines());
+                        self.inlines.set_spans(self.tree.take_inlines());
                         self.inline_parser = Some(inline::Parser::new(self.inlines.chars()));
                         let container = Container::from_leaf_block(content, l);
                         Event::Start(container, attributes)

src/tree.rs

@@ -14,12 +14,7 @@ pub struct Event<C, A> {
     pub span: Span,
 }
 
-#[derive(Clone)]
-pub struct Tree<C, A> {
-    nodes: Vec<Node<C, A>>,
-    branch: Vec<NodeIndex>,
-    head: Option<NodeIndex>,
-}
+pub struct Tree<C, A>(Box<[Node<C, A>]>);
 
 #[derive(Clone)]
 pub struct Inlines<'t, C, A> {
@@ -35,28 +30,34 @@ impl<'t, C, A> Iterator for Inlines<'t, C, A> {
 }
 
 impl<C, A> Tree<C, A> {
-    fn new(nodes: Vec<Node<C, A>>) -> Self {
-        let head = nodes[NodeIndex::root().index()].next;
-        Self {
+    pub fn root(&self) -> Branch<C, A> {
+        let head = self.0[NodeIndex::root().index()].next;
+        // SAFETY: tree must outlive the branch
+        let nodes = unsafe { std::mem::transmute::<&[Node<C, A>], &'static [Node<C, A>]>(&self.0) };
+        Branch {
             nodes,
             branch: Vec::new(),
             head,
         }
     }
+}
 
-    pub fn inlines(&self) -> Inlines<C, A> {
-        let start = self.nodes[self.head.unwrap().index()].next.unwrap().index();
-        let end = start + self.spans().count();
-        Inlines {
-            iter: self.nodes[start..end].iter(),
-        }
-    }
+#[derive(Clone)]
+pub struct Branch<C: 'static, A: 'static> {
+    nodes: &'static [Node<C, A>],
+    branch: Vec<NodeIndex>,
+    head: Option<NodeIndex>,
+}
 
-    pub fn spans(&self) -> impl Iterator<Item = Span> + '_ {
-        let mut head = self.head;
+impl<C, A> Branch<C, A> {
+    /// Retrieve all inlines until the end of the current container. Panics if any upcoming node is
+    /// not an inline node.
+    pub fn take_inlines(&mut self) -> impl Iterator<Item = Span> + '_ {
+        let mut head = self.head.take();
         std::iter::from_fn(move || {
             head.take().map(|h| {
                 let n = &self.nodes[h.index()];
+                assert!(matches!(n.kind, NodeKind::Inline));
                 head = n.next;
                 n.span
             })
@@ -64,7 +65,7 @@ impl<C, A> Tree<C, A> {
     }
 }
 
-impl<C: Clone, A: Clone> Iterator for Tree<C, A> {
+impl<C: Clone, A: Clone> Iterator for Branch<C, A> {
     type Item = Event<C, A>;
 
     fn next(&mut self) -> Option<Self::Item> {
@@ -188,7 +189,7 @@ impl<C: Clone, A: Clone> Builder<C, A> {
     }
 
     pub(super) fn finish(self) -> Tree<C, A> {
-        Tree::new(self.nodes)
+        Tree(self.nodes.into_boxed_slice())
     }
 
     fn add_node(&mut self, node: Node<C, A>) {
@@ -220,13 +221,15 @@ impl<C: Clone, A: Clone> Builder<C, A> {
     }
 }
 
-impl<C: std::fmt::Debug + Clone, A: std::fmt::Debug + Clone> std::fmt::Debug for Builder<C, A> {
+impl<C: std::fmt::Debug + Clone + 'static, A: std::fmt::Debug + Clone + 'static> std::fmt::Debug
+    for Builder<C, A>
+{
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        self.clone().finish().fmt(f)
+        self.clone().finish().root().fmt(f)
     }
 }
 
-impl<C: std::fmt::Debug + Clone, A: std::fmt::Debug + Clone> std::fmt::Debug for Tree<C, A> {
+impl<C: std::fmt::Debug + Clone, A: std::fmt::Debug + Clone> std::fmt::Debug for Branch<C, A> {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         const INDENT: &str = "  ";
         let mut level = 0;